300E ABS Failure On Highway
 

Hello All,
This happend to be quite a close one for me on the highway yesterday. Traffic came to a complete stop around a bend forcing me to slam on the brakes, the wheels screeched, locked up, and the car slid sideways. I let up off the brakes and countersteered into the brakedown lane, pampering the pedal while nearly missing the car in front of me. I suppose that I should have addressed this some time ago as the ABS dashlight has been on for over a year, yet all it takes is one quick stop!

I understand that a faulty OVP relay can cause the ABS light to go on but will it render the ABS system completley useless? Could this be a faulty ABS relay (Located under the black plastic cover on the ABS module) ? Any input would be greatly appreciated, this experince nearly scared the crap out of me, thank god I had my Intrax springs in for the twisties!!

If the ABS light was ON when this happened, the ABS system wasn't operational at the time. If not, the first place I'd look would be the wiring to the sensors or to the sensors themselves (in the front spindles on your car, probably).

Irresponsible. Sorry - I know this isn't OD, but somebody had to say it.

As far as I understand something as stupid as a faulty OVP relay could trigger the light. I'll accept not being responsible but that does not help me. In the W124 world OVP relay failures are overly common, even from just a jumpstart. I would like to think that the system would still function in the instance of an OVP relay failure? My sensor wiring is fine, I replaced my OVP relay, and I'm not made out of money :D so I'm basically asking if there is any way to troubleshoot to avoid "throwing parts at it". Any other suggestions are more than welcome :o.

Agree, thank god my Intrax's made up for my lack of responsibility this time no next time. ;)

When the light is ON the system is disabled. No way around that...it's as simple as that.

The only way to fix it without throwing parts at it is to diagnose the problem thoroughly. I'd be checking fuses, wheel speed sensors, and the relays underneath the plastic cover on the ABS unit for starters.

If the ABS light comes on before moving the car, it's OVP or a complete failure (open or shorted wire most likely) of a wheel/diff speed sensor.

If the ABS light comes on when you hit ~6mph, the pump/system is failing the self-test cycle.

If the ABS light comes on at different speeds and times, almost certainly a wheel/diff speed sensor, the most common being a dirty tone-ring on a front wheel.

The sensors can be measured with an ohmmeter.

Good luck!

Great, thank you all for clearing up some of my misconceptions. Next, I will be thoroughly checking the speed sensor wiring for shorts or opens. I may have bought a faulty OVP so that will remain suspect. I have read somewhere that a diode between the ABS relay contacts of 30 and 30C goes faulty, will check that.

What are good baseline resistance measurements for working sensors?

Thanks again fellas!