[W124] ABS query

[Frank Smyth]

Cheers for the ideas there! The cabling on my sensors appears to be big, thick coaxial.
 
Something strange just happened tonight, which points to an electrical problem: number 1 - the ABS light came on with the car idling and in Park. Number 2: the offside rear sidelamp failed. I thought it was simply the bulb, so replaced it with a new one. No go. Measured the voltage and watched as it dropped from 10v, 9v, 8v etc down to just under 1v. The sidelamp on the other side of the car is completely fine. 14+ volts there. Front lamps are okay. This is both with the engine running and not running. I have had the "bulb failure warning lit up since buying the car and am in the process of swapping out all the bulbs. Last night found that the "failed" rear bulb was 5w instead of 10w (despite it being physically same size) so thought that was the crux of the bulb-failure warning.
 
I curse the Haynes manual for not providing a proper wiring diagram - but any ideas why that single circuit might be facing a fall in volts ? Earth looks good back there. I'd love to see why the front and rear sidelamp circuits are separated. I wonder does this point to a failing OVP relay ? The 15A fuse on top of that is fine.
 
Not sure where to look here for OVP: http://www.kmsparts.com/more.asp?range=06&ModelLevel=MB133

________________________________

From: w124list-bounces at mbcoupes.com on behalf of Steve Nervig
Sent: Sat 23/09/2006 16:07
To: W124 CE Coupes and Convertibles
Subject: Re: [W124] ABS query




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


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