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Offline Flyin6

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Early Cummins powered Dodge computer removal and rewire
« on: May 04, 2016, 04:39:45 PM »
I have been assembling a compilation of general information about how to safely and smartly 'eliminate' the factory computer (PCM) from our classic Dodge Rams, particularly on the Cummins-powered Rams.  This information is specifically intended to help those doing a Cummins conversion, however I feel the information could be beneficial to anyone with an older Ram with a PCM, or who like me considers 'wiring' to be their nemesis.

First, as you all know, these older-than-dirt computers are dying every day.  The newest of them is now 20 years old.  With the three Ramchargers I've done conversions on, my goal was to eliminate the computer controls by replacing both the sensors and receivers for the factory gauges and switches with typical off-the-shelf components found at most local parts stores or online, while also trying to retain all of the factory gauges.  We got it to about the 90% solution, and we're still working on a couple items, but we're very close to being 100% computer-free.

Eliminating the computer on a Cummins conversion is fairly straight forward, and actually makes the wiring harness much less complicated for two reasons:  There are no ‘typical’ emissions requirements with an early Cummins, and second, no ignition system.  When we did Oxx, we also went through all of the legal requirements to officially convert the title to reflect the vehicle now runs on diesel.  For the most part, every single bit of factory Ram electronic ignition and emissions equipment was no longer needed.  The early 1st Gen Cummins Rams did not have catalytic converters, no 'fast idle solenoid’, no 'lean burn', no vacuum-assisted choke...none of that.  I'll address the main components individually.  You will see me refer to “12v power” – that is “switched” power that is enabled when the ignition key switch is turned to run (and start).  You may also see me refer to “B+” power – that is direct Battery Hot power.

Fuel Shut Off Solenoid (FSO).  The FSO is what 'allows' fuel to flow, or not.  When you turn the key to run (and start) the FSO is sent 12v switched power from the main harness via the key switch, which allows the injection pump to send fuel to the injectors.  When you turn the key off, 12v power is removed and the fuel is cut off, shutting down the engine.  This is the normal operating procedure for a Cummins.  Naturally, having 12v switched power going to the Cummins engine harness (and particularly to the FSO) is essential.  This doesn't require any special switches, but it can be overlooked during a conversion.  Also, the factory used a spade terminal to attach to the FSO – we highly recommend replacing the spade terminal with a proper sized fish eye terminal and using the proper nut to retain the 12v wire securely to the FSO.  Otherwise the spade terminal can wiggle off and cause a sudden and immediate engine shut down.

Grid Heaters (toaster).  The toaster is another matter.  From the factory, it is controlled via the computer, and received signals from a couple sensors in the intake manifold.  The computer is what tells the grid heater to cycle off and on during warm-up (only when it is cold enough outside), and after things warm up the computer tells the heater to stop cycling.  You will notice the volt meter drop significantly when this is occurring.  This is normal.  But, the engine does not need the toaster to cycle itself off and on for 5 minutes during warm-up.  Soon as the engine is started and running a few seconds, the toaster is no longer needed.  Many folks disconnect it all together.  It has to be really cold (0* or less) before the toaster is 'essential' for starting the engine.  But, I like having it, so I simply took the 12v signal wire to the toaster and redirected it to my dash to a momentary switch, which only activated when I push it, and I control for how long and how often.  Also, because we used the Ram (Cummins) dash and message center, the Ram [WAIT-TO-START] light is now wired into the momentary signal wire out to the heater.  So, when the heater button is pressed, the heater relays come on and the [WAIT-TO-START] light comes on.  It’s very cool.  On the coldest days we recommend using the heater for about 15 seconds, release, and turn the key.  It’ll start – they always do, though because the fuel is also ice cold it might immediately run a bit rough and blow a bit of white smoke.  That is normal.  Once the engine is running press/hold the switch again until it smoothes out, usually in about 10 seconds.  After that, drive on – no more heater required.

KSB (Kaltstartbeschleuniger).  I'm not a true expert on the KSB, but we know it is a German device that means ‘cold-start device’  i.e. – “choke” (the 1st Gen VE44 injection pumps were manufactured by Bosch, also a German company).  The KSB is the little round component about the size of a tall shot glass mounted on the drivers’ side of the injection pump.  The KSB is used as a cold start aid.  It will advance the injection pump timing when cold, and is designed to provide substantial injection timing advance in the low engine rpms so as to improve engine operation / emissions when the engine is stone cold.  When in the engine rpms come up, the substantial timing advance tapers off to a much smaller measure of advance.  The KSB does not function after the engine coolant reaches ~160* F.  There are two versions (the early non-intercooled, and the later intercooled), and it is important to understand the differences.

