'91 F-150 AOD

Maggie's son bought a '91 F-150 cheap. The guy told us upfront, that it had transmission problems. He told us that it lost reverse, then 1st but would go in reverse then. We had it towed to my house, put a battery in it and were able to move it around in the driveway. Brian said he felt a chatter, in first, but it was smooth in reverse.

So, first, we thought we would change the fluid and filter. If that doesn't change anything, what should we look at next?

Brian decided not to work on it today (rain & cold), so I'll report back, after he does the service.

Any help appreciated!