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· Outstanding Contributor
'17 GLS450, '14 GLK250 "Grandpa's Roadster" Project Car, 350SDL (Sold)
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The most common causes are an electrical failure of the sensor, corrosion on the tone wheel (the thing the sensor senses) or something (rust) forcing the two apart.

It isn't clear from your post, but, did you replace the sensor? Did it solve the problem?

As you might expect, the wires from the sensors go to the ABS control unit.

1.5 Ohms doesn't sound right. I have a working sensor from a different vehicle which I will measure.
 

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1989 560Sec
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Forgot to mention too, each sensor is very picky on its resistance, voltage and difference in resistance to one another, the wiring from the sensor to the abs unit can fail, so test the voltage output of both sensors when you spin the wheel.
 

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Smoke Silver '87 560SEC, Black Pearl '87 560SEL
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For reasons only known to Mercedes, my ABS light comes on for low electrical system voltage before my battery light comes on. Next time you get the light, measure the battery voltage. With the engine running, you should be about 13.6VDC and up a little.
 

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1988 Mercedes 500 sec
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Discussion Starter · #7 ·
the car has had a recent nut bolt restoration, so the hubs are clean zinc plated, no rust, sensors were also clean
I did try another sensor, this was not new and measured 1.5 ohms, this also caused the ABS light to come on when the right wheel was rotated
The battery is new, will check the voltage when I am with the car next
 

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ansel any idea what the voltage output should be
I believe they should be around 1.4-1.5 volts, here is a previous thread that will probably help out.
 

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1988 Mercedes 500 sec
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Discussion Starter · #10 ·
john350 did you manage to check the resistance on your working sensor, mine is showing 1.3 to 1.4 ohms
I just realised, when I restored the car, the suspension was sprayed with 2k primer and then few coats of raptor bed liner, is it possible the thickness of the paint is slighlty moving the end of the abs sensor away from the rotating ring abd this causing the light to come on
 

· Outstanding Contributor
'17 GLS450, '14 GLK250 "Grandpa's Roadster" Project Car, 350SDL (Sold)
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I completely forgot, so I just went out and did it. I measured it 3 times and got 966 Ohms each time. You aren't by any chance using the "x1K" scale, are you? 1.3 x 1K would be 1300. Also, are you measuring it when unplugged? Resistance measurements must be made with the component isolated from the rest of the system.

Measure one of the other sensors and compare it to the one you think is bad.
 

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Usually the sensor gets lifted off its mounting surface by rust or corrosion. I doubt any of the coatings you used could first seep in there and then expand enough to cause trouble.
 

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1300 is in the ball park. 1.3 is not. Unplug the other side and test it. If you get around 1300 they're both probably fine.

Try the next lower range on the meter and see if you can get a more precise reading.
 

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Discussion Starter · #15 · (Edited)
ok, went out today, disconnected the sensors from the car loom and got 1.495 k ohms on left sensor and 1.526 k ohms on the right sensor, voltage on both sensors was 0.0 mv, does this show whether my sensors are good or bad, the sensors were umplugged but not removed from the vehicle
the voltage from the ecu was 5.2v and 4.9v, each side
 

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· Outstanding Contributor
'17 GLS450, '14 GLK250 "Grandpa's Roadster" Project Car, 350SDL (Sold)
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When the sensor is disconnected you wouldn't expect any voltage. If you spin the wheel you should see a small AC voltage generated. Check the service manual for specifics.

Since the resistance is similar on both I think they are not your problem. Have you tested the rear sensor?
 

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91 420SEL, 72 350SL, 99 S600, 05 Ford F-350, 09 C300, 10 Boxster 987, 12 GL450 , 87 560SEL ,05 SL60
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Resistance measurement is a good starting point - it will tell if the sensor is roached or if it MAY be good. Corosion, bad connection, broken tooth, mispositioned, etc. will all cause sensor failure. The only way to tell if the sensor is working is to read the output on an oscilloscope. These used to be expensive items, but a laptop and $10 worth of parts and a free bit of software and you can have a 'scope perfectly adequate for automotive work - alternatively buy an oscilloscope add-on kit that works with your laptop for $50 - $100.
 

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Discussion Starter · #18 ·
I havent tested the rear sensor as when I spin the front right wheel the light comes on, will try test the rear tomorrow, I believe the plug is below the rear seat
 

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Hope you figure it out. My understanding is that both front ABS sensors are NLA. The rear I believe is still available for Gen II cars. For Gen I, all 3 are NLA, IIRC.
 

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Discussion Starter · #20 ·
Checked the voltage today, both front sensors are giving 0.001V, on the MV scale it was at 1.3 milli volts, the ohms though was in spec at 1300 ohms, does this show both front sensors to be faulty
The rear sensor in the diff showed 688 ohms which I believe is in spec
 
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