W123 dash brake warning lights, a review - PeachParts Mercedes-Benz Forum

dmorrison · #1 02-14-2008, 12:24 AM

Lets review the W123 brake light warning system.

In the dash, there are 2 warning lights associated with the brake system The first and most important is the Red " BRAKE " light in the dash. ( picture #1 ). This light has 2 functions. It informs you that the parking brake is engaged and more importantly it informs you that the hydraulic brake system reservoir fluid is at a low level.
The system uses a wire schematic that you can see in picture #2. From the #12 fuse, the circuit goes to the warning light in the instrument cluster. From there it travels via cables to both circuits in the brake fluid reservoir and then to ground G102 which is located behind the instrument cluster . The black buttons at the top of the reservoir are test buttons for the floats in the reservoir.

"Reseting" them as is sometimes posted, is not quite what you are doing when you press the button. If the light is stuck on and is caused by the float. When you "reset" it you are trying to disconnect the float from the lower "on" contacts. This is whats causing the false on indication. The float ( picture # 3 ) may be getting old and not move up and down freely. The older cars have a single chamber system and only 1 float and test button on top of the reservoir. Newer models have the dual chamber reservoir and 2 black buttons
The second circuit of the Red " BRAKE " light goes to the parking brake pedal assembly and lets you know when the barking brake pedal is in any position other that fully back ( parking brake off ) Picture #4 shows the Service Manual diagram of the switch location. To test the 3 circuits for a fault ( meaning your light is on ) you simply disconnect the wire harness going to the reservoir float or parking brake switch one at a time and check the light. If the light goes out, then it is that particular circuit. You can replace the parking brake switch but the reservoir switches cannot be replaced. A new reservoir is required. However the black rubber caps are available.
Of course first make sure that both reservoir sections are filled with brake fluid to the "MAX" line. To look at the rear chamber it is best to look on the passenger side of the reservoir towards the back of the reservoir. I usually wiggle the reservoir side to side, it makes it easier to see the fluid level change.

A second light in the dash is for the FRONT brake pad wear warning system ( picture # 1). As posted, the wear sensors are in the front pads only. 2 wires per wheel, one in each pad. The electrical circuit ( picture #5 ) goes from the #12 fuse, to the instrument cluster power line, through the warning bulb to the 4 sets of sensors. Then to the G102 ground for the left front hand wheel and the G101 ground for the right front wheel. The G101 ground is located RH front fender behind the headlights. Interestingly enough, the wear sensor tells you that your pads are worn out by grounding the circuit ( actually completing the circuit) when the plastic cover of the wear probe sensor is ground away and the exposed probe wire touches the rotor ( that is why it flickers initially ). I actually have not seen the wire for the wear sensor as it goes to either G101 or G102 as shown in the electrical schematic, go figure.
There is not a wear warning system for the rear pads or the parking brake shoes. You should check the rear pads for every 3 front pad replacements ( not a bad idea to check them every 2 sets of front pads ) The parking prake shoes should last quite a long time, I personally have never replaced a set of parking brake shoes in all the Mercedes I have owned (9 at last count). To prevent from replacing parking brake shoes you must keep the parking brakes in adjustment and not drive the car with the parking brake on.

I hope this helps anyone who has any questions about the dash brake lights.

Dave

JimmyL · #2 02-14-2008, 01:42 AM

Dang Dave's been helpful this last year or two!! Best thing about him is he adds pics......

Good info. Smart to do too.

Bill Wood · #3 02-21-2008, 12:22 PM

This DIY Article is now in the Wiki HERE.

Jordan G · #4 02-21-2008, 12:26 PM

Dave, I hope this doesn't come across the wrong way....but I love you man.

In all seriousness.....whenever I've been tackling a DIY job, I search for the Dmorrison version first......so many thanks for taking the time and effort to contribute your knowledge/experience.

I'll stop being gushy now - my brake light started flashing this morning - I just thought it was my non-functional e-brake gone awry....but now I'm going out to check my fluid levels.

lutzTD · #5 02-21-2008, 12:53 PM

i still had a strange one on the 84 300D I just sold. there was a white light just to the left of the seatbelt light and a buzzer when I started the car. I finally decided it was the seatbelt warning and the bulb was in the wrong place, does this sound right?

dmorrison · #6 02-21-2008, 02:05 PM

Quote:

Originally Posted by lutzTD

i still had a strange one on the 84 300D I just sold. there was a white light just to the left of the seatbelt light and a buzzer when I started the car. I finally decided it was the seatbelt warning and the bulb was in the wrong place, does this sound right?

Yes the holes can be used if you want. Or in your case someone put the bulb in the wrong hole. The non used holes are covered with black tape.

