PeachParts Mercedes-Benz Forum - Let the clues guide you!

My 91 SL has about 156,000 miles on the clock, so I accept the fact that things will break, and probably more expensive items as time goes by.

I'm driving on Sunday night on a 280 mile trip. Top down, 75 degrees out, beautiful. Decided to stop at a DQ for a vanilla shake. I get back on the highway, and a while later I notice a fairly annoying whirring. Coming from the rear. So I pull over, and touch the wheels thinking I may have a wheel bearing going south. The wheels were not too hot at all.

I get back on the road, and a while later the noise comes back, even louder than before. So I'm thinking crap, I have a bearing going out in the rear end. I pull off the road, but didn't smell the tell tail smell of hot gear oil, and the pumpkin didn't feel too hot. Back on the road I go. No matter what speed I drove, the noise was there. And it was not only annoying because it sounded like big money, but also an irritating whirring noise.

Another clue was one time, on flat highway, with the cruise on, the trans shifted into a lower gear, as if I were accelerating. So I am thinking whatever bearing is failing in the pumpkin is going to lock up real soon.

This am, I go to work, and the noise is there still.

So, I came home from work, jacked up the car, put her in neutral, and spun the wheels looking to see if there was any noise - none. Checked back lash by rotating the wheels - felt normal. The pumpkin had a oily coating, but it was just from being 15 years old. Pulled the fill plug on the pumpkin to see if it had enough oil - it was fine, and the oil smelled great and looked very clean.

Now I am stumped. Then I look at the driver's side rear wheel. And it dawns on me that for the last two or three weeks, as I was cleaning my car (at least twice a week), the left rear wheel had been really dirty lately. Now I was on to something. Pulled the wheel again and looked at the brakes.

Turns out the outer piston is stuck very hard. I can move it slightly with a c-clamp, but that's the noise, and the cause of the dirty wheel! The outer piston was stuck, causing the pad to barely ride on the rotor. When the brakes were applied, the outer piston could probably move just a bit, enough to have the brake make contact, but then not retract. The inner pad did most of the braking. All I need is some pads and a rebuilt caliper. I checked the other side, and there is at least twice as much pad left on both pads.

So, if you notice one of your wheels getting soiled much more than the others - look at the brakes!