2003 S600 with 61k - after doing front pads and rotors pedal Is very soft. Goes almost to the floor o
initially, but after the first pump comes up and feels more normal. The inner pad on both wheels was shot, and the outer pads about 1/2 way. Don’t see how I could have gotten air into the system, but I assume the brakes need to be bled (I planned to use the 2-man pump method). Then, the next day, I took the car for a ride to see how the brakes were responding. After awhile there was a thumping noise that seemed to be coming from the front driver-side wheel. Frequency varies with the rotation speed, and I can feel it through the floor board. It disappears when I depress the brake pedal. Popped off the wheel and everything looks fine. Comments/suggestions welcome before the car goes to the mechanic.

 

No positive answer, but a couple of comments...

With engine not running, pump the brake pedal slowly several times. This should exhaust the reserved vacuum in the brake booster. Let the pedal sit in the fully-released position for a couple of minutes. Push the pedal firmly once, and hold it. Any softness indicates either air in the system or a soft brake hose. The pedal should be hard and near the top. Press and hold the pedal with moderate pressure while starting the engine - the pedal should drop and get softer, which shows that the brake booster is working.

The pistons in disk brake calipers should not move back and forth. When you apply the brakes, the piston should move only enough to distort the rubber seals so that the pads press against the rotor. When you release the brakes, the rubber should straight out which releases the pad pressure against the rotor - but the pads should still lightly touch the rotor. When the pads wear slightly, the rubber seals will distort as much as they can, and if that isn't enough, the pistons will slide out a tiny, tiny amount until the pads press against the rotor.

If the rotor is warped, it will knock the pistons in as it turns. The next time that you apply the brakes, the pistons will have to slide back out so the pads press on the rotor, causing a long and soft pedal. The brake pedal may or may not pulse while you are stopping.

If there is a "hard spot" in the rotor (usually caused by parking the car with very hot rotors) the rest of the rotor will slowly wear more than the hard spot. The difference in rotor thickness will gradually cause more and more pulsation in the brake pedal, and will eventually make a noticeable pulsation in the deceleration of the car.

If your calipers have pistons on both sides of the rotor, uneven pad wear usually indicates a sticking piston or pistons.

Replacing rotors is not a difficult job.

 

Wally - thanks for the suggestions. Note that I did replace the pads and rotors a day or two before the "thumping" started. I did follow the break in procedure for the new pads/rotors. The sound I am hearing sounds like a tire issue (I immediately thought I had a flat tire). This morning I replaced the wheel with the spare and the noise is still there. I will post again when I know more.

 

One additional test that you could make - swap the front rotors. If the noise follows the rotor, obviously bad rotor or bad installation.

While you have the rotor off, spin the hub to make certain that rotation is smooth and quiet, and that there is no free-play in the bearings.

Make certain that there is no foreign material in the rotor , and that the rotor is seated correctly on the hub, with no foreign material between.

 

IF the 'tire thumping' is NOT a tire (you moved them around and noise/feel did not change), another possibility is a spalled wheel bearing. I have experienced 1 (one) in my 60 years of working on cars. The one was on a 80" Toyota Celica with the 6cyl engine. The noise was right rear and started suddenly. After eliminating everything else, I pulled the axle with its sealed bearing. It was immediately obvious when rotating the bearing that it was the source. I took the bearing off and replaced it and the noise and vibration was gone. I took the bearing apart out of curiosity ( to remove the bearing I first hit the outside race with a 3# ball-pien hammer to break the outer race, removed the ***** and race fragments and hit the inner race until it cracked and came off. .I used my century+ old 100# anvil as backing when hitting the bearing races. The outer race had a 1/2" x 1/4" spall in the race on the bottom (from impact or rough road (only 220,000 miles on it).

 

Uh oh. This potentially just got a lot worse.

