Heavy thunk under acceleration - PeachParts Mercedes-Benz Forum

Please Help · #1 11-01-2003, 08:28 AM

1989 560 SEL. Heavy thunk from rear (Driver side I believe) under acceleration, especially when car is shifting. I recently replaced two broken muffler doughnuts, but thunk is still there. ANy ideas???

tia,

Andy

Ken300D · #2 11-01-2003, 11:21 AM

Just speculating - perhaps the flex disks, perhaps the differential mounts - perhaps the differential itself is worn.

Do you have "slack" in the driveline under conditions of accelerating and then letting off the gas - or vice versa?

How long has it been since the differential oil was checked and/or changed? I'm a large advocate of differential care, having seen one worn out completely from harsh conditions and lack of maintenance (previous owner).

Ken300D

Please Help · #3 11-01-2003, 11:28 AM

Just found out, quite by accident, what the noise was. Was driving home from shopping with my wife when the rear end fell off the car.

Please Help · #4 11-01-2003, 11:50 AM

Ken300D,

Thanks for your reply. I believe that, since the car fell on it's a55, that it is most likely the diff mounts. Can you possible tell me how much this new problem will cosct me???

Ken300D · #5 11-01-2003, 11:53 AM

That's got to be pretty rare. I've been impressed with the typical differential mounting scheme.

Did it literally fall off the car (totally)? Fall down and start scraping the road? Or did you completely strip out the gears and are unable to move?

Does it seem like the bolts broke or the nuts fall off? If yours is the typical design, it has four large studs coming out of the top of the differential and is held on by four self-locking nuts.

I bought a used differential for the 1982 300D for $135 from Potomac German auto. They can be shipped. Nothing wrong with a used differential if you have the opportunity to check it somehow yourself or have return rights if it is worn. The main checks to perform on a used unit are the condition of the rubber seals for the pinion and axle shafts, and to look for any looseness in the gearing. You turn the driveshaft flange back and forth and make sure the axle shaft recepticals match that movement without much (if any) slack. Mine came from a 1985 300D with about 85k miles - just like new condition.

Ken300D

Please Help · #6 11-01-2003, 11:56 AM

It fell down and started to scrape. I was only doing about 7 -10 mph when it happened. I did not climb underneath as my wife and I were coming home from shopping and I had to get the leg of lamb into the fridge.

The car was still able to move (limp) under it's own power. And the wheels were canted inwards.

Ken300D · #7 11-01-2003, 12:02 PM

Yes, the wheels will splay inward if the differential falls, due to the way the differential itself forms part of the rear suspension. That in itself is not serious.

What you have to do (if you want to attempt a fix yourself) is get the back of the car up on jackstands and find out what failed in the mounting. It probably came loose and then rattled against the mounting studs until they failed. You may be able to re-stud the differential and bolt it back up. If you do, check the mounting holes for wear - you may have to use (heavy) washers if the holes have deformed.

I used jackstands with wooden blocks (cut from a 2x4 and custom wood chiseled) under the rear subframe mounts. On a 300D that's near and just inboard of the rear jack points. You can also support the car from the rear frame rails (or whatever they really are called) near the rear sway bar mounts.

Disclaimer: This is dangerous work and you could be killed if the car falls off the jack stands. Have to mention that.

Ken300D

Please Help · #8 11-01-2003, 01:19 PM

Ken,

Thanks. I've got a flat bed on the way. Going to have it towed from Warrenton to Luxus in Manassas. I wish I had the time to do it myself. Thanks for letting me know about the seriousness of the collapse. I feel a lot better just knowing that. I've had this car barely a year and have put >3k into it and put <3k miles on it.

Andy