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Help with 4L80E shifting problems

Blackbart

By Blackbart
in Ask A GM Technician

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Blackbart Enthusiast

Blackbart

Posted
Posted

All:

 

I was hoping to get a bit of advice and input on a tranny problem I am having.

 

Vehicle is a 1991 Suburban 2500 4X4 that is basically stock. Had the 4L80E rebuilt about 3 years ago now and it has under 20K miles. The rebuilt was a hevay duty one with improved clutches, steels, etc. Importantly, it also had a Trans-go shift kit (not the the reprogramming kit) installed. It ran and shifted perfectly well for about the first 1.5 years.

 

For approx the last 1,5 years it has developed a shifting problem that ONLY surfaces when the trans is fully warmed up and is worsens to some degree as the ambient temp goes up. Also shoudl note that I have a big B&M trans cooler mounted.

 

The shifting problem is this: When you come to full stop then accelerate, the trans seems to be in 2nd (like it never downshifted to 1st) then in hesitates down shifts to 1st then upshifts to 2nd in a very erratic manner. Usually I drop it to "D" (from "OD") and it shifts fine. It may be diagnostic to also point out that when this happens, if I leave the shifter in "D", or "3", and drive it around it shifts perfectly well I just up shift after it hits 3rd then the converter locks up and it shifts into OD as per normal function.

 

I haven't driven the truck much in the last year, but now want to fix this issue and not exactly sure where to start.

 

It seems as though a pretty obvious place to start is with the shift kit maybe getting another one and going through the valve body, but while I am pretty good shade tree mechanic and have done a few shift kits in the past, I would really like to get some expert opinions as to where the issue may lie with this shifting problem.

 

Thanks much !!!

Bob

 

 

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted

you need a hydraulic schematic and go from there ,,,,,remember ,,,,internet and auto transmissions should never be in the same sentence !!!!

 

good luck

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