Hey guys I got a new one. I know quite a bit about 4L60E transmissions and even overhauled a few myself but I'm stumped. I got a 98 Jimmy 4.3.
Several weird symptoms:
-Will not downshift even with pedal to the floor, only manually with the shifter.
-Will take off in 1st gear only if I come to a complete stop and wait a sec.
-Then does not want to shift out of first if I floor it till it hits the rev limiter and shifts directly into OD with lock up converter at only like 20mph
-If I make a rolling stop it will stay in OD even at under 5mph and I have to manually drop down the gearshifter into at least 2nd to be able to accelerate
-If I run the trans manually ex: start in first and shift to 2nd it shifts to 2nd great and shift to 3rd it shifts great and then shift to OD it shifts great and shifts at the right RPMs and engages correctly.
-Sometimes I can unhook the battery overnight and then it will work great for a day or so and everything does what its supposed to do. And sometimes just out of the blue I will start it and it works great till I start it again.

I have all my gears and they hold firmly and can shift good.
Its like the computer doesn't know what is going on or what speed its at or something but it is not in the default 3rd gear and I am getting no codes so you would think it is not a computer problem but maybe. I have driven thousands of miles like this too without losing one gear in the trans.

This is what I've done:
-Changed filter twice
-Changed every solenoid and the in pan wiring harness (all came as a kit)
-Tried every kind of supplement: Wynns, Lucas, Trans X
Nothing has helped oh and the fluid is clean and not burnt.
Any ideas? Thanks

 

If you find anything out let me know man lol. My 95 blazer seems to be experiencing the same problems.

 

Have you checked the TPS sensor?

 

Hey guys finally figured it out. I did not say that I scrapped all of my electrical shifter unit for the 4x4 and converted it to a manual floor shifter. Well since about then I've had my problems. Well I found out that the computer does many things to the trans when it is in 4x4 and especially 4x4 low. Well since the computer did not know what the transfer case was in sometimes it started thinking it was in 4 low or 4 high or 2 high. I hooked back up my push button shifter in the dash and when the 4 low light is lit up I have all these problems. I just push the 2 high button and all my problems go away untill I start it again. I've driven another 6,000 miles since and no problems. I just need to program the computer someday to run without 4x4.
Thanks for the help.

 

The computer will still need to know when it is in 4LO so that it can alter the shift timings. On the older NP231 equiped vehicles, this was accomplished via the selector switch in the shifter.