VE44 Non-Intercooled KSB.
Pre '91 engines used the ‘wax motor’ type KSB, called so because a pellet of encapsulated engineering wax/plastic expanded when heated to move a plunger, which internally advanced the timing.  1988 emission requirements required the timing to be advanced when cold, to reduce white smoke.  The non-intercooled trucks have a "wax motor" that opens the bypass passage in the KSB unit.  When the ignition switch is turned ‘on’ an internal heating elementbegins to heat up the wax, the expansion of which opens the bypass.  It may also have a voltage reducing resistor, but I am not sure.  The wax motor style does not open instantly, but relies on an approximate warm-up delay to simulate the time required for the truck to warm up.  The KSB is located on the driver's side, low, and toward the front of the VE44 pump.

The earlier 89-91.5 (non-I/C) wax motor type KSB holds the valve open when cold.  When the engine is started the valve is already open, increasing timing.  As the engine warms the pellets melt and the valve closes.  This type requires 12v to close and function correctly.  Without 12v power, the KSB will remain on, which can be hard on the pump.

VE44 Intercooled KSB.
The 91.5-93 KSB works in conjunction with a thermistor sensor in the head.  When the ignition switch is in the ‘on’ position, 12v power is present at the thermistor.  When cold, the thermistor sends 12v power to the KSB solenoid, which opens the KSB.  Upon starting the engine, injection pump internal case pressure will advance the timing.  12v switched power is required for the KSB to open and function correctly, and will turn off when the thermistor sensor warms up and opens the 12v circuit, thus removing 12v power to the KSB.  If unplugged, the KSB will simply not function, which will only possibly make a difference when cold.   Under 90 deg., voltage is applied to the solenoid, blocking the fuel return path, and using internal pump fuel pressure, advances the timing slightly.  Over 90 deg., no current is applied to the KSB solenoid, fuel is allowed to return via the normal operating fuel path.  The timing advances normally thru internal porting, and sliding plunger.  Also, between the switch and the solenoid is a resistor, mounted to a bracket on the side of the head that reduces the voltage to the solenoid down to ~ 8V (when current is flowing through it & the solenoid -- if you just disconnect the wire at the solenoid you will get battery voltage).  The solenoid-operated KSB works instantly when you connect & disconnect the voltage to it -- when it is working you can hear the engine speed pick up and drop off connecting & disconnecting it.

It is not necessary for the KSB to be powered up, but it can help by advancing the fuel injection a bit.  I am working on exactly how the factory powered the KSB, and so far it is via a sensor in the intake manifold behind the turbocharger air inlet.  It is a temperature sensor that ‘allows’ 12v power to go to the KSB until such time as the sensor warms up, killing power to the KSB.  If I understand the factory wiring harness correctly, the same 12v power wire going to the FSO is also going to the KSB, only the KSB wire is routed via the temp switch.  In other words, 12v power goes to both the FSO (and KSB when cold), and then when warmed up the KSP power is switched off at the intake manifold sensor.

Speedometer / Cruise Control.  The factory electronic speedometer is tricky.  The factory used a sort of ‘Mopar-only’ signal to tell the speedo what to do with itself.  Meaning you cannot go get an aftermarket speedometer signal generator (from Autometer or Stewart Warner, for example) and tap it into the speedo – won’t work, at least we haven’t been able to make it work.  So far, my speedometer is still using the computer to tell itself what to register.  We do have other possibilities we’re looking at right now – just haven’t been able to get much further into it.  As well, the cruise also gets signal from the speedo, so the speedo and cruise require the computer to work.  I haven’t had cruise since the conversion, and I like cruise, so I am looking into a single way to both power the speedo and cruise without the computer.   

Edit:  Word on the street is the computer only provides a ground signal for the speedometer circuitry to function.  We (the RCC 'collective') are looking into how to determine exactly which wire this is etc and will post back once we isolate it, ground it, and prove functionality.  If so, then the speedo only needs 12v power and a rotating tranny shaft, and the speedo and trans hardware and wiring.

