Got new rotors - now the ABS kicks in all the time

After 200,000 miles, splurged for new rotors, pads, and a timing chain for the 420. The cars was running and braking fine.

Question: When I got the car back and did a test drive, the ABS system kicked in everytime I applied the brake a little harder than normal. Before the ABS kicked in only when I slammed on the brakes when I was on snow/ice, never would it kick in on dry pavement - never.

I called my Tech and he told me to bring it back in for probably some "Dust" got on the pads during installation and he has to clean the pads off. Does that sound right?

He also said that you don't bleed the brakes on Mercedes unless you absolutely have to. I was curious because the pedal was not that much firmer after the new rotors.

I called a dealer about the ABS kicking in after new rotors and he thought that was something that would go away with time after the pads get better contact with mileage.

I hope to get the car back today. Was wondering what you thought?

[dpetryk]

Sounds like a line of s***. Dust on the pads has no effect on the ABS system. The ABS system uses magnetic pickups on the rotors to sense wheel motion. I would suspect that the problem is related to the pickups somehow.

I cant understand the statement "don't bleed the brakes on Mercedes unless you absolutely have to". However, the pedal firmness should not be any different before and after the new pads assuming that there was no air in the brake system to start with. It is not resonable to expect the pedal to more firm after a brake job assuming everything was working OK before and you just needed new pads.

I dont understand why the dealer would think that the problem will just go away with time. The ABS system is not sensitive to the condition of the pads or how well they contact the rotors. ABS only reacts to the slippage of one or more wheels in relation to the others.

Personally I dont buy what these people are telling you.

[Ali Al-Chalabi]

I don't buy it either, I don't understand how pad cleaning could possibly affect the ABS.

The pedal should also feel no different. If the brake lines were not disconnected, then there would be no reason to bleed the system.

[Jim Anderson]

Maybe some metal dust got near the sensors and is confusing them. A good cleaning, even with compressed air to blow out any dust may help and won't hurt.

The new magnets may be stronger too. Is there an adjustment on the pickups?

[sixto]

More likely they didn't properly attach the ABS wheel sensor/s or tweaked the wiring. Strange because I don't think you have to touch the sensor or wiring when pulling the hubs to change rotors.

FWIW, the ABS wheel is part of the hub, not the rotor on a W126.

Sixto
91 300SE
87 300SDL
81 300SD

[vinamg]

Your mechanic probably damaged the speed sensor wiring. I have seen this many times. The wires get very brittle with age and constant shock movement. You have to be extremely careful when removing caliper not to move it around too much and to place it correctly. I have seen them placing it at the top of the hub while smashing the speed sensor wire and then trying to compress the pads. In your case only to remove rotors since the pads can be changed with the caliper in place. BTW, you always need to bleed them; you should never compress the oil back into the system. This can also cause damage to the system.
Have a tech check your speed sensor resistance while tugging on the wire.


Vin

Let me know if you found the cause of the ABS engaging on dry pavement. I too, changed rotors and pads, in this case to aftermarket to reduce break dust and found that the brakes would begin to squeal and engage the ABS at slow speeds after periods of stop-n-go traffic. I lived with it for about 6 months until I decided to go back to MB OEM pads. I have 3 times the dust, but still experience the ABS problem, although its more random. It can even happen while driving slowly first thing in the morning. It is always accompanied by a very loud squeal. Also, the brakes don't squeal everytime, just randomly.

I wondered if it had something to do with the combination of the ABS sensors reacting to the vibration when the brakes squeal. I haven't returned to MB yet. Any thoughts ?

I just back the car to my tech and asked to re-look at it. If I remember correctly, it had to do with not installing the new sensors properly, i.e., dust on them during installation, loose, etc.

In the last several months it only did it about once or twice and the ABS light comes on about once/month, but nothing like before. I think this time the ABS light comes on due to the low voltage of having all the electronics on now and then..

[kowached]

I don't know why anyone would say that, especially a mechanic. Brake fluid is "Hygroscopic" meaning that it absorbs moisture over time. Most manufacturers agree that every 2 years you should change your brake fluid.

On another note, Coolant flushing recommendations seem to vary from every 2-5 years.

[csnow]

In response to Addicted's problem:

As you discovered, the pads are not related to the ABS problem you describe.

These are some possible sources:
1) Dirty wheel sensors- these are magnetic, and pick up rust fragments. (Easy to remove and clean).
2) Dirty ABS 'sprocket'. These can get fouled with grease (sometimes when the hubs are overpacked) or misc crud. Can be observed through the sensor hole, and cleaned with compressed air or brake cleaner.
3) Excess play in hub- this can alter the clearance between the sensor and the sprocket. In really bad cases, the sensor can hit the sprocket.
4) Damaged wiring to the wheel sensor.
5) Failing wheel sensor.
6) Warped rotors, though there would be other symptoms with this, such as vibration when braking.

I would start with 1 and 2.

[stevenstevensteven]

These are some possible sources:
1) Dirty wheel sensors- these are magnetic, and pick up rust fragments. (Easy to remove and clean).
2) Dirty ABS 'sprocket'. These can get fouled with grease (sometimes when the hubs are overpacked) or misc crud. Can be observed through the sensor hole, and cleaned with compressed air or brake cleaner.

Where does one find these sensors on the 124?

[csnow]

Wheel speed sensors are mounted inboard of the caliper, and 'poke through' the splash shield.
They can be identified further by the 'thicker' black electrical cable that is attached.
Only 2 electrical cables run to the hub assembly.
One is the pad wear sensor, which goes to the caliper.
The ABS sensor is attached to the other cable...

Hope that helps. A picture would be more helpful, if I had one...