G30 6.2 Diesel Trans Slips 3rd and OD

I recently bought a 1991 G30 with a 6.2 diesel from an auction. The transmission slips in 3rd and OD. Shifts and works fine in 1st and 2nd. Sounds like it is in neutral when it shifts into 3rd and OD. Fluid does not look or smell burnt. Fluid level is good.

Any ideas what could cause this? What should I check?
What trans would this have?

Thanks

What transmission?
Is it just slipping on gear change or all the time?
Does it pull any at all in 3 and OD

Check all adjustments and vacuum to it.
Has the filter been serviced and fluid level been checked?

I beleive it is a 4L80E based upon the information I found on the internet. The fluid level is full. It does not pull at all in 3rd or overdrive. The fluid looks clean. The vehicle has 53,000 miles on it. I'm not sure if the fluid has been changed recently. I don't see any signs of the pan being off recently.

I beleive it is a 4L80E based upon the information I found on the internet. The fluid level is full. It does not pull at all in 3rd or overdrive. The fluid looks clean. The vehicle has 53,000 miles on it. I'm not sure if the fluid has been changed recently. I don't see any signs of the pan being off recently.

If the 4L80E
Pull codes and post back codes.
May be a shift solenoid or wiring.
MT

MT

The trans is a 4L80E. I don't have a scanner so I read the codes by flash method. Code 87 is what I found. From what I have read this may indicate a 3rd/4th shift solenoid problem. I'm hoping its just a solenoid vs a burnt trans. what do you think? If so, should I replace all the solenoids or just one?

thanks

MT

The trans is a 4L80E. I don't have a scanner so I read the codes by flash method. Code 87 is what I found. From what I have read this may indicate a 3rd/4th shift solenoid problem. I'm hoping its just a solenoid vs a burnt trans. what do you think? If so, should I replace all the solenoids or just one?

thanks

The codes does not say the part is bad.
It says CHECK IT OUT.
Get a good repair flow chart and check wiring and solenoid and power and grounds.
Lot po wiring problems on them.
Good Luck
MT

Good news. I didn't see any problems with the external wiring so I replaced the 2-3 shift solenoid. Trans now works like new.

The solenoid was easy to replace andscheap. The part only cost $28 from the local Chevy dealer. NAPA wanted $42 for the same part and didn't have it in stock anyway.

The part number for the 2-3 solenoid is GM and Delco # 24230289.

The only minor problem I had was in getting the retaining screw out of the valve body. The torx "teeth" rounded off with the first twist (maybe I used the wrong size bit), so I had to use vise grips to get the screw out.

I'm glad I tried the repair before I bought a trans!

Thanks to all that helped in diagnosing the problem.

You are welcome.
And thanks for posting back how it went and the fixed.
A thank you and a fixed makes my day.
Glad all is well.
And yes dealer OEM parts are better and sometimes cheaper.
Good luck
MT