HELP!!!! 91 GMC Tranny trouble

[Robyn]

OK I went back through and reread this thread again.

The issue may well be as Maverick mentioned, the "Coolant temp sensor"

The Converter clutch will not lock if the ECM senses too cold of an engine temp.

The idea is to allow things to warm up fully (tranny and engine) before the TCC is allowed to operate.

This is most likely a driveabilty and or emissions thing.
Some trannies had a temp sensor in the pan bolted to the VB to accomplish this too.

You need to get the code if you can or if no hard codes are stored then get it scanned real time to see whats happening.

You can try replacing the temp sensor in the water crossover and see if this fixes the issue.

I hate the replace and see program, BUTTTTTTTTT if you do this and it fixes the issue then you may be ahead $$$$$$ rather than paying to get it scanned real time.

This could also be (as mentioned) a TPS issue (Throttle position sensor) but usually the tranny will act wierd as this is the control used to tell the ECM what the driver is asking of the beast and where it needs to hqave pressures and such set as well as what gears to use (Throttle pressure)

Best

Robyn

[andyvillen]

I'll have to find someone with a scanner, when i flash the computer my self it just flashes 12

[andyvillen]

I was looking around under the hood while my oil was slowly pouring into the motor. Its -17 here.

I was trying to find the coolant sensor on the coolant crossover pipe.. there isnt one. the only spot to put one and there is a square drive pipe plug in it.

Did the 91's have one here, the only other sensors i can see that are in the cooling system are, One on the side beside cyl# 1, and one in the rad that is probably a coolant low level.

[Robyn]

The sensor up front by #1 is the gauge sender

They could have used either the rear side port on the RH head or possibly the rear upper port area that usually has a blockoff plate on it.

There has to be a coolant temp sensor to tell the ECM what to do with the cold advance as well as other things.

Usually the sensor is in the water crossover ?????????

You may want to get the thing scanned real time while its showing the ses light.

A real time scan will allow a visual on whats going on.

The lockup is controled by the ECM and several conditions have to be met in order for things to work right.

Coolant temp is only one of them.

We may be barking up the wrong tree here. The early trucks had very rudimentary controls and obviously the thing is not setting a hard code in memory if you get a 12 flashing out.

Get it scanned real time and have it done warm so the tech can see whats happening.

There could be an issue with another sensor such as the input or output speed sensor on the 4L80.

The temp sensor will cause a code if it reads outside of normal parameters after a certain amount of run time.

Keep us posted

Robyn

[andyvillen]

I'v had a bit of trouble starting the truck latley and I thought that it was just the cold weather because i have never driven this truck in the winter before. But i checked the water level in the batteries while looking for the coolant temp sensor. and i found that i had one frozen cell in one of the batteries.

So i put in a good used battery i had kicken around and then i fired it up and did a bit of plowing around the yard.

the SES light came on as usual once the truck warmed up a bit, then after a bit it went out and stayed out tell i turned the off, i didnt get a chance to blast down the highway yet though to see if it will stay off.

Could a low voltage have been screwing up the computer.?

[andyvillen]

seems to still have the problem.. im going to go buy a coolant temp sensor.

[andyvillen]

Does any one know if the Two coolant temp sensors are the same part #?

I found out that when the check engine light is on if i put it in park or neutral and floor it the light goes out..

Could this be a engine speed sensor?