Flickering brake-wear light.

[dieseldude]

It's about time for some news pads on the E420 ('95) as the brake pad wear light came on about a week ago. Already got the parts ordered and should be set to go in a couple of days.

One question: the light flickers off and on (sometimes very faint) even when just driving around with no pressure being applied to the brake pedal. How sensitive are these sensors? Does the flickering indicate a sticky caplier, or warped rotor perhaps?

Braking performance is sweet. Nice and linear, no shuddering or other associated shaking is discernable.

Thanks all!

Regards,
- Ryan

[donwo]

I just experienced the same problem with my 300E. Light would flicker on and off. After inspecting the pads I decided to replace them. The pads I removed still had lining on them but light was indicating it was time to replace them. After replacing the pads and sensors problem was solved. Pad replacement was very easy to do. I don't think the set up is the same for the E320 but I'm sure you can follow the instructions on this site.

[J.HIDALGO]

They do go bad plus they are cheap!

[A. Rosich]

The flickering on the brake pad light sensor is due to the extremely basic and rudimentary design of the pad sensors:

A simple copper wire with some insulation is fitted in a hole into the brake pads. When the brake pads reach its limit as they are being consumed by the brake discs, the insulation of the copper wire is disolved. The copper wires are exposed very slowly, creating intermitent ground as they touch the pad or the disc.

That is why at the beginning the light comes on and off. As the wire consumes itself, is more exposed, making constant ground. By this time the brake pad warning light should remain constantly on.

A. Rosich
S320, 1998
E320T, 1995

[MikeTangas]

I had a flickering brake wear lamp too. Found when I changed the pads that one pad had worn enough to expose the sensor end slightly. The other three still had a hair or two of material left leaving the sensors fully covered.

I was surprised at the amount of pad left once the sensor made contact. Nice feature, no reason to ever get metal to metal.

[dieseldude]

Thanks A. Roisch! That was just the info I was looking for. I figured there wasn't much to the 'sensing mechansim' but wanted to make sure. I noticed that if I applied the brake, even somewhat hard, at freeway speeds, the light wouldn't come on at all (I'm assuming from enough airflow to keep them cool), whereas coming to a 'hard' stop at a light, or braking due to an inattentive driver ahead of me, the light would come on bright and constant.

Yeah J. Hidalgo, I did get the new sensors, I've heard they can be replaced but the one's on my 123's were so brittle (or melted) by the time they were due for changing, they didn't come off in one piece

Regards,
- Ryan

[A. Rosich]

The way the bristles make contact with the brake disc is very opportunistic, to say the least.

You will notice, as you say, that if you slam the brakes hard on the highway the light will go off (when it started just flickering). I don't know why it does this (maybe to much friction cannot ground the wires properly).

Other times, when the brakes are wet, the wires become more sensible (due to the water making ground) and the brake pad light will come on more often, or stay on constantly. Once the brakes are dry, the attitude changes.

Anyway, it is interesting to see how simple and logic the sensor systems is layed out. It makes you wonder why not everyone else installs brake pad wear sensors like these.

If you change your brake pads once the light starts flickering, you still have good life left on your pads, according to the local M-B dealer. Once you first see the brake pad light go on, you usually still have about 1,000 kms. of life on the pads (it all depends on your driving habits and terrain conditions).

A. Rosich
S320, 1998
E320T, 1995

[G-Benz]

I too, replaced my pads around the time the light started flickering during braking. Noticed that there was still just under 1/2 of good meat left on the pads, but what the hey!? Cheap and easy job, and another several thousand miles of sure-footed braking ahead!

[Ken300D]

Phases in the Life of a Brake Pad Wear Sensor:

Phase I - Lots of pad between sensor and rotor.

Phase II - Sensor barely makes contact with rotor under firm braking. Brake light does not flicker when running along, but will come on under firm braking.

Phase III - Enough pad material has worn now that part of the sensor is wearing away too. Brake light will flicker running along.

Phase IV - Sensor is worn completely away. Brake light does not come on at any time.

Beware: Do not mistake Phase IV for your brakes having had a miracle recovery!



Ken300D

[blackmercedes]

Originally posted by Ken300D
...

Beware: Do not mistake Phase IV for your brakes having had a miracle recovery!



Ken300D

LMAO!! I needed that!

(And it's good advice, too)

[mossimmo]

HELP,

I replaced all my brake pads and rotors including the front copper brake wear indicator wire on my 1993 300E. Last week I noticed the brake indicator warning light was flickering, what gives ? All the parts are brand new, I haven't pulled the wheel yet, has anyone experienced this problem?

Thanks,
Ed

[MikeTangas]

Check the routing of your sensor leads. If routed incorrectly the lead could be making contact with the spinning rotor. This will expose the wire, which will make contact with the metal of the rotor and give you a lamp.

[glenmore]

mossimmo,

Hah! Exact same thing happened to me and in my case the fix was obvious. I neglected to tuck one of the sensor wires under the spring clip and the rotating wheel shorted out the sensor. There is NOT much clearance there!

glenmore
1991 300CE