1979 Mercedes Shuddering while driving mystery

I bought a 1979 Mercedes 300SD Turbo Diesel. The car was in great shape and only had 45,000 miles on it, no rust and the engine runs strong. It did have a brakes problem.

The owner that I bought it from had been driving it around for some time (he never did tell me how long) with toasted brakes. I didn’t know they were toast at the time but the car was really cheap, so I bought it.

The big issue with the car is that when you reached around 50+ mph it would start to shake and shudder violently. So much so that it was very unsafe to drive. I could also smell burning brakes. The first thing I did was get 4 new tires on it, and an alignment. I suspected it needed much more then that so from there it was to a repair shop.

It turns out the previous driver had cooked the brakes. He had been driving around with a bad master cylinder, the shuddering was the brakes being applied on and off while driving. That explained the burning brakes smell (from the front) as well. I was a strapped for cash so they replaced the wheel bearings, rotors, pads and master cylinder. After that, it drove ok for about 40 miles or so, then the shudder started again, only not as bad as before.

I parked it in the back yard for the next two weeks. At that point I took it out again for about a two mile test drive to see if I could spot anything that set it off… about a mile into the drive, the brake light came on and I lost about 50% of my braking power. Parked it again.

About two weeks after that, I brought it to the repair shop again for round two (This time when I started it, the brake light didn’t come on, and the brakes behaved normally…???) . This time, they replaced the front rotors (and possibly another item but I can’t remember off the top of my head). It drove great for another 30-40 miles again and then the burning brakes smell came back. I could also feel a little bit of the shaking coming from the steering wheel again. I decided to have the tires balanced again. While there the burning smell was strong, the tire and brakes guys said it was probably just everything re-seating and not to worry about it to much.

All went well for the next little while. Several days went by where I would feel the vibration start, but then go away again before it became to bad. It was just kind of annoying. Then Friday, it started coming back like it was originally. I commute 70 miles each way for work.On the commute home Friday, it started the same old original shaking and shuddering again. It was about 25% of its original strength, it only lasted a mile or two, then vanished again. By the end of the long weekend, I was starting to think maybe I was over estimating it, so I drove it to work again. Same thing, once on the way it did its shuddering and shaking, stronger this time, it does it for a few miles then vanishes. Over lunch, I took it the dmv and the shuddering was just as bad as it was when I bought it. I got about 2 miles down the freeway and it started shaking violently once it was up to 60mph, the shaking comes back under control somewhat under 55mph, but the problem persisted for most of the 12-13 miles to the dmv. When I parked, there was the strong smell of burning brakes again. On the drive back to work, it behaved normally, although I didn’t go faster then 60mph. The shaking seems to start at the beginning of the commute, lasts for 10+ miles or so, then goes away.

Obviously, I will be driving it home and then parking it once more until I can afford another round of repairs. As it currently stands, I’m not sure what could be left to replace on the brakes. It stands at around $2200-$2300 of repairs so far that have not gotten rid of it.

I am completely mystified to what is going on? Any ideas would be really, really appreciated. Its incredibly frustrating and I am starting to feel like I am throwing a lot of money at a problem that just won’t go away.

Thanks and sorry about the long post.


Nobody has replaced the wheel cylinders? I’m bewildered!

I’ll bet luch that you’re fighting a sticky wheel caliper or two. I’ll bet that
(1) one of your rotors is again warped, and probably purple and/or glazed.
(2) if you were to lift each corner of the car seperately and spin the wheel by hand, perhaps having a friend pump the pedal too as a part of the exercise, you’d discover by the obvious drag at least one dragging shoe.

IMHO the odds are about 95% that you have a sticking brake caliper. Honestly, if you have the original calipers and this turns out to be the case, I’d consider changing them all. They’re OLD!.

As I usually do, I agree with mountainbike, but I want to add another bit of advice.
The flexible brake lines on this 33 year old car also need to be replaced!
Internal deterioration of these lines can lead to all sorts of phantom brake application that is often (incorrectly) attributed to other sources.

Truthfully, I would not ride in that car until every part of the brake system has been replaced.

Thanks for the reply!

I did have new calipers and rotors put on.

Last Friday when I had the tires balanced (again) the mechanic didn’t seem to think the new rotors looked bad. I saw the left rotor and it still looked new. I didn’t get a chance to see the right one. He thought the same thing you did and thought it might be a sticking caliper until he saw the new ones. Then he just figured they needed to seat themselves a little.

I forgot to mention, at times, it feels like the brake pedal is a little bit hard, just a little bit though, then it goes back to feeling ok.

Once you get the brakes working, then have the valves adjusted (I think it still needs it for this year), that can perk those old MB diesels right up.

The plot thickens!

Your 300SD has a vacuum brake booster (I looked it up). If this is true, and the booster is defective internally, it is possible for the vacuum to be pulling the rod that goes into the master cylinder while you drive, constantly and mildly applying the brakes while you drive. An intermittant failure could also cause your brakes to be occasionally hard.

Ask you shop to consider this as a possibility and test the booster.

Excellent advice on the brake line. I do not think that was done. I think that’s about the only thing that hasn’t been done on the brakes.

I am uncomfortable driving it home, unfortunately, I am 70 miles from home without any way to get back. I just plan to drive very slow in the right lane and pull over whenever I suspect or feel anything odd.

Excellent advice!!

Thanks for taking the time to reply to my posts. I have new hope now and feel a bit less like I am tossing money into the wind.

I will definitely bring up replacing the flexible brake lines and checking the vacuum brake booster.

Don’t “bring up” replacing the flex lines…You should just replace them ALL…This has probably been the problem since day one…