Maybe a good location for a low oil pressure light with a buzzer. I may look into that.

Dave

Orkrist2 · #7 03-24-2008, 09:39 PM

This is exactly what I was looking for. I just had the weirdest experience. I noticed my car smelled a little different today after some highway miles, it smelled like clutch or brake smell, you know? Its an auto, so I figured it was just hot. Anyway, I drive home and then leave home when its dark and all three warning lights are on.

The red brake warning light is on, the wear waring lightis on, and the battery light is on. They all fade at higher revs but are still very bright. Tomorrow AM I wil check the brake fluid level and top up if necessary. I probably need new front pads, too. One week ago, they gave me the squeak warning but it was also wet so I couldn't tell if it was water, rust or the pad warning squeak.

Anyway, that leaves me the battery light. I hope I don't need a new alternator, too. We'll see. Anyway, any more input on this please let me know.

Thanks

bodyart27 · #8 03-24-2008, 10:12 PM

Just to add on to the write up for illustration

Pic #1 - front caliper on 300sd showing white wear sensors wires inserted into pads

sensor part # 140 540 12 17

SmokersTees · #9 04-29-2008, 10:21 PM

Thanks, the picture helped a lot. Now I know what needs to be fixed before I take it to the shop.

compress ignite · #10 04-30-2008, 01:07 AM

bodyart27,

That shot of your brake is unreal...I know you do not drive that MB.
Too much chance of dust on that caliper!

Simpler=Better · #11 05-21-2008, 05:59 PM

Quote:

Originally Posted by Orkrist2

.....all three warning lights are on.

The red brake warning light is on, the wear waring light is on, and the battery light is on. They all fade at higher revs but are still very bright......

My car is doing EXACTLY the same thing; I'm going to tear it apart tonight and see if I can figure anything out

**Update**

I swapped out the front pads, flushed the reservoir with brake cleaner, and installed a new master cylinder. I still get the lights at idle, but they fade completely once I get past 1000rpm. Braking has improved however

durado · #12 11-06-2008, 11:29 PM

I've looked and looked and can't find the answer ... perhaps somebody here can help me out ...

I have an 83 300D Turbo (the 4 door not the wagon)

BOTH brake warning lights come on when I start the car (the pad wear indicator and the parking brake/brake fluid level light ... orange and red) ... both lights STAY ON until I move FORWARD for a little distance ... if I go in reverse they do not seem to go off (but I haven't backed up all that far when they've been on) my brake pads are new with new sensor wires ... HELP ...
Thanks
ps the brakes function fine ...

dmorrison · #13 01-07-2010, 01:11 AM

Well the post was 11-06-2008 ( it' now 1-06-2010 and I'm still coming back to the forum even though I have not really worked on the Mercedes for a couple of years. And Dorado has not posted since 2008). But the problem with all the lights coming on is usually the warning buzzer relay. All those lights go through the buzzer relay. I had this problem and replacing the relay fix it.

Dave

PS I am going to work on the SL and get it going now that I'm feeling better. I had Hypothyrodism and the medicine is really making me feel better.
The " auto shop" is being turned back into a wood shop.

JimSmith · #14 01-07-2010, 02:05 AM

Odd when that much time goes by and there are some long time unanswered questions in the thread. Like the two guys with failing alternators (or voltage regulators) who have multiple lights on until they get up to speed.

If you notice, when the car is electrically powered by the battery - engine not running - all the lights are lit with key turned to the run position. When the alternator and voltage regulator take over, the lights go out, unless there is a specific problem to report or, in the case of the high beam indicator, you turn the high beams on. When the brushes in the voltage regulator get worn they make relatively poor contact with the commutator (rotor) and add a higher resistance to the charging circuit. This reduces the voltage below the level where the dash lights want to come on because they "think" the engine is not running. Initially, at higher rpm the alternator puts out a higher voltage at the commutator and enough voltage will make it to the other side to dim the lights, or even shut them off. As the wear continues the lights come on brighter and don't fade out as rpms increase, until they stay fully lit all the time and your radio shuts off and the headlights fade out.

The point is they are not providing any indication of a brake or other problem described in the little icons that light up - it is a sign the regulator brushes are failing and that you should replace the brushes or install a new voltage regulator.

Jim

layback40 · #15 01-07-2010, 05:51 AM

Jim,
You know when you try & explain that to some, they just wont have it. How can the alt brushes cause the brake light to stay on ? But then when it fixes it, its just a co-incidence. I had an alt once that wore the rings right off. When the brushes were replaced it didnt fix the lights. Concluded it was time to junk the alt.