[W124] ABS query

[Steve Nervig]

On Sep 23, 2006, at 4:23 AM, Frank Smyth wrote:

> Hi folks
>
> Been tracing the wiring on my '92 230CE to see why the ABS warning  
> light comes on sometimes (when driving fast, especially in the wet;  
> although it has been coming on more frequently recently). It all  
> looks okay, and the connectors are sound. Also check the relays  
> with an ohm-meter and they appear okay off the vehicle
>
> All of the online guides to ABS I found (generic, not MB-specific)  
> say that the next step is to read the trouble-codes from the ABS'  
> ECU. This will report "voltage low" problems and such like
>
> Can that be done on the Benz ? Would hate to just start throwing  
> parts at the problem (relays, sensors, and then the big-money-stuff)
>
> Thanks
>
> Frank
> _______________________________________________

First thing I would check is the over voltage protection relay.  
Making sure that the OVP fuse is good (replace) is a start. An  
intermittent fault suggests the OVP.

I would then check for nicks, cuts, corrosion, etc on all of the  
cabling and connections, as much as possible. I think there is a  
terminal block under the rear seat. The wiring from the OVP relay.  
Cabling to wheel-speed sensors. Connectors on all wiring.

ABS without ASR uses 3 sensors; one on each front rotor and one on  
the rear drive pinion. The ASR system utilises an additional sensor  
in the rear, as it has sensors on both left and right axles. I would  
make sure that each sensor itself, is clean. They, and the trigger  
wheel they ride next to,  get packed with crud and or metal filings.  
Pull each sensor and make sure they are clean.

I was talking to my indy the other day, as I had had my ASR go into  
limp mode. The ASR system uses the same wheel-speed sensors that the  
ABS uses. He said that some cars came with thin wires to the sensors,  
and these seemed more prone to failure or spurious signals. MB  
improved the design with a thicker, coaxial cable, that shielded the  
wire better and protected it better from nicks and cuts, etc. He said  
that the later design rarely gave the problems of the earlier.

Hopefully, you can find the fault is one of the above ancillary  
components, rather than in the ABS control unit or hydraulic unit.

Regards,
Steve
1992 300CE Sportline