W123 M102 230E 1985 Fuel Pump Issue

Discussion in 'Engine' started by DRHurst, Nov 28, 2016.

  1. DRHurst

    DRHurst New Member

    Nov 28, 2016
    West Sussex
    W220 & W221

    Hoping this forum will help throw some light on a frustrating issue I have with a ‘barn find’ W123 M102 which is refusing to pump fuel. The car incidentally has been sitting for a few years, however it does crank with compression and every indication would suggest it should run with a little tlc.

    I have used this forum and maintenance manual to complete the following steps in an attempt to fix this issue without luck and so now have come to the point which every bloke dreads which it to ask for help!

    Here is a précis of the issue:
    1. Pump makes a brief sound (like it’s about to run) when ignition is switched on but does not run/energise.

    Diagnostics Performed:
    1. DVM across the pump connectors shows a very brief 12v (a second and no more) then nothing.
    2. Cleaned tank and applied fresh fuel.
    3. Connected a separate 12v supply to the pump and it runs fine.
    4. Pump Relay tested and even replaced; click heard when ignition is turned, followed by a further click.
    5 Removed relay and bridged pins 7 & 8 with same results as ‘1’ above; Pump makes a brief sound when ignition is switched on but does not run/energise.
    6 Removed, cleaned and refitted fuel sender unit.
    7 Removed, cleaned and refitted all fuses.
    8 Searched high and low for the OVP Relay but can’t find one??!

    I have to assume the relay is shutting the pump down due to further circuit logic or indeed failure, however I have exhausted my expertise and now throw this to the mercy of the experts and enlightened out there.

    Any help that can offered would be very gratefully received.


  2. nick mercedes

    nick mercedes MB Enthusiast

    Jan 8, 2003
    far far away
    Ford Expedition
    No OVP on a 123, broken wire somewhere?

    Should work fine if it's bridged, loads of 123s drove about with no fuel pump relays at all.
  3. roastbeef

    roastbeef Active Member

    Jan 15, 2014
    mercedes190e w124s
    Good idea to check ignition side of things I believe the fuel side of things needs to see a ignition pulse to keep the fuel pump working
    Happened to me on a w124 230 had a faulty dizzy cap ?
  4. alabbasi

    alabbasi MB Enthusiast

    Feb 18, 2008
    You're probably focusing on the wrong end of the car :)
    The fuel system can clog up and prevent the car from running even though the fuel pump is delivering fuel. Disconnect the fuel line that goes into the fuel distributor and check if it's getting fuel. If so, I suspect that the fuel distributor or injectors are gummed up and will need cleaning.

    I've had to do this job a number of times on CIS cars that have been sitting a while.

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