Help...Brake Pad Light Stays On... - PeachParts Mercedes-Benz Forum

lindajane · #1 03-10-2008, 06:45 PM

Hi,
I have an 83 Diesel 300D. Lately, the brake pad light is Staying on!...
This would be the 3rd light from the left, the circle with the spokes..Not the brake light.
I took the car to the mechanic and he said he could not find anything, a leak, etc. the brake fluid is topped off, even above the line. He said he checked the brake pads...He charged me $200 to tell me this so don't want to take it back to him.
Usually the light comes on as soon as I start the car, and stays on. BUT for some reason, the light did not come on immediately, but came on after a couple minutes when I drove it yesterday.
It seemed to star when there was a heavy rain, and my car was hit hard with alot of water when a truck pulled in front of me on the freeway a few weeks ago.
I'm hesitant to drive very far when I don't know whats going on.
Any suggestions?
Thanks very much,
Lindajane

Ara T. · #2 03-10-2008, 06:49 PM

Either your brake pads are wearing down, or the brake pad wear sensors are bad (the wires usually tear). Or both.

toomany MBZ · #3 03-10-2008, 06:55 PM

I don't blame you for not wanting to take it back to the rip off artist you've been to. $200 for not being able to diagnose your problem. That light is indicating your front pads have reached their limit, time for new pads, and measure the rotors while you're there. This information is in the owners manual, if you still have it.

mobetta · #4 03-11-2008, 12:50 PM

orone of the wires to the sensors is broken and grounding out.

I'd take it back and demand a brake job for the $200 he already charged you for.

at least post up the shop so no one else goes there.

patbob · #5 03-11-2008, 04:40 PM

Don't take it back to that clueless mechanic. That indicator is telling you that the brake sense wires, or something in their circuit, is being grounded out. The nominal cause is that the pads have worn down to the point that the sense wire down in the pads is now rubbing on the rotor and grounding the system out to turn on the light -- this is what is supposed to occur.

Other causes are that the system got wet and is grounding out through the wet path, which is probably what happened to you since it started just after going through a puddle.

Check the sense wires to make sure they haven't worn through their insulation by rubbing on something. Check that the sense wires, and the wires they plug into, don't have a break in the insulation. If none of that shows any problems, buy a set of 4 new sense wires and replace them all.

There's a little rubber cap on the end of the wires, and if you pull them out of the hole in the pads, the cap can come off. Once it happens, it is then possible for water to seep down the hole and cause a conduction path through the pad to the caliper/rotor, thus illuminating the little light. This exact thing happend to me after I forded a local creek a few years back.

Palangi · #6 03-11-2008, 09:11 PM

The harness for the brake pad sensor wires can get worn through and cause a short at the point where it goes through the fender on each side.

daw_two · #7 03-12-2008, 12:22 AM

This thread is quite timely.....Coda is having a similiar issue. When braking, sometimes the brake pad light (is that what it is?) will come on and sometimes it won't. I looked in the owner's manual last night and while the warning lights are described.....I couldn't find a picture. It was late, were the pages stuck together?

So, mine must be a short........maybe???

RichardLSU · #8 03-12-2008, 12:36 AM

When you change your brake pads you need to change both sensors on each front caliper as well. There is a little plastic piece that fits down into your brake pad and when the pad wears down and through the plastic piece there are small wires that make contact with the rotor itself and complete a circuit, turning on that light on your dash. Indicating that your pads need changing.

Yours could be bent or frayed and rubbing against the caliper the entire time your driving. In either case, replace them and your pads and your problem should go away.

Its also a good idea to replace your rotors as they can't be machined down, only replaced.

turbobenz · #9 03-12-2008, 02:31 AM

the wire grounds out on the rim, stick your hand in there ans bend them toward the caliper

tangofox007 · #10 03-12-2008, 09:52 AM

Quote:

Originally Posted by RichardLSU

Its also a good idea to replace your rotors as they can't be machined down, only replaced.

Have reference for that?

vstech · #11 03-12-2008, 09:57 AM

Quote:

Originally Posted by tangofox007

Have reference for that?

ok, lets let the ones who believe it can't be machined believe it, and the ones who believe it can believe that. as long as the rotor is above min limits, I don't care how it is maintained.
for the record. I replace the rotors when they wear down.

Jordan G · #12 03-12-2008, 10:00 AM

Quote:

Originally Posted by patbob

Don't take it back to that clueless mechanic. That indicator is telling you that the brake sense wires, or something in their circuit, is being grounded out. The nominal cause is that the pads have worn down to the point that the sense wire down in the pads is now rubbing on the rotor and grounding the system out to turn on the light -- this is what is supposed to occur.

Other causes are that the system got wet and is grounding out through the wet path, which is probably what happened to you since it started just after going through a puddle.

Check the sense wires to make sure they haven't worn through their insulation by rubbing on something. Check that the sense wires, and the wires they plug into, don't have a break in the insulation. If none of that shows any problems, buy a set of 4 new sense wires and replace them all.

There's a little rubber cap on the end of the wires, and if you pull them out of the hole in the pads, the cap can come off. Once it happens, it is then possible for water to seep down the hole and cause a conduction path through the pad to the caliper/rotor, thus illuminating the little light. This exact thing happend to me after I forded a local creek a few years back.

I've got nothing to add here - everybody is right on - pads or sensor shorting out. I do enjoy the rotor debate too.

Patbob - you forded a stream in your 300D? Did you want to?

#13 03-12-2008, 10:21 AM

Quote:

Originally Posted by vstech

ok, lets let the ones who believe it can't be machined believe it, and the ones who believe it can believe that. as long as the rotor is above min limits, I don't care how it is maintained.
for the record. I replace the rotors when they wear down.

I don't know if there is a reference, but I've never seen any shop machine them. I doubt it would be cost effective anyway.

mobetta · #14 03-12-2008, 11:53 AM

Lets start another one- I got new rotors for $18/ea. cant beat that w/ a baseball bat.....

so far, so good........only about 7K on them yet......

back to topic- the sensor wires are on the front axle, 2 per side. they cost about $1/ea, and are easy to replace by only removing the wheel. I dont think any other tools are necessary, but maybe a needle nose pliers.
maybe someone else can chime in- seems like if they function as designed, the light will only come on when the brake pedal is pressed? I have yet to wear down front pads to that point.....

RichardLSU · #15 03-12-2008, 04:07 PM

where did you get your rotors for 18 dollars?

And yes, theoretically they could be machined down as long as they were within factory specs. However, like someone previously mentioned, its not cost effective when you can just get new rotors.