'89 GM Tranny troubles

On an '89 Grand Marquis, 5.0, automatic, Car is slipping @ highway speed while in OD or High gear when accelerating (like to pass). We've noticed that this seems to coincide with unrequested changes in the Heat/AC air deflector doors. Could this indicate a failure of some sort of vacuum modulator or something similar? If so, where is this part located? And will replacement require R&R of the transmission?

On an '89 Grand Marquis, 5.0, automatic, Car is slipping @ highway speed while in OD or High gear when accelerating (like to pass). We've noticed that this seems to coincide with unrequested changes in the Heat/AC air deflector doors. Could this indicate a failure of some sort of vacuum modulator or something similar? If so, where is this part located? And will replacement require R&R of the transmission?

A few things to check befor going to the tranny.
The throttle linkage cables at the throttle body,the OD/kickdown cable is attached with a plastic clip that has a tendency to break which can give the symptons you describe
The other is a bad EGR valve believe it or not,try removing the vacuum line from the egr valve and plugging the line,if the cars stops doing what you describe then change the egr valve.
One more thing is the TR Sensor if this car has one(Transmission Range Sensor).If it doesn't then it can be an adjustment of the cable attached to the throttle linkage.
I just replaced it on my 93 GM which was doing the same thing you described.

Pickup a Haynes manual for about 20 bucks and it will show you all that I just mentioned along with adjustment and replacement procedures.
The chnges in the air deflector doors is definetly a vacuum issue but should not affect the tranny,You car should have the AOD tranny which is electrically controlled but I could be wrong ,you will have to verify which tranny you have in your car

I could be wrong but electronic trannys did not start in the GM's until the '93 model. Perhaps some '92's but not '89. I have a '93 and '87 and have had the plastic grommet go on the '87. I would look at that before anything else!! MB

P.S. If the grommet is the problem (and you will know as soon as you look at it) I would replace it IMMEDIATELY or the tranny WILL fail and need a rebuild!! If it has been defective for days and has been driven it may have already failed and the grommet won't help.

Well, the verdict is in,, the high gear hub is busted, anybody want a nice GM that needs a tranny? CHEEP! Make offer... contact [email protected]