LB7 Allison transmission issues

[TexasRedNeck]

And an update on the LB7 transmission drama. As you know it was built by Calvins 3 years ago and only has 7,000 miles on it.

Drove home from the weekend place and it ran fine for 130 miles at 75mph. Getting off the freeway and pulling up to the light it would not move when the light turned green. I was able to get it to barely move by revving it and I limped it the 1/2 mile to the house. No movement to speak of forward but reverse was solid and fine. (remember that)
A buddy with really expensive software (read actual Allison software) helped me scan it and we got a solenoid C error code. We dropped the pan and tested the solenoid and the manifold pressure switch while we were at it. Bought a solenoid and pressure switch at the dealer.

Dealer swears the old pressure switch that has 6 pressure switches has been superceded with a newer model (found on the 2004+ year models) and even though it only has 4 switches it is the correct switch and will work with no programming.

Install the parts and it throws different codes??? Moves forward but in limp mode and has reverse. I forget which codes, but they didn't seem to make much sense. So we assumed it might be the internal wiring harness which was original and the wires were pretty brittle. Amazon to the rescue.

Drop the pan again and install the harness...no dice. Same codes and still in limp mode (4th and reverse)

So we start questioning whether the newer pressure switch manifold is correct. Some googlefu and we find articles saying, "yes" its the right part and then we call an "expert" at Stewart and Stevenson who says yes, right part, but TCM needs to be reprogrammed.

Since I still have the old manifold pressure switch and we never really determined that was part of the original problem, we decide to reinstall the original PSM. Drop the pan again (did I mention I'm getting really good at this by now???) and install the old PSM.

Fire it up and no codes! Yippe!....but wait. I now have no reverse, no codes and it drives normal in all forward gears. NUTS!

Start running the diagnostics....looking for TCM connector pins out of whack, 30 pin harness connector on the transmission, etc. Nothing abnormal. It can only be the TCM, the connections in the harness, or the PSM.

So I order another PSM off Amazon and this time make sure it is not the new superceded part, but the original 6 switch style.

Comes off the big brown truck and, you guessed it, I drop the pan again. Did I mention I can get that thing off like a NASCAR tire change?? The fact that its got a 12 inch lift and I don't have to do anything other than roll under it helps.

Install the new PSM, and all of a sudden....it runs normally. Reverse and all forward gears. At this point I'm clueless as to why, and what fixed it, but that was a lot of drama chasing gremlins.

At this point, I really believe there is something not right about using the updated PSM on the older transmissions. No one can explain why, but once I changed it out, the codes went away and the reverse gear came back.

[Sammconn]

Glad it's sorted. Hopefully for good.
That is a whole lot of practice. So how fast are you now? Lol.

Good tidbit of info too to avoid stealer intervention.

[Dawg25385]

Glad you got er back up and running!


Sent from my iPhone using Tapatalk

[TexasRedNeck]

That is a whole lot of practice. So how fast are you now? Lol.

fluid out, pan dropped, internal filter off, PSM off, new PSM on, filter on, pan on, filled with fluid. less than 20 minutes.

[Sammconn]

That is a whole lot of practice. So how fast are you now? Lol.

fluid out, pan dropped, internal filter off, PSM off, new PSM on, filter on, pan on, filled with fluid. less than 20 minutes.

Well, that is fast. NASCAR style for sure.

[EL TATE]

Dang Gina! I'll remember this one for sure, thanks for going through the hassle for me.

[Nate]

RN, I am not saying that calvin does or does not know what he is doing, and that mike l does or does not know what he is doing, but have you tried to give him a call and see what he says?

[TexasRedNeck]

I didnt talk to mike l but my buddy called him.  I'm not sure they communicated effectively because the answer didn't seem to fit. All in all here is what I think happened.

C solenoid tanked.  Caused slippage and clutch material was deposited in the PSM causing the switches to fail.  When I replaced the C solenoid and the PSM, the C Solenoid was fixed but the new style PSM caused a 1713 code, which is related to the PSM.  When I reinstalled the original, the reverse switch was not functioning due to the clutch material.  Replacing with the new "old style" PSM solved the problem

[KensAuto]

That sounds right. The whole "new style updated blah blah" never made sense to me. Having to reflash the tcm to accept the new style would make more sense tho.

[TexasRedNeck]

That is a whole lot of practice. So how fast are you now? Lol.

fluid out, pan dropped, internal filter off, PSM off, new PSM on, filter on, pan on, filled with fluid. less than 20 minutes.

Well, that is fast. NASCAR style for sure.

it helps when the truck is tall enough that you can place a 2.5 gallon used amsoil jug with a funnel in the mouth directly under the drain so you can capture the fluid without jacking up the truck. . Then just take that jug and use it to refill.

[Sammconn]

Yes the 12 inch lift makes the bottom stuff a whole lot more accessible.
Probably a bit more of a pain having to do all the top side off of a ladder.

But Dang 20 minutes is fast!

[OldKooT]

Congrats on the fact it drives properly now. That said this is why I refuse to buy anything with wires LoL

[Flyin6]

Congrats on the fact it drives properly now. That said this is why I refuse to buy anything with wires LoL

Really!
Such a good idea!

[JR]

Yeah, when ALLYs run, they run good. These darn things have more wires and sensors than a 20 years old car!!

At least you have the selector switch out side your pan, the LBZ and later are inside.

Good job and I was thinking MikeL all the time too.