4L80E Intermittent Shifting Problems

TheJackal

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Hi everyone!

I seem to have a problem where my 1995 Gmc Suburban with a 6.5L Diesel randomly will drop into second gear and stay there while traveling at highway speeds (55-70MPH). This even usually seems to be triggered by going up a grade for a prolonged period of time, or if a load is being pulled behind it. But it has also had this symptom completely randomly.

It is rectified by shifting into neutral while rolling, turning off the engine, and then restarting. Then after I turn the engine back on, I shift back into drive and it works as it should, no problem. I keep coming to the computer. I don't see how the transmission could be at fault. It also has an Intermittent hard shift from 1-2, but that problem is also fixed by turning it off and on again.

Has anyone else experienced this? I've looked at several forum posts at similar issues, but I keep coming up with computer issues. But those are all on OBDII vehicles... I have OBDI. What should I do?

Thanks in advance for any and all advice!!
 

stutaeng

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Can you remove the main electrical connector and see if you have any ATF fluid in there?

Leaking fluid on the connector is supposed to be a common problem, and will cause weird, unexplainable erratic shifting...

If issues seem to go away after you shut off engine and restart, I think you are in the right direction about it being an electrical issue.

Edit: the hard shift from 1-2, that is resolved if you turn truck off and back on may be the TCC valve wear. It will give code P1870 though if that's the issue. Does it do it after getting off the highway after driving for like 30 minutes?
 
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TheJackal

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Can you remove the main electrical connector and see if you have any ATF fluid in there?

Leaking fluid on the connector is supposed to be a common problem, and will cause weird, unexplainable erratic shifting...

If issues seem to go away after you shut off engine and restart, I think you are in the right direction about it being an electrical issue.

Edit: the hard shift from 1-2, that is resolved if you turn truck off and back on may be the TCC valve wear. It will give code P1870 though if that's the issue. Does it do it after getting off the highway after driving for like 30 minutes?
No fluid on the connector. I do remember reading about that.

I don't have OBDII, so it will not tell me the DTC you specified. Is ther any other way to diagnose the TCC valve wear? I reckon visual inspection is needed.
 

stutaeng

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No fluid on the connector. I do remember reading about that.

I don't have OBDII, so it will not tell me the DTC you specified. Is ther any other way to diagnose the TCC valve wear? I reckon visual inspection is needed.
Oh, sorry. I forgot your truck is TBI/OBD1 :rolleyes:

And again, I'm getting these units confused because the TCC valve wear issue I described is apparently an issue of the 4L60e.

I rebuilt a 4L80e (1995 model) last year and this is the kit that I did to the Actuator Feed Limit Valve: https://transgo.com/product-details/4l80e-afl-valve-kit/

The valve body needs to be removed for this; but it was pretty easy to do with the reamer on a handheld drill. If you read the description, it sounds like some of the symptoms you describe?
 
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1997

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No fluid on the connector. I do remember reading about that.

I don't have OBDII, so it will not tell me the DTC you specified. Is ther any other way to diagnose the TCC valve wear? I reckon visual inspection is needed.

OBD1 can set codes without turning on the check engine light, have you checked for any codes?
 

udidwht

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No fluid on the connector. I do remember reading about that.

I don't have OBDII, so it will not tell me the DTC you specified. Is ther any other way to diagnose the TCC valve wear? I reckon visual inspection is needed.
Disconnect it and look inside the connector. There should be no signs of fluid.
 
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