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300E 1991 Mercedes. The ABS light stays on perminantly

Discussion in 'Suspension, Steering & Brakes' started by sladedestiny, Aug 2, 2012.

  1. sladedestiny

    sladedestiny New Registration

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    Location:
    Bournemouth,UK
    Your Mercedes:
    300e,1991,3litre
    Hi.I have just joined.I an trying to get help with a problem on my 300E 1991 Mercedes. The ABS light stays on perminantly.I have checked relays( including the constant voltage one with the fuse on top) all appear ok. sensors look ok although I have not taken them out or cleaned them..So I have no ABS brakes.Any pointers would be great.
     
  2. television

    television Senior Member

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    Location:
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    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    Re the OVP relay with the fuse on top,,,the only way to check it is by measuring the outputs on all pins 87 on the relay, you cannot do this in situ without a break out box. the other way is to check that you have 12 volt driving the ABS ECU

    A simple blink code reader would help you find the fault
     
  3. whitenemesis

    whitenemesis Senior Member

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    Your Mercedes:
    CLS55 AMG '05
    You may have been trying to post in the DIY section. No permission for users there.

    I have moved your post out of the FAQ .
     
  4. Alex Crow

    Alex Crow Senior Member Authorised Forum Supporter

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    Your Mercedes:
    W169, W124, w202, W203, KTM 250 EXC, VW T25 camper and a Polo in a pear tree
    I don't believe there was ever any diagnostic fault code facility in the 124 series ABS systems, unfortunately.

    AS has been said, it is usually an OVP or wheel speed sensor, and these should be tested with a meter - a visual check is pointless.
     
  5. television

    television Senior Member

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    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    Yes you are right, only on the 124034 and 036 with the 38 pin box
     
  6. wireman

    wireman Senior Member

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    Your Mercedes:
    nice 201 2.5D 1993 & very nice 129 SL500 1994
    In my experience with ABS the OVP is a the most frequent culprit.

    !2v is supplied by it to the ABS control and hydraulic units (OVP pin87, red/yellow wires), on petrol models it also supplies 12v to part of the injection system (OVP pin 87E, red/yellow wires). 12v power arrives at the OVP on a red wire at pin 1.
    !2v ign on power arrives at the OVP pin 6 on a pink/red wire.

    The red/yellow wires from pin 87 go to pin 1 of the ABS control and pin 10 of the ABS hydraulic units. Pull the plug on the hydraulic unit and look for 12v on pin 10 (ign on). Pins 6 & 12 (red wires) of this plug are hot all the time, I think directly from the terminal block in front of the battery.

    Check all the brown earth wires for integrity and freedom of corrosion at the body studs.

    The sensors can be at fault, if the warning occurs as soon as you switch on an open circuit in any of them will cause a fail.
    The rear sensor connects to a terminal block beneath the rear seat on the right hand side, blue and yellow wires are connected to it, lift them and check for 500 to 2000 ohms between the ones that disappear towards the axle.
    The front sensors are connected by coaxial plugs near the top of the suspension turrets under the bonnet, pull the plugs and test for 500 to 2k ohms.
    The sensors should generate more than 100mv AC when a wheel is spun by hand, this test would only be required if the ABS lamp comes on as you move off.

    My bet is the OVP relay, they can get dodgy due to poor internal connections which can be repaired by a deft hand and soldering iron. Giving the OVP a sharp tap can sometimes put them back into order but it will fail again.
    Europarts stock the relays for about £50.

    You might try as a temporary measure bypassing the OVP by linking pins 1 and 87&87E of the OVP socket, if it works its definitely the relay to blame, don't operate the car like this, there will be no circuit protection and your battery will go flat if its left any length of time.

    Welcome to the forum and please do let us know how you get on, good luck.
     
    Last edited: Aug 6, 2012
  7. dlicheri

    dlicheri Member

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    Your Mercedes:
    190D 1992 2.5D
    Hello wireman,

    So if sensors on wheel are open circuit (resistance is infinite) will light be permanently on (from ingnition switch on to starting engine and running )?

    I ask as I seem to have this condition on a 190D 2.5 1991. From checking, The only thing that the ovp relay seems to feed is the abs Ecu.

    Thx and great write up

    Appreciated!
    Dean
     

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