Air Conditioning.   The A/C is actually pretty simple.  Basically, the PCM tells the A/C compressor to turn on, and instead of the computer doing that, just set those wires to a simple toggle switch on the dash.  I prefer a blue LED switch so I know it’s on.  Now, the trick is not leaving it on all the time – it can ice up.  The PCM cycles the compressor off and on throughout the time the A/C button is pressed in.  We simply cycle the toggle switch manually. 
But, there is another option we’re looking at – a timed-delay relay.  The timed relays are designed to remain on for a predetermined amount of time.  Some are 2 minutes, some are 20 minutes, some are adjustable 0-10 minutes {cool}.  They are a ‘momentary’ relay per se.  Once you push the button, it turns on and remains on for however long the internal timer is set.  Install such a button with a blue LED on the dash and you will know it is on, and it will automatically turn itself off, letting the compressor warm up a bit.  Then, when you notice it is getting a bit warm, push the button again…and the cycle repeats for as long as you want A/C.

We like this simple adjustable unit from Waytekwire.com, #75543  -  http://www.waytekwire.com/item/75543/SOLID-STATE-OFF-DELAY-TIMER/ .  It has a 0-10 minute adjustable timer – pretty slick.

There also one we’re looking at from Wolsten Tech, http://www.wolstentech.com/products/timedelayrelay/basic-instructions.php

Voltage Regulator.   On the 90-93 Rams, the computer actually ‘is’ the voltage regulator.  There is no classic old-school voltage regulator, so, we install one.  The wiring is fairly straight forward.  Please see the attached diagram.  The same 12v switched power (originating from the fuse block) will also go the center post on the regulator, continuing on to one of the alternator field wires.  The other field wire will go to the outer post on the regulator.  That’s it.  We also highly recommend installing a completely fresh heavy gauge cable from the alternator to the battery.  The factory utilized a ‘dis-connectable’ main wire, and liked to splice all kinds of accessories into that wire.  The factory connecter joint (buried deep inside the wiring harness near the brake booster) likes to corrode and short out – not a great way to fly.

Fuel Filter Heater.  Fuel is critically important to any engine, especially true for diesels.  Having clean, warm fuel is important as diesel fuel flows much better when warm, and can get thick (and even gel up and not flow at all) if very cold outside.  To that end, the factory utilized a simple fuel heater element.  On the early 12 Valve engines, the factory fuel heater looks a lot like a hockey puck with two little wires sticking out of it.  It is attached to the bottom side of the cylinder head via a dual-thread fitting, and the fuel filter attaches to the heater, at the bottom of which is the Water-In-Fuel (WIF) switch/drain assembly.  However…as is the case with the factory PCM, the fuel heater element and WIF sensor are also becoming extinct, and there are no new versions being reproduced.  We have taken a good hard look at the actual ‘requirement’ for the factory fuel heater and WIF sensor, and have determined that fortunately...neither of them are necessary.

When very cold diesel fuel can ‘gel up’ and can actually prevent fuel from flowing.  This happens because wax crystals inherent within diesel fuel has solidified and clogged a port.  Heating the fuel helps eliminate fuel gelling.  But, the wax collects at the bottom of things, not the top, so having the fuel filter heater at the top of the filter actually provides very little real benefit.  If the heater was at the bottom of the filter, it might help some, but that’s not where the fuel is drawn from – it is drawn from the top where the fuel circulates through the cylinder head just after coming through the mechanical the lift pump.  In practicality, the cylinder head does a much better job of heating the fuel than the factory fuel heater ever could.

There is a very simple conversion that makes everything simpler, and is all on the shelves at local parts stores.  First, detach everything associated with the fuel filter/heater/WIF sensor assemblies and discard.  Next, installed into the cylinder head is a dual-thread nipple – this is what retains the original fuel heater unit to the cylinder head and also which the fuel filter screws onto.  Remove it, and replace it with another similar, shorter nipple, Cummins nipple #3925954.  Next, thread on a WIX #33472 or NAPA #3472 fuel filter (or your preferred equivalent).  These newer filters have a built-in water trap, and by simply performing normal water draining procedures at every oil change (open the drain and let a couple seconds-worth of fuel...and any water trapped at the bottom...bleed out) you'll never have a need for the factory WIF sensor or dash light.  When complete, the fuel filter will now be directly attached to the cylinder head, thus eliminating the fuel heater and WIF sensor all together. 

If you are a new diesel owner, you may not know of the need to replace the fuel filter at every other oil change.  As well, draining the fuel filter a couple times a year can help eliminate collecting water in the bottom of the filter (which is where water collects when sitting for a long period of time).  Ironically, the cylinder head warms up faster than the fuel heater, and stays at a nice stable temp all the time – perfect for circulating clean fresh fuel to the injection pump. 

