1990 Buick LeSabre trans pops out of gear/ won't shift.How to fix?

Hi Seanolo! Here's a tranny troubleshooting guide to help firm up your diagnosis. At that point if you are still pointed at the VVC, take a trip to your local library where they will probably have the Chilton's manual for your Buick. That should have the procedure for the switch out. Good luck! http://www.procarcare.com/includes/content/resourcecenter/encyclopedia/ch21/21TSautotranny.html

try replacing the vaccuum modulator(about $100) and maybe the connnector hose too if it is over 5 yrs old. if that doesn't help then check the manual for more diagnotics. how is your tranny fluid level??