I'm not sure if this is allowed but I'm double posting this. Also posted in "Sights and Sounds", but I think I may get more help here...

So I am wondering if anyone has experienced anything like this. My girlfriend has a 1991 240 Wagon. I don't know most of the history of the vehicle unfortunately as she bought it with at least 200k miles (the odometer stopped working an unknown amount of miles ago).

A few potentially-related known problems.

1. The overdrive is out, it has kicked in maybe 3 times over the past year and a half so she has just tried to avoid driving on the highway as much as possible.
2. When she bought the car the rear differential was leaking and noisy. So a year ago she took the car in and the mechanic apparently replaced the leaky rear differential with another used (I assume) diff. Apparently this fixed the leak and the noise for a while.

The other day I took it out to run a couple errands. Regrettably, I took it on the highway. I was trying to see if I could get the Overdrive to kick in and in the process, reached probably about 60, maybe 70mph. I began noticing a whirring sound that seemed to be coming from the differential. It very quickly and progressively got worse within a few miles of highway driving. I think I also noticed a burning smell - this is hard to know though as they were repaving the highway. I pulled over and it looked like the diff was leaking. The liquid was burning off from the hot metal of the differential.

In its' current status, the 240 has a slight whirring sound when accelerating. It seems to be coming from the main driveshaft > u-joint > rear differential area. However, the most alarming sound happens when you let off the gas and coast at speeds above 30 or-so mph. There are a couple small clunks and bangs as you let off the gas, followed by a loud whirring and squeal, coming from the rear diff area. It also vibrates and feels like the car is going to come apart.

I was able to jack up the rear end and record a video under the car. With the rear wheels jacked up, we put the car in drive and gave it some gas to see what is going on under there during accelerating and decelerating. I recorded video and have posted it below. The quality makes it difficult to distinguish the different sounds happening, so I made some equally crappy graphics that probably don't help much. I'm hoping a seasoned mechanic can maybe see something in the video and help give me a diagnosis?

Any help is much appreciated! Thanks!

You Tube

 

Need a rear u-joint. Easy diagnosis is to take off the 4 bolts that attach the drive shaft to the differential and check if the u-joint is flexible. From what it sounds like it has dried up and is binding. Typical symptoms of this is vibrations while decelerating and braking that on bad cases can cause severe bucking. While you're down there, you might want to drop the entire shaft and check the center support bearing. You mentioned in your post some smell when you heard the noise which makes me suspect that the center support bearing maybe binding as well which will cause it to spin inside the rubber mount - which would cause the smell. As for the overdrive, most likely it is the overdrive relay - which is accessed by removing the glove compartment and is just to the left. I believe it is gray. The typical problem with this relay is the solder joints that hold the little solenoid crack over time from vibration. You can actually repair the relay by re-soldering it.

 

Act1292 - Thanks for the super helpful information! That seems like it would make sense since it was such a quick progression of symptoms. I’ll try this out hopefully this weekend and report back.

 

When taking off the driveshaft, mark the the shaft so you assemble in the same position. The driveshaft is balanced as a whole and if not reassembled the same way it could be out of balance.