IAC maybe?

[wrangler94]

Originally Posted by ChevyTech

Glad you are making progress.

I would not replace any sensor just to replace parts as you have funds to do so.

The quality of replacement parts is poor. I would prefer a used good original parts over much of what is being produced today.

When you buy a new oxygen sensor I suggest staying away from BOSCH brand.

Just adding another update. Truck has not shut off at random now for 2, maybe 3 weeks now. Thank you again for all the help with that.
I did replace the O2 sensor saturday and it stays in closed loop as it should now, mostly at least. Not sure how much it should cycle to open, but it's almost always closed now.
When replacing the fuel filter i found hacked together wiring for the transmission plug for the 4L80E and wasn't sure what to think, until I did some digging and the plug has been updated, but it looks like a monkey with a butterknife did the upgrade. At this point I'm happy and not gonna chase it so hard, but the scanner does show that the torque convertor never locks up.
Obviously the bad wiring job is a prime suspect, but research has shown that a bad brake light switch can cause it too, and when the scan tool is plugged in i throw a code (not on scanner, in flashing brake light warning) of stop light switch. Scanner shows that brakes are off or on as i press the pedal, and brake lights work, but is it possible that the switch is somehow not giving the transmission what it needs? The computer seems to get signal, but that code does keep coming up, even with resetting the computer. Again, only when the scan tool is hooked up. I unplug scaner and brake warning light goes off again.

[ChevyTech]

You're welcome! Glad you are making even more progress.

The brake switch has multiple circuits through it and the brake lights can work while other circuits going through it don't work. The brake switch is a weak point on these trucks. The brake switch could cause the lock-up converter to stop locking up. The brake switch can cause anti-lock problems while the brake lights still work.

Does your 3500 series truck have rear disc brakes?

There are different brake system and I am not sure what your trucks has as for brake issue while using the scan tool. Some trucks have 4WAL some have RWAL and if your truck could have ZPRAWL if it has rear disc brakes.

By the time your 1992 was built some of the RWAL and ZPRWAL codes were soft codes which need a scan tool to read. The hard codes can be read without a scan tool.

The anti-lock issue can turn into a long thread just by itself. If you want to go into that you should start a new thread for it, and if I don't reply just send me a private message to get my attention. I have posted on the RWAL system several times over the years and a search should be able to find these. The ZPRWAL has some different procedures, like how to clear codes.

[Just call me Sean]

I just deleted the RWAL on my mother's 89. The brakes work way better now.

[wrangler94]

Originally Posted by ChevyTech

You're welcome! Glad you are making even more progress.

The brake switch has multiple circuits through it and the brake lights can work while other circuits going through it don't work. The brake switch is a weak point on these trucks. The brake switch could cause the lock-up converter to stop locking up. The brake switch can cause anti-lock problems while the brake lights still work.

Does your 3500 series truck have rear disc brakes?


By the time your 1992 was built some of the RWAL and ZPRWAL codes were soft codes which need a scan tool to read. The hard codes can be read without a scan tool.
.

It has rear drums, and the brakes seem to work fine, and as said, only throws code when the scanner is hooked up. No anti lock brake issues as far as I can tell, just the computer saying that the torque convertor isn't locking up.

[ChevyTech]

If the computer was trying to lock the converter and it did not happen I would expect a trouble code would set. My guess is some criteria / sensor reading is not reaching the computer.

Spend some time looking at it with the scan tool.

The scan tool can show if brake pedal is pushed or released. If not released the torque converter will not lock.

The converter lock-up is dependent on the coolant temperature sensor reaching a threshold.

Converter lock-up is TPS reading dependent.

Check the PRNDL select as the shifter is moved through the ranges.

The TCC will lock up in third or fourth gear.

If the TCC is locking you should be able the feel it release if the brake pedal is tapped.