Questions and Answers

Harsh Marquis Q. I have a 1991 Mercury Grand Marquis with 55,000 miles that has a vibration that baffles both me and my mechanic. I hope you can identify it. The car has a 5.0 liter EFI engine and an AOD transmission. It has a vibration under HARD coasting conditions (going down a steep hill with absolutely no throttle applied), but only at speeds above 60 mph, either in third or overdrive.

The application of the slightest amount of throttle (not even enough to start acceleration) makes the problem disappear. It seems to be worse in overdrive, but that may be only because the car tends to slow down faster in third so that it's above 60 for a shorter period of time. The vibration can be felt in the floorboard and feels almost like an unbalanced tire or a vibrating drive shaft, but I can't locate the source of the vibration. No vibration is noticeable on coast above 60 mph if the car is raised on a hoist and the back wheels are off the ground. It only occurs when the weight of the car is pushing the drive line.

The wheels have been balanced, the rear tires replaced, the U-joints and the transmission mount have been replaced, all shocks have been replaced, front brakes and calipers have been replaced. I've even had the EGR valve checked for sticking and/or leaks. Nothing seems to help. The car is very quiet and, except for this problem, extremely noise and vibration free. I like the car very much and would like to be able to get it fixed, if I can find out what's wrong. I hope you can point me in the right direction.

Thank you in advance for any help you can provide.
Dallas

A. I think your problem is in the transmission valve body.

The 1989 and later AODs have been known to damage the large (5/16") orange check ball in the valve body. In many cases, this damage results in the check ball blowing through the separator plate. The check ball will either lodge in a passage or be digested by the transmission altogether. This results in a very harsh 4-3 kick down.

Since this is a soft check ball, it leaves no mark on the separator plate. An assumption can be made that the ball was left out by design. All AODs were designed to include this check ball.

A transmission shop can drop the valve body and check this for you. If it is indeed the case, the check ball needs to be replaced. An effective measure against repeat failure is to use the slightly harder AXOD dark brown check ball in this location. The Ford part number for the AXOD check ball is E7DZ-7E195-A

Additional Information provided courtesy of ALLDATA