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  #1  
Old 09-08-2002, 11:29 AM
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C36: ABS light on, speedo dead

Yesterday while cruising normally, my ABS light went on and at the same time I noticed that the speedo and odometer were not working. I tried shutting off and starting 3 times but still not working. Brakes seemed fine so I continued on my trip with the ABS light on and no speedo.

This morning when I started the car everything was back to normal but later tonight the same thing recurred, twice. The light went out and the speedo worked again by itself but when I shut off the engine and started it again, the light came on and stayed on up to now.

I did a search of the forum and checked the voltage as suggested but it was 13.5 VDC nominal.

Please help, I am far away from the dealer right now and am worried the car might conk out and strand me and my family.

Thanks

Bong
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  #2  
Old 09-08-2002, 12:41 PM
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You are either loosing the speed signal of one of the front wheels or you are loosing the ABS system in general.

The vehicle speed signal is taken from one of the front wheels via the ABS wheel speed sensor. The raw AC signal is processed by the ABS controller and passed to the various users of vehicle speed (the speedo is one) as a pulsed DC signal. This is all easily verifiable with a scan tool using actual values.

Stated differently, the ABS will recognise a bad sensor and close down ABS function (and turn on light). The vehicle speed signal should be checked on a wiring diagram as I am not sure what happens to that signal if the ABS control unit is turned off.
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Continental Imports
Gainesville FL
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33 years MB technician
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  #3  
Old 09-08-2002, 01:24 PM
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Thank you for the prompt reply. To help our techs diagnose, I was able to borrow a Fluke digital tester and did some voltage measurements again. Here are the readings:

1. Engine off before starting 12.45 VDC
2. Idle RPM, no accesories turned on 13.40 VDC
3. Idle RPM, All lights & stereo on 12.01 VDC
4. Idle RPM, All lights, stereo & A/C 11.80 VDC
5. same as #4 except 2000 RPM 12.13 VDC

After the engine had been shut-off for 3 hours, the ABS light went off when started and the speedo works. What stumps me is why I lose the speedo function when the ABS light turns on and why they return to normal after resting a while? All fuses in the fuseboxes are OK, also OVP fuse.

Thanks again.

Bong
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  #4  
Old 09-09-2002, 12:19 AM
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those voltages sound a bit low to me alternator??
Willam Rogers.......
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  #5  
Old 09-09-2002, 09:23 AM
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Any output less than engine off voltyage (12.45 in this case) means the current is leaving the battery rather than into it as should happen with an alternator. The ABS is sensitive to the alternator to establish a running engine. It will shut off at voltages below 12v.

The alternatot on your car should be able to handle all those loads at idle (not the case with cars before 1990). handling means current flow into the battery, which means voltages higher than engine off battery voltage.

As I pointed out the ABS controller processes the wheel speed signal. When it shuts off the signal output dies. I think you have a bad alternator or regulator/brushpack.
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Continental Imports
Gainesville FL
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  #6  
Old 09-09-2002, 11:48 AM
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Steve, Yes I think you are right - my alternator is causing this problem. It only manifested itself while we were driving at night and it was raining hard, so all lights, wipers, stereo and A/C were on. The following morning it was ok again probably because I had no lights & wipers on.

I will take it in to the dealer as soon as I get back to the City. Thank you very much for the kind assistance.

Best Regards,

Bong
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  #7  
Old 09-10-2002, 01:11 AM
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FWIW I had a very similar problem with a C class very recently in the shop.
What to remember is that only the left front wheel speed is utilized for the speedometer input from the ABS control unit.
I checked this out on the car I worked on and had a good LF wheel speed input. The input from the ABS control unit to the instrument cluster is serialized data.
The problem was the ABS control unit.

Gilly
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  #8  
Old 09-10-2002, 11:25 AM
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Thanks Gilly. I will ask the dealer to trace the sensor output from the LF wheel when I get back.
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