Trans experts step inside, 4L80E help in a pickup

[Warwagon]

I am not having luck chasing down an issue in an antique 1993 6.5 turbo diesel pickup with a 4L80E. This year the computer only controls the transmission by using a TPS, speed sensors on the trans, and engine RPM as the rest of the engine is mechanical.

When I am in 3rd gear the transmission will shuttle shift between 3rd and 4th extremely fast under heavy to full throttle sometimes - until the MPH is high enough to prevent a 3rd downshift. It has started to shuttle shift in the parking lot between 2nd and 3rd. It is possible that it is a shudder and not fully grabbing the next gear with the speed of the shifts.

Also I have compression braking in manual 1st and 3rd but it will let the engine RPM drop to idle in manual 2nd if you let off the throttle. In other words manual 2nd will not compression brake at any time. Is this a separate issue?

Using a TCC override switch causes is to shudder and slip in 3rd during compression braking during higher RPM operation. This switch is a known weakness and a bad thing to do, but, during acceleration WOT TCC lockup gains you 1-2 MPH during a hill climb at 45 MPH... Acceleration is fine in all gears other than the shuttle shift.

I have used a analog voltmeter to test the TPS several times including borrowing a scanner to trace the TPS. No flat spots or drop outs were seen.

Trans temperature runs about 180 tops.

Transmission lasted a week when I first bought the pickup before OD scattered and I had it rebuilt. It has 25K on it since and is beat like a redheaded stepchild pulling trailers with the engine power turned up. Yes I abuse it, but, I would like some ideas on what needs to be fixed as a shop handles everything in the transmission and I take care of the things like wiring on the outside... Keep in mind this is a 6.5 so turned up the power is not anything to be impressed with.

[Greg Ducato]

I think you might have multiple problems with this trans from possibly the solenoid feed orifices being too large to maybe an intermediate /third clutch problem such as a worn center support bushing or poor sealing rings. These early 4L80Es respond very well to modification, I like the trans-Go 4L80E-HD2 kit as a means to improve both shift quality and accuracy and durability. If you end up removing the trans for a re-inspection you would be well advised to check into the installation of this kit.