So, to recap, remove the factory fuel filter heater/filter/WIF sensor assembly.  Remove factory fuel filter nipple and replace with Cummins nipple #3925954.  Then install a WIX #33472 or NAPA #3472 (or your preferred equivalent).  These conversion filters are functionally the same as the original, only the water drain is incorporated directly into the bottom of the filter itself and not a separate attachment.  Yes, you will lose the ‘Water-In-Fuel’ sensor ability, but if you drain your fuel filter for a few seconds twice a year you’ll never need it.  So by doing this conversion you can eliminate the requirement (and wiring) for two items that cannot be sourced anyway, and have simplified the overall operating nature of your Cummins.

Transmission Overdrive (OD) and Torque Converter Lock-Up (LU)::  1st Gen Cummins Rams had two versions of the automatic transmission; the good old ‘diesel’ 727, and the ‘diesel’ A-518 (46RH).  I highlight the word ‘diesel’ because there was also a similar (but very different) version of the 727 and A-518 available for the gasoline-powered Rams and cars.  This difference has been a source of confusion for many years. 

The ‘diesel’ 727 is just that – a 3-speed automatic.  It had a 23-spline output shaft and was only available with the non-intercooled 6BT Cummins from 1989-1991.  Those model Rams also came with a front D61 axle (if 4x4) with 3.07 gears and a rear D71 axle with 3.07 gears.  These D61 and D71 axles were somewhat unpopular because of the very deep ‘airplane’ gearing.  Great for the street and highway, not great for off-road.  The diesel 727 is a very simple and very strong transmission, and without the OD tailhousing, because it is really short, it is a great option for serious Cummins-powered off-road rigs.  In 1991 the diesel A-518 (46RH) was introduced and was in use through the 1993 model year.  It also has a 23-spline output shaft.  The A-518 is also a very good transmission, both the gasser and diesel versions.  The diesel version has a bad reputation for weakness, mostly attributed to the lousy factory torque converter available at that time.  The A-518 is literally a 727 with an overdrive built into the tailhousing.  It is a non-lockup torque converter - no diesel A-518 had a lock-up converter.  Only the gasoline/small block A-518s were available with a LU converter. 

When it comes to electronics and controls for the entire transmission, the diesel A-518 had three electrical plugs:  the 3-pin neutral safety switch, the speedometer electro-mechanical sending unit (2wd was electronic, 4wd was mechanical from the NP205 transfer case that was converted to electronic), and a very basic 2-pin plug, just behind the gear selector/kickdown levers.  This 2-pin plug is the one we will focus on.  The rear pin is 12v source power, and the front is the ground, which when grounded/ungrounded activates/deactivates the overdrive.  The factory PCM controlled that functionality.  We do it manually via a very simple 12v toggle switch or button ‘on the dash’.  In other words, you can control the overdrive at the touch of your finger.  We actually found a fantastic location for the overdrive button using the factory 2nd Gen (94-2002) column shifter lever.  At the tip of which is where the factory put the on-off button for locking out the overdrive.  We simply remove that button and install one very similar from Del City that is a very basic on-off button; #73556 from www.delcity.com.  This is what we use to control the ground circuit for the overdrive.

In 1994, Mopar introduced the diesel A-618, a.k.a. the 47RH.  It was literally the same thing as the A-518 but with two main differences:  It had stronger guts, and it had a LU converter.  The diesel 47RH was only around for two years, when in 1996 the 47RE was introduced.  The 47RE was available up to 2003, when the 48RE was introduced.  The major difference between the 47RH and 47RE, is the functionality of the OD and LU converter.  The 47RH and 47RE each have three similar electrical connectors on them, similar also to the 46RH, and it is important to be able to visually determine which is which when you are shopping for a transmission for your truck.  Each has the same old-school 3-pin neutral safety switch on the drivers side, just above the pan.  Each has a speedometer signal unit in the tailhousing.  And each has either a 3-pin inline or 8-pin round plug just behind the shifter/kickdown lever assembly.  This 3-pin/8-pin plug is what distinguishes a 47RH (3-pin) from a 47RE (8-pin).  Many, many people selling a transmission think their 47RE is a 47RH because (when asked) they think the 3-pin neutral safety switch ‘is’ the OD/LU switch.  It is a very common mistake, and can be an infuriating problem for buyers.  Get pics before you buy a transmission.   

