Expert: Brad Sears - 11/22/2010

Question
My overdrive is on a 52 Plymouth and works. Only when in overdrive at speeds above 60mph, a pulsation sound occurs. When out of overdrive up to 75mph, no pulsation sound is detected. Bearings and planetary gear have been replaced with no fix for problem. A few people have said the problem occurs as a result of wear to the OD case from the ring gear. What is the fix for this?

Answer
I would start at the engine to make sure that I did not have a weak cylinder that could set up a harmonic when the engine is loaded. And in overdrive the engine is loaded. A simple compression test will check the engine. All cylinders should test to within 25 pounds of the highest cylinder. Then I would look at the motor mounts to make sure that I did not have collapsed mounts that again could be letting the engine rest on the frame under load and setting up a harmonic vibration. The last thing I would check before condemning the OD is the output shaft bearing in the back of the transmission between the transmission and the OD. If all else fails, there is a tool called something like the electronic ear. It is a small microphone that can be Velcro's to various parts of the car with a wire that runs to a small amplifier and ear phones. Then drive the car with it attached to various parts. I have one and it is a great aid when chasing something like this. I doubt very much that it is wear in the case that is causing the problem. Oh, just one other thought, check the shackle bushings on the rear springs.
Brad