1991 GMC Safari Auto Trans chatter

I have a 1991 GMC Safari (Astro) Van with 158000 miles. The transmission has had fluid and filter changes at the recommended intervals at dealer service centers. The current fluid has less than 20,000 miles, is at the proper level, has a nice clear red color and does not have any burned smell. There are no transmission fluid leaks on my garage floor.

Recently, as I am going up a hill the transmission will sometimes shudder or chatter as it attempts to downshift from overdrive to drive. Letting up on the accelerator stops the chatter immediately.

Any ideas?

TIA, -Joe A company by the name of ATP makes a transmisison fluid additive to reduce chatter. I think it is part number AT203??? Ask your local auto parts people, they should have something like it on the shelf.

I am assuming you have ruled out a spark plug wire breaking down causing an engine missfire? I get lots of "transmission problems" that turn out to be engine missfire problems instead.Car Repair Talk's forum.