The 47RH is essentially also just a 727 with an overdrive bolted onto the back.  The valve body/1-2-3 shifting etc are all controlled by a hydraulic governor assembly located within the tailhousing unit behind the overdrive.  The overdrive and converter lockup are controlled via the 3-pin connector.  The center pin is 12v and the two outer pins are ground (front is OD, back is LU).  The OD and LU are typically controlled via the PCM – taking drivers inputs, heat, all sorts of signal inputs etc, and eventually grounding the two outer pins when the PCM says so.  Well,…that can be done manually via two little toggle switches on the dash.  Meaning, you can eliminate every single sensor and the PCM, and have a toggle switch (or any other switch of your choice), and manually control the overdrive and lockup to engage and disengage exactly when you want.  If you never hook up switches to the OD and LU the transmission will still shift through 1-2-3 just like any other 727.  But, the 47RE does not operate that way – it requires the factory PCM for it to operate normally because it has an electronic governor.  This is the main reason why most people doing a diesel conversion prefer the 46RH (A-518) and 47RH (A-618) over the 47RE (A-618).

To control the OD and LU simply requires two little toggle switches somewhere within easy reach of the driver.  When you want the overdrive to engage, hit the switch.  When you want to engage the lock-up, hit the other switch.  It’s primitive, yes, but very easy and 100% effective.  Just don’t forget to turn them off when you start slowing down for a red light, especially the LU.  If you don’t disengage it, it could stall the engine. 

Now, what about an ‘automatic’ way to control the OD and LU?  That…is one of the items at the top of our list, and we think we have a way to do it, using off-the-shelf hardware from B&M.  We are still verifying it will work, but the theory is sound.  B&M produces a ‘Torque Converter Lock-up Controller’ #70244 and 70248 for a Chevy application, which simply sends a 12v signal to the trans at a pre-determined speed.  The signal comes from the transmission speedometer signal generator, and at a certain speed the controller sends 12v signal to the transmission which activates, in this case, the LU.

The difference between the Chevy application and the Dodge application, is that Chevy's are factory engineered to use 12v signal to activate their OD and LU, whereas a Dodge is factory engineered to use a ground signal to activate their OD and LU.  It's essentially the same difference.  We are confinced, however, that the Dodge 2 and 3-pin plugs can very simply be 'reverse-wired', so that instead of the center pin going to 12v power, the center pin can be set to ground, and then the two outer pins for OD and LU can be triggered via input 12v signal, which is exactly how the B&M controller is designed to operate.  The question of whether or not the Dodge solenoids can be reverse-wired seems sound, because all they are are electromagnets which close when triggered, and it should not make any difference whether or not the pins are reversed - the same effect should happen.  We have yet to do this mod and therefore cannot say it works for 100% certain, but the math adds up.

A518 diesel side picture (yellow is the 2/3/8-pin plug location, green is the nutral safety switch location) -

2-pin plug (A-518/46RH 'diesel'), removed from tranny housing -

3-pin plug (A-618/47RH 'diesel') -

8-pin plug (A-618/47RE 'diesel') -


Low Fuel Light.  The [Low Fuel] light is an important feature.  On a Ramcharger (gasser) harness, the low fuel feed wire from the gas tank comes up into the chassis/body harness and is originally supposed to go to the fuel pump.  According to the wiring diagrams, the diesel and gasser tanks have the same wires coming off the sending unit, but one of the wires has dissimilar functions - on a gasser it goes to the fuel pump, on a diesel it signals the low fuel light.  A diesel-powered Ram does not use an electronic fuel pump, so we repurposed the wire direct to the [Low Fuel] display on the dash message center.  Worked perfectly.

High Beams:   One of the ‘other’ upgrades we did was to ‘re-purpose’ the high beam switch from our 1993 Ramcharger.  I dislike having the high beam switch on the turn signal - I much prefer it on the floor, like in the older Rams and Power Wagons.  After sourcing a floor-mount high beam switch/wiring from the junk yard, we spliced the required 12v power (from the floor switch) into the same wire as the original Ramcharger turn sig switch (in the steering column wiring harness, 18g green wire), cut the two output low and high beam wires coming out the column switch, and wired the floor switch into those loose wires.  Doing this re-directed the Hi/Lo beam function down to the floor switch, while using the original 12v feed wire as original.  But, the Ramcharger turn signal switch is still a perfectly good switch, so we decided to power something else with it - the fog lights.  What was the "high beams" is now my fog lights.  The original low beam wire now has no use and is just snipped off, and the "high beam" output wire now powers the fogs.  Each of those – Hi and Lo beam, and fog lights are actually powered by relays under the hood, which will in turn power the lights.  It also means the “Flash-to-Pass” feature of the turn signal switch now ‘flashes’ the fog lights momentarily, unless I pull back all the way and actually latch the switch, at which point the fog lights remain on, just like how the high beams would otherwise function.  See below for an explanation of relays.

