Brakes Pulsing
 

I had the front and rear rotors, F&R pads, and front calipers changed out on my 85 300TD. Unfortunately, I put the wrong pads on the front. I used the thicker pads for Bendix calipers and I had ATE calipers on the front. This caused the front brakes to drag. I immediately figured out my mistake and got the correct pads put on the front. The dragging stopped but I had a definite pulsing as I came to a stop. So....I figured I had warped the new front rotors and ordered another set. Had them put on this weekend and it did get rid of 75% of the pulsing but it is still there enough to be annoying. Any ideas on what to look at next? I hate throwing parts at it. The rears should be fine since I put new rotors and pads on and they were the proper type.
Frustrating as everything worked fine before I had it "fixed" :o

Thanks
John

More rotors.

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They are brand new rotors all around> MB parts also...not aftermarket

Then drive for awhile on the warped new ones until the pads are worn a bit more. Otherwise swap them out now with the correct pads this time.

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Oh I see the pads are correct now. In my experience all it takes to warp the fronts is a good hard stop for a red light change. Sitting there for a few minutes seems to cause enough differential cooling under the pad to warp them.

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1) Suggest removing the front hub/rotor assemblies, unbolting the rotors from the hubs, thoroughly cleaning the mating surfaces of the hubs and rotors to insure that nothing is holding them apart, and reassemble.
2) Take the assemblies to a shop that has a brake rotor lathe, and have them trued.

I'm thinking the same thing. The tech did not wire brush the mating surface of the hub. Might be a little rust there that would make the rotor not seat perfectly?

The rotors certainly need to seat well on the hub so I'd check that. I have a cup-type wire brush in one of my HF 4 1/2" grinders and it works really well in that sort of situation. If you decide to do this, stand back as they'll throw wires especially when turned on for the first time. This is unpleasant or worse depending on where the wires decide to fly (wear eye protection!).

Also check for concentricity on all 4. If you don't have one buy or borrow a dial indicator and check both for alignment with the hub and (less important) roundness on the outside diameter on the rotor. These CAN be off in rotors fresh out of the box.

BTW - highly unlikely that your rotors came out of a Mercedes factory. Like all manufacturers they use any of a number of third-party suppliers (ATE, Bendix, Girling, etc. to name a few) so even though they came in a Tri-Star box they're likely the same as rotors from a number of other suppliers who put those rotors in THEIR boxes. My point is that although the manufacturer no doubt tries to make a good product there's no absolute guarantee that ANY replacement rotor is all well and groovy so if you run into an issue like you have you have to measure everything.

Dan

Dan
I'll give the wire wheel a try. I suspect that might be the problem. I agree about the rotors being made by someone other than MB. The fact that I have put 2 new sets on and still have the problem leads me to believe that the odds are against getting 2 sets of bad rotors and more likely an uneven mating surface.
John

Pulsation can be caused by 2 things:

Either the hub is rusty and gross and the rotor is not sitting correctly on the hub

Or the pads were not bedded in correctly.

The rotor can be removed and the hub can be scrubbed clean. If the pads were not bedded in correctly, the first high power stop you did heated them up and then welded the pad to the rotor. That left a lump of material that pulsates each time it comes around. The only fix is to turn the rotor or replace it.

Rotors don't "warp" as most people think. They either don't sit flush in a big way and wobble, they don't sit flush in a tiny way and start an uneven wear pattern that causes a wobble, or they get pads welded to them in a panic stop and are left with a lump. Those are the reasons for pulses.

Whenever I do a brake job, I clean the rotor of cosmoline, clean the hub of rust, break in the pads using sequentially higher speeds and never coming to a complete stop, and then I avoid holding the brakes after a panic stop. It's easy for me with my 5 speed since I brake till I'm just shy of my stopping point and then allow the car to roll slowly to a stop. That let's the pads cool off and prevents them from coming to a stop on the rotor and welding. I don't have any pulsations whatsoever.

Since you had the front hubs off how did you set the bearing play?

New rotors come with a waxy coating to prevent rusting in the box. You need to clean the surfaces with mineral spirits or brake cleaner before installing. If that's been done, then have them turned.

If car has abs you might want to think this through a little more. Just a thought. If not carry on.

Update
 

Well, I changed the front rotors again! 3rd set. The pulsing is gone except for just before coming to a complete stop under semi-hard braking, At about 10mph until stopped there is a definite pulsing. No pulsing during normal braking. I'm at a loss. I could change the pads again although these only have about 2000 miles on them. The car stopped perfectly before all this work. The 2nd set of rotors were probably warped by putting in the wrong pads, but I changed the pads to the correct ones. Still pulsing. So I changed the rotors but kept the pads....still pulsing when coming to a stop! I cant figure out why the pulsing could still be there. Could changing the pads again be a solution. Wouldn't the existing pads wear in to the new rotors and take off any high spots on the pads. The tech now says I need new hubs. The hubs were fine to begin with so I cant see how they would be bad now. Any suggestions? I hate to keep throwing parts at this.

Thanks

Does your car have ' ABS ' ?