Hopefully I'm not regurgitating a question that has already been asked before. I have a 89 RS automatic. Yesterday I pulled out of the garage and went to take off in overdrive like I normally do and the transmission engaged but slipped when I gave it gas. I pulled it down to First and it kicked in just fine. Pulled back into the garage, checked the fluid, and it was just under full with no noticeable discoloration. I pulled back out, same thing. So I manually shifted it down to the local AutoZone, picked up some Trans-x 75k, put it in and drove around for a while with no noticeable difference. Today was the same, but I noticed I don't have to go down to First for it to kick in. It engages just fine by placing it in Drive. No slip at all. Has anyone ever had this happen and what was the diagnosis? Sorry in advance for this question being so long

 

Start diagnosis by overfilling an additional Quart of ATF and report back.

 

I did a lot of research on the web prior to asking my question. It appears mine is the first to ever encounter this problem in the history of transmissions. It seems possible I'm getting a fluid restriction when placing shifter in overdrive but somehow I'm bypassing that restriction when I drop it down one more notch into drive. Is that why you're advising me to overfill? Possibly create an overpressure situation that might clear it out? If that is the case, we are on the same page, which is why I went with the trans-x 75k to possibly eliminate the blockage but it did not help at all. The one thing I found consistent throughout my research was "do not overfill". Is there a danger of blowing a seal if I do what you're suggesting? Thanks in advance

 

1 extra Quart no harm... 3 or 4, that's a problem

 

Unless there are clogged Filter Elements, or a clogged Filter/ Sump Pick-Up...
Restriction is not usually an issue.

What we Custom Builders and Engineers have observed on the Transmission Test-Stand (What you guys call a Transmission-Dynomomter)...
Is that the ATF Level in the Oil-Pan often drops too low from the Filter/ Pick-Up (loosing suction/ fluid volume and pressure) while in 4th-Gear or in Lock-UP.