Additional headlight switch tech -

One of the things to point out is that the 1993 turn signal/high beam/flasher switch/lever/assembly is really three switches in one - four if you count the washer...and they each apparantly have their separate power sources. 

If I've decoded the switch correctly...there are 5 separate power sources into it - 12v ignition (presumably) for the turn signals and washers (red/white and a blue wire), headlight power (green), flash-to-pass (green/white), and hazards (red). 

The issue was, the solid green wire coming into the turn signal lever switch is powered only when the headlights are on.  That same green wire also powers the foot pedal switch for the hi-lo beams, and works as it should, when the headlights are on.

What I needed was for the high-beam switch to be powered when 12v ignition is on, not just with the headlights.  So, I cut the main green wire into the switch [after the splice for the foot pedal], and wired the remaining input green wire to 12v ignition.  Now, the factory high beam switch powers the fog lights any time the key is on.  It could power any 'feature' you wish - in my case I wanted it to the fogs.

But we also discovered something else.  The flash-to-pass function comes from a separate green/white wire (which is B+ hot), and when the lever was pulled all the way back it was sending B+ hot to the ignition harness, which isn't exactly what we want to have happen .  So, I cut/terminated the green/white into the turn sig lever....and now everything works exactly as I want.

If my figuring is correct, there is a pink wire which is the hot 'feed' for the hazzard flashers.
There is a red-white and a blue wire each that are 12v ignition feeds, presumably for the turn signal switch and washer switch. 

So, in the end, what I wired up was a foot-mounted high-beam dimmer switch, using the green feed wire as the [headlight-on-only] power wire which sends signal to the the low and high beam wiring via original red/orange (high) and voilet/white (low).

Then I wanted the original high-beam lever to turn my fog lights on and off.  To do that I cut the green power wire and sourced it to 12v ignition power, and ran a separate wire for the fog lights.  And to keep the 'flash-to-pass' green/white B+ battery power from back feeding into the 12v ignition system I cut the green/white B+ wire from coming into the turn signal/high/beam switch.

This now works perfectly, and there are no shorts or unnecessary power feeds.

Batteries.   We always recommend dual batteries to all 1st Gen owners.  The factory only had a single battery, which is ‘okay’ for most climates, but if your area can see 0* temps a second battery can make all the difference between a good start-up and a bad start-up.

Chassis and Dash Wiring.  When we did the conversion on Oxx, we wanted to build the truck so that the wiring and chassis were ‘factory-appearing’, to have the truck function and look as if the factory had actually built it.  In the end, we got very close to that goal.  Mechanical is pretty basic; electrical is the trick.  To that end we retained the Ramcharger’s dash wiring harness and ‘augmented’ it with the ‘diesel’ stuff, mostly the dash and message center, and associated wiring. 

We decoded the wiring harnesses of both the Ramcharger and the donor truck, in our case a 1991 Cummins Ram W250.  We were pretty sure we could simply and literally plug in the diesel engine bay harness to the Ramcharger firewall bulkhead connector and have the truck run.   We discovered the firewall bulkhead connector is fairly ‘generic’, meaning the ‘diesel stuff’ does not have a ‘gasser’ equivalent, meaning the firewall bulkhead connectors on the gasoline-powered Rams (and Ramchargers) are vacant where the diesel wiring would otherwise be.  The gasoline and diesel connectors are the same for things like the lights, wipers, door locks, horn, cruise control, etc - gas or diesel doesn't matter.  About the only thing we eliminated altogether was the anti-lock brake stuff - it never worked anyway.

One interesting feature; within the message center on our 1993 Ramcharger there is a [GATE OPEN] light, whereas on the diesel version of the same message center, because they were all pick-ups, there was the [ANTI LOCK] light for the brakes.  Same bulb location, different function.  Since the Ramcharger wiring was designed to recognize the gate is open, we simply wired up the Cummins message center so that the anti-lock light will come on when the lift gate is ajar.  In other words, when the Ramcharger’s lift gate is open, the signal still goes to the original location at the message center, but instead of displaying [GATE OPEN] it now displays [ANTI-LOCK].  It has actually been a very nice feature to tell me the Ramcharger lift gate is not fully closed.  It works great.

By combining the diesel dash harness (and dash) with the original Ramcharger harness, and by using the diesel engine bay harness, the Ramcharger now functions as if it came from the factory with a Cummins – like it had originally been built that way – just how we wanted it.

