yevgenievich

iTrader: (1)

I have been stumped by an issue with 4th gear and lock up in my 91 V2500 Suburban. All original with 125k miles. Problem started with random shifting, that new TPS fixed. Next the computer decided to completely disable torque converter lockup. I have put a manual switch in to test the lock up, and it works great when manually engaged. Around the same time, I found that if it downshifts to 3rd while going up a hill or passing, it will never shift back up to 4th. Shutting the engine off and restarting it will make it work normal again. In city driving and stop and go shifting down to 3rd and back to 4th is no problem and works correctly.
The things that I have checked:
Verified that harness plug is new design, clean and verified splice is done correctly. Fixed one wire that was not spliced correctly.

Replaced TPS

Replaced front speed sensor

Verified proper operation of the temperature sensor in the transmission and on the engine coolant

Does not show any codes when checked (except for the test code 12)

Looking for some advice and what could be the culprit

yevgenievich

iTrader: (1)

bump
thinking of replacing output speed sensor

dubwise

How exactly are you pulling the codes?

My guess would be that the PCM is setting a Code 68 (won't illuminate SES) - which is a TCC Slip code. I say this because you mentioned the PCM is disabling 4th gear. It does this because it detects the slip in the TCC and disables it as a safeguard.

Manually enabling the TCC takes the PCM monitoring out of the loop so you really have no way of knowing if it is truly slipping w/o a datalogger.

You can try replacing the Output Sensor, but I'm not confident that will fix it.

Make sure the Tan/Blk TCC wire is pushed in the PCM all the way. If the PCM cant ground the circuit it will also set a Code 68 and disable 4th.

I would suggest taking a look at the TCC Solenoid, but it sounds like its working ok by grounding it manually.

This is the absolute cheapest and easiest thing to try: Dr. Tranny Instant Shudder Fixx It has good reviews. For $5 or whatever it costs - try it out. It's cheaper then replacing the TCC and may get you by. Otherwise it sounds like the TCC is on its way out.

You may want to invest in a ALDL to USB Cable and use Tunepro or TTS Datamaster (If you have the 7060 PCM) to datalog the event so you can see whats really happening.

Let us know how it goes..

yevgenievich

iTrader: (1)

Thanks for response with few ideas. I have already purchased a USB UART chip that I was planning to use for data logging. I have lost an old serial cable that i made many years ago when playing with tpi. But in the end did not even use it to fix the issue.

I have realized that I still have a spare good speed sensor laying in the garage, so it was easy check to put it in in place of the rear output speed sensor. Transmission is now shifting like it did before any problems started.

I will still get in a data logging session just to make sure, but so far, I think it should be ok for a while longer.