First - many thanks for the comments/suggestions on my post. I don’t think it’s a bearing issue. I’ve had a bad bearing on other cars, and it typically is more like a grinding noise. Besides, when I put on the spare to confirm it’s not a tire issue, I spun the wheel when it moved freely with no noise. I know it’s not under load, but I’m pretty sure I’d hear something if it was a bearing. I thought the idea of swapping the rotors was a good one, though it will have to wait until the weekend when I have more time.

But...

I went out tonight to stretch the legs of one of my corvettes, and noticed the 600 sitting down in the back, practically on the rear wheels. I’m no stranger to ABS issues, and sure enough, started the car and got the dreaded orange “ABC Failure - Visit Workshop”. About a month ago I had the motor mounts replaced and one of the ABC lines that was crushed when the mount(s) failed. I’m concerned that this is going to get expensive really fast.

Does anyone see a connection between the “thumping” I heard with the ABC warning I’m now getting? I don’t see how they are connected, as I’m pretty certain that the frequency of the thumping varied with the tire rotation speed. Thanks in advance for any/all comments.

 

The ABC hoses must be securely fastened or they CAN vibrate against the car frame/body and cause all kinds of noises. Check the oil level in the ABC reservoir.

 

I don’t think it’s a loose hose - it would have to be banging against something every time the wheel rotates, regardless of rotation speed. The frequency of the noise is consistent with how fast the wheel is spinning. Similar to putting a baseball card in your bicycle spokes when you were a kid.

when I out the spare on to see if it was a tire issue, I checked the ABC fluid level. It was just at the low level, so I did top it off. Not related to the wheel sound - but could overfilling throw the ABC warning light?

 

The level of ABC fluid will not cause 'wheel speed thumping,' A wheel speed thump you can feel is not to be ignored or driven on. SOMETHING is causing it. A failing tire belt or tread separation on a tire would. A large bolt or other object lodged in the tire tread is another possibility. A brake hose clipping a wheel balance weight will make a noise but unlikely to be a 'thump.' Am puzzled.

 

My first test would be to swap the brake rotors side to side.

 

Wally - I agree with you - and thanks for the suggestion. As per my earlier post swapping the rotors will have to wait until the weekend. Unfortunately the ABC "visit workshop" mssg will take priority; I need to understand/resolve that first. I don't see how they could be connected - so I am assuming they are not - but I don''t want to move the car at all with the ABC mssg in fear of damaging the pump. For the record, I have not been driving the car with the wheel "thiump" - other than to back it out of the driveway and drive the 40 ft or so forward to see if the thumping sound is still present after making a change (e.g. tire replacement). Again, thanks for the comments.

 

Did the car come back up when you started it? Also, when was the last time you flushed the ABC system? It may be time to do that now and put in some new filters on both the PS and the ABC side.

As for your brakes, I would take the rotor and caliper off on the suspect side and see if you find something wrong. You may have missed something the first time. Also, go ahead and bleed the system if you still haven't. Check the rubber seals around the pistons too. Good luck!

 

Re the ABC issue - I honestly don’t remember if it started to come up or not on its own. I instinctively pressure the raise button and it came up. The next day it was sitting down about 1/2 way in the back, I started it and it came up. No ABC malfunction, it it was only idling in the driveway. The ABC fluid was replaced about 1500 miles ago when the motor mounts and ABC line were replaced.

re the wheel thump - the more I think about this the more I think it has to be a tire. Although I was convinced it was the front driver side - I’m going to start swapping out the other wheels 1x1 with the spare to see if it goes away. Then swap the rotors. If no luck with either of these, it’s going to the mechanic. I’m also working on the stuck shifter issue but will post that separately.

I love this car, but sometimes...

 

Thanks to all who’ve replied to my post. After resolving my stuck shifter yesterday, I swapped out the pass front wheel w/ the spare and the thump I was hearing disappeared. It’s raining (again) so I’ll have to wait a day or so to bleed the brakes. The ABC malfunction has not returned (fingers crossed) so after new tires I’ll be onto the remaining nagging issues with my car.

Thanks again.