Accessory Wiring.  It is very common to see factory wiring harness with additional accessories tapped into them.  Sometimes there are so many other things being powered by a single glass fuse you wonder how the fuse block hasn’t melted.  In some cases, they have.  In worst cases, they’ve caught fire.  Wiring harness fires are no fun, and easy to avoid.

Every vehicle has its own factory wiring harness.  We recommend those remain 100% ‘unmolested’.  If you want to install additional accessories like gauges, power ports, heated seats, selectable lockers, or independent ignition systems (in gas engine vehicles), we fully recommend installing a fully stand-alone wiring harness (or two) to ‘power’ all of your accessories.  Most times, we fabricate a stand-alone electrical power center (we call them a ‘sub-station’), where we have solid B+ and 12V power sources and a good ground.  From these ‘sub-stations’ we get all power for all of the additional accessories, and biggest of which are powered via relays.  Doing this isolates the factory fuse block and leave it alone so it can continue to properly power the items for which it was originally intended.  This commonly prevents overheating conditions and can obviously help prevent under hood/dash fires.

Relays.   In general terms, a relay is simply a very stable ‘alternate source’ for battery power – a source other than the factory fuse block.  You may have noticed that in the older cars and trucks, when driving at night with the headlights on, the headlight switch actually gets pretty warm, even hot to the touch.  Well, ever wonder why?  Ever think ‘that’s just not right’?  The reason this happens is because 100% of the power needed to power the headlights…is all coming through that little 35-year-old headlight switch, and with a little glass fuse somewhere in the fuse block saying ‘…please kill me’.  When the headlights are switched on, the headlight switch takes fuse block power and sends it all the way back out to your headlights.  That is a lot of amperage coming from the battery through the firewall bulkhead connector (via the fusible links), into your dash harness, into/out of the headlight switch, and back out through another wire through the bulkhead connector…and then finally on to the headlights.  There is a lot of resistance going on through all of that re-routing, and it creates a lot of heat, and it can all be simply eliminated...by using a little $5 relay.

It works simply like this.  Headlights (for example) require a lot of amperage to ‘turn on’ – a relay requires almost none.  When wired up properly, a relay is wired in-line with the output wire to the headlights, and instead of your headlight switch sending lots of amps to the headlights, it will instead be sending very few amps to a relay, which will turn ‘on’.  When turned on, the relay takes another pre-wired heavy gauge input power source (usually directly from the batteries), and sends that power directly to the headlights.  The headlight switch turns on the relay, which turns on your headlights – directing fresh battery power straight to the headlights, bypassing the entire factory wiring harness. 

We have relays on both the Hi and Lo beam circuits, as well as the fog lights and air compressor.  You have noticed that at night the headlights will ‘dim down’ when you come to an idle.  That is because the alternator is having a difficult time powering everything on the whole vehicle from the same source – the factory fuse block.  This is a common cause of electrical fires, and is compounded every time another accessory is added to the factory harness.  By utilizing independent ‘sub-stations’ you can completely eliminate over-stressing the factory harness, and power all of your accessory items from a separate power station.

Most of our builds have three separate wiring harnesses.  The factory harness, a sub-station under the hood, and a sub-station inside the vehicle.  Each has dedicated power cables coming direct from the battery, with a mega battery-disconnect switch on each power cable so we can kill power to the vehicle in the event of an electrical fire or overheat condition.  Being able to manually disconnect battery power to the vehicle can make all the difference in avoiding a vehicle fire. 

- Sam

Here are a couple basic schematics for relays and voltage regulators

Voltage Regulator wiring:

Relays - standard Dodge headlight wiring using relays:

« Last Edit: May 04, 2016, 05:08:31 PM by Flyin6 »
Site owner    IS 6:8  Psalm 91 
NSDQ      Author: Distant Thunder

Offline Dodgeman

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #1 on: May 30, 2018, 10:15:23 AM »
Greetings all; I appreciate being on the forum!

I have a 1992 Dodge Cummins AT that I am in the process of rewiring/gutting the computer in it. I have cut out the computer in it already, since it was already 'blackened'/burned & I didn't trust it anymore. I did not cut out ALL wires, just the ones that went to the computer

What would be very helpful to me are 2 things:
1) knowing what each wire going out of the firewall harness goes/went to, & the simplest/easiest way to rewire everything to get things to work again.
2) since you have a pretty good knowledge of rewiring this truck, I would very much appreciate your input on what wires should be on one 'circuit', or what wires should/should not be spliced into others. This is a work truck, & probably will not be anything other than that, so cherrying out the electrical isn't necessary. The basic items I would like to work are (in order of importance) on/off AT overdrive switch, alternator, headlights (not even necessary for high beams to work), power windows, all gauges in instrument cluster, and fan/heater controls. If I had these working I could skip the rest, or add it later if need be.

