126 Rear End "Clunk"

Hi! I'm thinking of buying an '82 300SD. Drove it yesterday, and the only thing I'm concerned about is a loud "Clunk" noise from the rear end when slowing down--possibly on downshift from 2-1? The owner says it's been doing it for a year since he bought it about 20K miles ago... Is this something common to these cars, or is this something I should be concerned about? Any help would be greatly appreciated. (Car seems to be in great shape otherwise.) Thanks
Phil

[stevebfl]

It is probably a vacuum shift control problem. If it is, then I wouldn't worry about it. It probably can do it forever. As to whether to buy the car, the early Turbo-diesels are notorious for poor shifting. We usually make adjustments at least every 30k on these systems.

If you can find someone knowledgable on that trans, determining if this is the cause is simple. Fixing it and not causing new problems is not necessarily simple or achievable.

[MikeTangas]

Following Steve's advice on a possible vacuum shift problem, find a knowledgable MB Tech who knows this tranny and have him do a pre-purchase inspection. May cost $100.00 or so, but he will be able to give you a good idea of the cars condition and possible problem areas.

[longston]

Steve,

Is there any chance that this could be a U-Joint going out? My 300SD has always shifted "hard" and does seem to smooth out somewhat after it warms up. I just had a thorough going over by a local independent garage, and they didn't seem to find that there was anything wrong with the trans, nor did they think that the shifting on my car was unusual.

Phillip,

I also recommend the pre-sales check. Mine cost me 1 hr, @ $78.00, and the owner of the garage was willing to "absorb" the charges if I had a certain amount of the work done by them that they suggested was required. You might also ask the current owner if he would split the cost with you. Or you might be able to work out a deal where you will pay for the inspection if you decide to buy the car, and he pays if you do not. Understanding that your decision would be based upon the outcome of the inspection.

The results of my inspection were quite surprising. To me, the car seemed to be in outstanding condition, but I was amazed what a professional MB tech can find that even someone with a reasonable amount of experience wouldn't. That's why we're so lucky to have access to Steve and the other pros on this forum!

[stevebfl]

Scott,

The key to the vacuum shift "klunk" is that it occurs while slowing with ones foot off the accelerator. The problem is really independent of other play in the drivetrain.

The problem occurs because the vacuum for one reason or other drops and shifts are made under pressures that normally should only occur at full throttle. It can easily be diagnosed by monitoring the vacuum to the modulator. With no throttle the vacuum should be above 10 inches AT LEAST. Properly functioning it will go over 20 inches. The shift becomes objectionable at vacuums levels below 5 inches.

Actual play in the drivetrain is more likely to be felt when changing driveshaft rotation: reverse to drive. Usually a bad u-joint will become a vibration long before it becomes a "klunk".

[longston]

Steve,

Excellent! I see why you get "the big bucks"

As you mentioned, I usually look for U-joint problems by changing from drive to reverse and back again at a dead stop with the brakes on and listening for the clunk. But I thought Phillip's clunk might have been a U-joint because of it occuring "on downshift from 2-1".I had a car once where that was the problem, and the same symptoms were evident.

[toms]

Could also be the flex disc.