I'm new to this site and I'm not sure if I'm posting this in the correct spot. I'm having some issues with my 4L80E transmission. I've searched and searched but I haven't found anyone with the same issues. I drive a 1991 GMC 2500 Suburban with a 6.2 liter J-code diesel. This is my daily driver and I put a lot of highway miles on it. Over the last couple months the tranny started acting funny on the interstate. While in overdrive around 55-65mph it would, what I thought, try to decide between 3rd and 4th gear. Every second or so it would kinda shift back and forth between the two gears. If I shifted it into 3rd gear the symptom would almost go away but I can still feel it trying to shift or maybe it's the torque converter trying to lock, I don't know. This would not happen until I had driven for a least 45 minutes to an hour. Then when I would be stopped at a light or stop sign and go again it would kind of slip for a second then clunk into 1st and drive just fine. Well today I realized it was not slipping from a stop because now it will not go into 1st at all, it starts off in second. If I shift it into 1st with the shifter it wont go and it will not rev up very well, almost like I was pulling something really heavy. The transmission goes into all the other gears just fine. It cruises great at 75mph but sometimes has that shifting symptom at that speed too but not very often. About a month ago I changed the fluid and filter. There was not any metal shavings in the pan on the magnet.

 

there is no possiable way for a 80 to electronically have neutral while the manual valve is stroked out of the neutral or park position. Its time for it to come out plain and simple, we could go deeper into this if you want but the outcome will be the same. Rebuild or replace it. The hunting at interstate speed could very likely be a TPS problem. Sounds like you have atleast 2 problems.

 

What do you mean "there is no way for the 80 to electronically have neutral while the manual valve is stroked out of the neutral or park position"?
I never said anything about neutral. Maybe I'm misunderstanding what you're saying.

 

Only mechanical failures can cause these symptoms. there is no way for a electronic issue to cause these style problems.

 

Update,
In one day my transmission fluid went from pink to brown. I drove it to work and back for a couple more days. Dropped my son off at school and I got first gear back. I can also manually put it in first and it no longer binds up. The transmission is working just fine. A little odd and I know I need to get it repaired. I'm getting it remanufactured and I'm also installing a True-Cool cooler. I was told to keep the Tru-cool connected to the factory one that is part of the radiator because of the sub zero temps here in the mountains of Colorado.