4l60E loses 1st gear & won't shift automatically when hot

I have a 1994 C-1500 4.3L with a 4L60E transmission. After driving for 30-40 minutes it loses 1st gear and you have to manually shift between gears. R, 2, D and OD can be manually selected but there is no automatic up or down shifting. There isn't any gear slipping prior to this. After sitting for 25-30 minutes everything works fine. I have noticed that sometimes before the problem occurs that if i manually select 1st gear it will stay in 1st & sometimes it will automatically shift to 2nd even though the gear selector is in 1st. Also it usually shifts into D at about 30 mph but sometimes it stays in 2nd until I manually shift down to 2nd and back up to D. It isn't giving any relevant codes (use jumper & flash codes). I'm thinking possibly faulty solenoid(s), but not sure. Any help would be appreciated.

Wow JFD444! I thought mine was for sure a unique situation. My 99 gmc suburban is doing the same exact thing. I'm a seasoned auto tech so it is truly driving me bananas, especially since my truck is in superb shape and runs perfectly otherwise... not to mention we are in a blizzard and getting 18-24 in of snow (I'm limited to short trips only). I did some trans rebuilding in tech school and was always one of the best students (according to the instructors). I'm gonna replace shift solenoid A and see what happens. I suspect it is losing power thru the coil field windings once it heats up or just plain old binding up. After all it is just an electrical part. I'll post results.

Wow JFD444! I thought mine was for sure a unique situation. My 99 gmc suburban is doing the same exact thing. I'm a seasoned auto tech so it is truly driving me bananas, especially since my truck is in superb shape and runs perfectly otherwise... not to mention we are in a blizzard and getting 18-24 in of snow (I'm limited to short trips only). I did some trans rebuilding in tech school and was always one of the best students (according to the instructors). I'm gonna replace shift solenoid A and see what happens. I suspect it is losing power thru the coil field windings once it heats up or just plain old binding up. After all it is just an electrical part. I'll post results.

replacing both solenoids is a common repair. when you do this only use DEXRON VI tranny fluid as this is now the approved product . GM made this change in 2006 because it caused huge warranty failures , so use the proper fluid. since DEXRON III is not an approved fluid anymore you can buy a qt for $3 USD .., the VI is $10.


replace both not one .