4l80e Advice

[DTrain]

Picked up another truck a couple weeks back. 2000 Chevrolet k2500 CCSB.

All my other trucks are and have been manual trans trucks. This truck has is an auto, 4l80e.

Couple things that have happened...

Truck has stalled when I put it into reverse real quick. Doesn’t happen every time. Has only happened once or twice.

The truck shifts really well across the board, but once I’m cruising at like 55-60 it’ll suddenly like downshift or upshift or something pretty hard. It’s only around that speed. Slower is great and faster seems to well too.

Curious if I’m doomed or just need a service or something.

 

[1ton-o-fun]

Are you still moving forward when you "put it in reverse real quick"? That'll kill (stall) most vehicles and isn't good for the trans, mounts, u-joints, rearends...

 

[thegawd]

run a test on the tps, it can be graphed with an obd2 scanner.

Al

 

[andy396]

First guess is torque converter related, but more info would help.

 

[stutaeng]

Picked up another truck a couple weeks back. 2000 Chevrolet k2500 CCSB.

All my other trucks are and have been manual trans trucks. This truck has is an auto, 4l80e.

Couple things that have happened...

Truck has stalled when I put it into reverse real quick. Doesn’t happen every time. Has only happened once or twice.

The truck shifts really well across the board, but once I’m cruising at like 55-60 it’ll suddenly like downshift or upshift or something pretty hard. It’s only around that speed. Slower is great and faster seems to well too.

Curious if I’m doomed or just need a service or something.

About the reverse engine stall: My K3500/5.7 does this when I put it in gear, even letting it warm up; for the first mile sometimes it stalls at stoplights, but after that it's okay. There has been a recent question regarding this, and I created a thread back in November or so. Mine feels like it goes into neutral until I give a good gas push or it stalls. I initially thought the TC was locking up at idle, but don't know enough about these units myself. I read on performancetruck.net that if you rev the engine in R with your foot on the brake for a minute or two that causes the fluid to come up to temperature. I forget to try that when I drive it.

Unfortunately, I don't drive that truck daily to cause me to try to find the problem. At the minimum a fluid change and new filter is my plan. I have 2 slightly newer trucks with the same tranny and those don't have that problem.

I'm not sure if the torque converter issue on the highway is related.

 

[yevgenievich]

The random shifting might be because of the input speed sensors. My 91 did not know what to do when at speed because of one of those. And tps sensor as well causes it to act up as throttle position would jump all over the place as far as ecu is concerned

 

[thegawd]

exactly....

 

[stutaeng]

I forget what "limp mode" does to these tranmissions. But I've seen some diagnostic strategies where you unplug the main connector and you could isolate the problem electrically from mechanically.

 

[MIHELA]

These have several well know issues with reverse that all can thankfully be corrected. The reverse servo can crack, often caused by high line pressure from a sticking reverse boost valve. The valve can be accessed from the pan and Sonnax has a good replacement valve that fixes the line pressure. If the reverse servo cracks it won't apply the reverse band all the way and over time that can damage the drum as well.

 

[Dropped88]

Just from what you wrote without more details sounds like the the tcc circuit when the bore wears and the valve hangs. They make a repair kit but involves valve body removal.

But needs a proper diagnostics dont throw parts at it