I appreciate your time; I would be happy to add any details should you need them. Thank you.

Offline EL TATE

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #2 on: May 30, 2018, 10:53:50 AM »
Welcome! I'm sure at some point Koot will log in, see your post and give you everything you need, but I'm sure that someone on here will be able to get you pointed in the right direction shortly.
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Offline Flyin6

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #3 on: May 30, 2018, 11:14:18 AM »
Although I cannot point to exactly what each wire does, I used this process on SquareD:

For every system, I attacked them one at a time. FOr each system just solve for the issue. Rewire the automatic transmission according to what you can find on the web. Run wires for the other systems that stopped working and just hard wire things like the grid heater. A hot wire to the solenoid set to a twenty second timer will fire that up for cold starts. I however have found that the 12 valve easily starts under almost any condition.

So although I am giving you a generalized answer, the approach I advocate is really the most simplistic way of getting around computer control and back to simple manual control.

Welcome, and good hunting!
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Offline Dodgeman

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #4 on: July 17, 2018, 01:15:48 PM »
thank you, & COUPLE more requests.

The 2 things I need to get going immediately are the alternator and electronic OD for trans. I am an automotive electronic noob, so I will ask a couple of very 'novice' electronic questions :smiley:

regarding the alternator, i read your voltage regulator example above (you indicated a  separate voltage regulator must be added since if was regulated by the ecm, which is now gone). how is this wired? do you solder wires onto voltage regulator, then onto battery? where would the best place for this to go under the hood? what is best or cheapest or both:) place to get voltage regulator, at automotive store, or online?

Regarding the OD switch, I also read your trans OD section above, my year (1992) truck had the non lock up converter, so would only have the 2 pin connection type. this looks like my situation. if the ecm was needed to operate the electronic OD and is now gone, was is needed wiring wise to get back the missing gear? also can the 'OD off' switch still be used in the truck, or does that have to be rewired?

Sorry for all the questions, but I would like to get this truck at least operational again. If you had 3 or 4 pics to send of any related work done this would be a big help, or I would be happy to send an email address or even a phone number if you had the time, would really speed things up for me, thank you.

Online BobbyB

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #5 on: July 17, 2018, 02:08:40 PM »
but I would like to get this truck at least operational again.

Good luck on yours.
So, Bobby...being the calculating trained warrior NCO that you are.  Take the appropriate action, Execute!
your standard grunt level CQB is just putting rounds and rounds on scary stuff till it stops scaring you!


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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #6 on: July 17, 2018, 02:53:15 PM »
The charging system is very simple. All you really need to do is wire in a external regulator using the diagram posted previously. To source the regulator go to any parts store and ask for a Dodge pickup regulator 1990 or older.  The transmission is also quite simple.

Using the factory OD switch can be done, or a even simpler way, is to use a old school dimmer switch on the floor.

As for the transmission....it has the two pin plug...rear pin is 12VDC the front pin is a ground. You control the OD function by switching the front pin/ground with any on/off switch...like I mentioned I use a old floor mount dimmer switch.

« Last Edit: July 17, 2018, 02:56:43 PM by OldKooT »

Offline Dodgeman

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #7 on: May 27, 2019, 10:23:24 PM »
I still have this truck, ended up gutting most of the factory wiring & started over

Offline wyorunner

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Early Cummins powered Dodge computer removal and rewire
« Reply #8 on: May 27, 2019, 10:47:34 PM »
I still have this truck, ended up gutting most of the factory wiring & started over, was a long process & would not do it again.
I’m either going to sell it, or it’s to the point where I’m confident to drive it. I was wondering if you ever figured out how to get the speedo to work? Everything else I’ve about got, but would really like that to work, I appreciate it, thx.

Isn’t the speedo mechanical? I must seriously be missing something here. My 92 trans seems to have a mechanical speedo on the side of it.

I could be completely clueless too.

Online JR

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Re: Early Cummins powered Dodge computer removal and rewire
« Reply #9 on: May 28, 2019, 03:50:06 AM »
Need to keep track of this thread. My 85 Sub has the mech tach and hopefully keep the 46R behind it.

For now the 6.2 turbo is doing fine.
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