Hey all. Need your help please. Wifey reported that the ABS light came on my family's grocery-getter (a very clean and tight '97 C280). Went out and checked. Here are the symptoms. Maybe you can help me fast-track the problem:

* Key in. Foot on brake pedal.

* Key to position #2 (pre-start). All idiot lights come ON.

* Key to position #3. Engine starts instantly. All dash lights go OUT, including ABS.

* Foot OFF the brake pedal. ABS light comes ON. (Note: if I start the engine with foot OFF brake pedal, ABS light stays ON.)

* Checked battery connections (disconnected for 10 minutes), then brake fluid, brake lights, fuses, and inspected (but did not replace) front wheel sensors, brake pedal switch, rear differential speed sensor, multi-pin connectors in trunk. Unplug, inspect, plug back in. All look clean and tight.

* Drive the car. Everything works ... except and CRUISE CONTROL. That has to be a clue.

* Note: the same day Wifey also says the driver's seat belt chime/warning isn't working -- the one that warns you if you drive without being buckled in. Is there a connection? Hard to see the relevance.

Any ideas? Much obliged if you can share them. Thanks a million.

 

PS: From what I have learned, the common culprits are:

1 - rear diff speed sensor
2 - front wheel speed sensor
3 - brake pedal switch

The first two are straightforward, but if the brake lights work, is the brake pedal switch okay? Seems a major hassle getting the old one out. The hood release gave me a lot of trouble.

 

I would go with the brake light switch. Just because the brake lights work does not mean the switch is OK. It has several functions and just because one works does not mean the switch is OK. As for the seat belt chime/lignt I do not believe it is related and could be that the Pass seat weight sensor needs attention or recallibration.

 

Follow-up: You were right. I had the codes read by my local MBZ wrench. Apparently, the culprit is the brake light switch. Ugh, looks like a PITA to get to in the foot-well. And time I buy an OBDII reader!

 

Hi. As mentioned, the fault codes stated brake switch. So I had it replaced. The ABS light went out permanently, but I just found that the cruise control still doesn't work. Any ideas? Thanks.

 

If you used an aftermarket brake light switch that still could be the problem, If not and a MB switch was used then a fault could still be in the computer and thats causing the problem. Have the faults read out and find out what the cause is.

 

Hi. Thought you may be interested in / able to lend advice on my ongoing problem: an intermittent ABS light in my '97 C280:

Action 1: Two weeks ago I had a MBZ mechanic replace the brake switch. Actually, twice. First one was faulty! It got stuck in the "on" position. Drove it like that for 30 miles or more.

Result: Although the ABS is off when I start the car, it will comes back on a few miles later. It's not a repeatable problem, but sometimes it comes on after a stop sign/right or left turn.

Action 2: Last week I had the error codes read by the same MBZ mechanic:
- ABS: "C-1200 stop switch"
- Engine: "P1584 stop switch"

What? It's a new swtich in there!? The mechanic checked the switch and said it's working fine. Note: Since the first stop switch had been stuck "on" for a while, I changed all the brake light bulbs because of the excessive heat (one was black inside). Just a precaution in case a bad bulb can have an effect on the ABS system (a stretch, I know).

Result: Drove it all week ... same problem exactly. Looks good for a while, then it comes on. Typically while going slow and turning sharply. IS THIS A CLUE?

Action 3: Today, I changed both front ABS wheel speed sensors.

Result: Oddly, the light was on when I started the car this time. It went out after about 100 yards of driving. Then, as usual, a few miles later, while navigating the neighborhood streets, it back came on.

The question: Should I have reset the system when I changed the sensors? Do I keep moving, changing the REAR sensor next?

Thanks for any advice.

 

There are two different brake light switches- both look the same- one of them burns out almost immediately if you put it in the wrong car. Sorry, I don't have the part numbers. I've been burned by this myself when the parts guy is too lazy to look it up and just goes and gets you the switch. And god knows if you got an aftermarket one!!

 

Thanks for the tip, MB Tech!

This is starting to make my head spin. I know these things can happen (faulty new parts) because I remember a 911 Porsche that had a new starter put in, then it wouldn't start. It'd spin, but the engine would not catch. After HOURS of searching why, it was determined that the starter's windings were REVERSED! Who would have thunk it!

Anyway, I guess I need to get down in the foot-well to read the PN on the switch? Oh joy. Getting access through that panel is a real pain in the butt. Maybe there's a trick, but I can't see it.

 

ABS light problem is finally fixed. Here's a review of my findings.

First Mercedes "specialist":

- First new brake switch installed was faulty. It failed thirty miles from home. I drove back not knowing my brake lights had been on the entire time (remember this part).

- Second new brake switch installed. Seemed to fix the problem -- for a while. Then the ABS light came on again. Same basic problem as before. Codes read "brake light switch." Weeks later I started getting a bulb warning light. Replacing the brake light bulbs turned it off, but the problem would surface in a few days.

The shop showed no real interest in helping further. They said: "Make an appointment next Thursday. We'll have to keep it for a few days -- maybe we can find the problem. Probably an ABS control unit."

New Mercedes specialist -- a real one:

- Called a new mechanic. "Hans" asked for me to come over immediately. Took the time and interest in the car. Spent an hour right then to read the codes, drive car, re-read the codes. It came up every time "Brake light switch." He put in a third new switch. I also mentioned the brakes lights had stuck on for thirty miles. We inspected the rear multi bulb housings very closely. They appear to have slightly warped/burned areas where the bulbs are held. Possibly this now causes a high resistance situation, hence a bulb out warning when there isn't one. We replaced both units and installed all new bulbs.

End of problem! I have driven the car for about 150 miles and all lights stay out.

The problem got complicated when the fist shop installed a questionably "new" first switch. I say this because they showed it to me before they put it in. The bag had already been opened. My sense told me it's not 100% new. It was just a hunch, but look at the result. Keep this in mind when your ABS light goes on and you think replacing a suspected part with new instantly solves the problem.