Used 1990 Buick Riv trans slps in & outta gear; restart to engage

THE SOLENOID PACK IN THE TRANS IS BAD

This sounds like an internal problem .. possibly a broken hub on the clutch pack Standard GM L460 problem..call a couple of trans shops in your area and ask their assessment and price a rebuild.

Definitely not good. The transmission is slipping internally due to worn gears and clutch packs. It will have to come out for rebuilding, IMO. Have a transmission shop check it out.
Good luck.

it sounds like the problem started with the brake shift iterlock switch by forcing it he now broke the trans shift cable mech at the column

I would begin by replacing the filter if its not new, 2 possibility of issue, faulty shifting module or defective transmission rebuilt. just because it has been rebuilt does not mean the individual work on it did a good job. the less cost effective way is to locate used modulator from ebay or junk yard, if that does not rectified the issue than more likely it will be the tranny. the bad part is that I dont know of a company offer a bench test for the tranny modulator.

Sounds like the transmission is worn out. It could be rebuilt or call around to the junk yards and see if any has a good clean used trans.If you get a junk yard one change the fluid before you drive the truck.

possibly 2 solutions 1. manual transmission --probably clutch fluid. find out where it is, fill it , and make sure the cap is closed exactly like you took it off. if it's not closed clompletely, pressure won't be able to build up. 2. auto trans-- transmission fluid..note there is no dipstick to check fluid.

Time for a rebuild, or used trans.

I had the same problem. I changed the transmition by an used one. It was fast, money and future problems save.

is it manual or automatic transmission?