GM 4L80E tranny

otto_kretschmer · 05-19-2019, 09:30 PM

Hi Folks,

I have a 93 GMC Suburban 4wd 3/4 ton with a tranny that won't shift into OD (4th) when its at operating temperature.

The truck has 250k miles. I had the tranny replaced 12 years ago by an independent GMC guy and he put in a GMC rebuild. The tranny has 38k miles.

I changed the oil and filter a year ago. There was a normal amount of metallic gunk collected on the magnet on the oil pan. The previous oil change I used silicon on the gasket and there was some silicon that was squeezed out still in the pan.

The tranny shifts into all gears and reverse when cold but as soon as it gets hot it won't go into OD.

I don't tow with this truck. Its just for running to Home Depot and scuba diving trips to Mexico.

I'm reading things about possible causes; bad brake switch, bad TPS, bad solenoid valve to name a few.

Anyone have any insight to automatic tranny issues?

thanks

otto_kretschmer · 05-20-2019, 01:02 AM

after further research I think its the TPS

find out tomorrow

LWJ · 05-20-2019, 04:34 AM

I had one on my suburban - a long enough while ago.

Not sure if this is a causal thing but I recollect a sensor could leak onto an electrical connection?

This may have caused a different fault. It's the only thing I remember about this trans.

Good luck! Lots of these installed. You will find the answer in your research for certain.

JackDidley · JackDidley's Garage

You should really hook up a scanner and check codes. 4l80s have a reputation for being tough.

otto_kretschmer · 05-20-2019, 04:45 PM

I swapped batteries a day ago and that cleared the codes

I tested the TPS this morning

.5 volt throttle shut
4.5 volt throttle open

that seems to be in spec so the TPS looks ok

maybe I should just let the 93 go

Arizona_928 · 05-21-2019, 01:11 AM

how's the fluid? do you know the temp of when it gets hot? Sounds like a simple fix, and not rebuild territory yet.

https://www.thetruckstop.us/forum/threads/4l80e-no-overdrive.39765/

JackDidley · JackDidley's Garage

Quote:

Originally Posted by otto_kretschmer

I swapped batteries a day ago and that cleared the codes

I tested the TPS this morning

.5 volt throttle shut
4.5 volt throttle open

that seems to be in spec so the TPS looks ok

maybe I should just let the 93 go

I just remembered, it takes a special scan tool to get trans codes from a 93 anyway. Not likely you could have access to it. It would be easy to hook a pressure gauge to it to see what is happening. May be time to let it go though,.