hakongroven

Hi! I have a 2002 Suburban 2500 with 8.1 l engine.
I have a problem where the transmission will not shift out of 1st gear (might be that there is an issue shifting from 2nd to 3rd as well). D, P and N works as expected and 2wd, 4wd, high/low is ok as well.
In the beginning this happened only rarely, but now it is so often that the car is useless by practical means.
Diagnostics show these two:
DTC P1860 : TCC PWM Solenoid Circuit Electrical
DTC P0758: 2-3 Shift Solenoid Circuit Electrical
A+B solenoids have been replaced without any improvements.

Additional symptoms that might be relevant:
1) Overdrive button does not work
2) Dashboard/instruments sometimes does not work immediately at start up: gear position indicator doesn't always immediately have ligth/is not visible at start and fuel level indicator show correct level for a sec, but then drops down to "no life" in sync with the gear shift indicator.

To me it looks like the gear shift issue is present when I have the dash/instrument panel issue at startup (and vice versa).

I've read quite a lot on the web and asked different experts, but I'm struggling with the issue as it seems very hard to pinpoint exactly what the problem is (hopefully I don't have to replace ignition switch and potential other components without having a clear indication that it will solve the problem).
My current main theory is that there is an issue with the ignition swithc, but I'm afraid it is only a guess...

I will be very happy for any input!

mulmur1

It sounds like you have been fighting with your transmission problems for a while. Hopefully someone will have a few suggestions that may help solve the problem. I believe that with the 8.1 litre engine, you have an Allison 1000 trans. Based upon your description, you are having a number of electrical related issues.

A couple of common issue with the early Allison transmissions are:

- Neutral Safety Backup switch. This is located on the driver's side of the transmission. On the early transmissions it is black and the updated unit is tan in color. It is connected to the automatic shift linkage and signal gear shift position. It also tends to have an impact on the cluster lights. The early NSBU units often get moisture within the switch and fail. The newer units are sealed better. If you decide to purchase this, go directly to Allison for the part. (29540479). In my case the dealer was $254 and Allison was $90.

- Ignition Switch. This often is replaced along with the NSBU. It may be your problem, as low voltage can case all sorts of problems. It isn't a huge job to replace it and it isn't very expensive. They are around $57.

Unfortunately, electrical problems do take a lot of effort to track down and sometimes you will have to throw parts at it to be certain that things are working properly. You may want to also check the main 20pin connector on the transmission and the Transmission Control module for any signs of corrosion.

If you are still pulling your hair out, it may help to have a Allison tech look at it. Hopefully you can figure it out.

Good luck with the repair.