Hello, I am new to the forums, and not sure if i'm posting in the right place. Sorry in advance if so!

I recently got a 1987 G20 350TBI. It's a fixer-upper, and I have fixed many odds, ends and engine issues, but I have no experience with transmission repairs, and need some help diagnosing an issue.

I don't know the specific model of the trans. It's 1987 auto with OD on a 350tbi engine. It was working fine until a week ago. I noticed it slipping in reverse only when ice cold. Now it slips badly in reverse and 1st until the engine is warm. It will not shift into 2nd, until high revs and while I manipulate it into N, and back in D while letting off the gas. It will not shift into 3rd until reaching high revs. Once quite hot, there is no noticeable slip, and it will shift from 1st to 2nd, and 2nd to 3rd without manipulating the shifter, but still does so at very high revs. The dipstick reads too much fluid in the trans.

I've never opened an auto trans before, but am fairly familiar with their operations. I'd like to hear your ideas on what's wrong and how easily fixable this is. Thanks!

 

If you are sure the tranny is over filled the van should not be driven until you drain it down to the proper level. If you do a fluid and filter change and fill to the proper fluid level your issues may be resolved assuming the tranny is not already damaged which could be the case.

I believe to check the fluid level the tranny should be warm the van parked on level ground with the gear selector in neutral. Important don't forget to set your parking brake if the ground is not as level as it seems, you'd hate to explain getting run over by your own van.

 

Yea engine needs to be running to check the fluid. in park, not neutral.

 

I think its a 700r4 tranny. Not cheap to rebuild.
Take it to a tranny shop.