[SOLVED] W124 fuel pump relay help requested

Discussion in 'Engine' started by Phileas Flash, Sep 2, 2013.

  1. Phileas Flash

    Phileas Flash New Member

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    Apr 4, 2013
    Location:
    Portsmouth
    Car:
    1994 E280
    Hi all,
    My 1994 E280 won't start. It turns over nicely on the key but doesn't fire at all. I'd like to check the fuel pump relay but can't work out which it is!
    Under the plastic cover behind the fuse box in the engine bay there are a few relays...

    A big module, part # 126 542 03 32, can't remove it without more work.
    A smaller (cigarette packet sized), black module, part number 201 821 00 47. I've removed this one, taken the cover off and see no problems or burnt bits on the PCB.

    There's a small green relay, part # 126 820 20 26

    Another green relay with a fuse mounted on it (fuse is fine), part # 001 542 96 19.
    A blue relay with a fuse mounted on it (blown, I need to replace it), part # 002 542 01 19. I tried shorting the fuse holders for these last 2 to force them on, but they both activated the fans.

    Lastly, there's an opaque covered relay, same size as the last 2, part # 001 542 78 19.

    I can't hear the fuel pump because a retractable ariel comes up making a nice noise as soon as I turn the key.
    I'm sure I can make some progress on this if any one of you knowledgeable guys can tell me where to find (and ideally, how to bridge) the fuel pump relay.
    Any help much appreciated,
    Thanks.:thumb:
    Phileas.
     
  2. GP801

    GP801 MB Enthusiast

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    Take out the fuse for the aerial or disconnect it to let you listen for the fuel pump. Both the OVP relay and fuel pump relay are involved in making the fuel pump run. When ours had this issue it was the OVP that was at fault.
     
  3. boomtings

    boomtings Active Member

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    Car:
    95 e320 cabriolet ,93 300te_24v Turbotechnics
    Fuel pump relay is not near the fuse box.

    On a 6cyl 320 (think it should be same for 280) its behind the battery (remove plastic cover), and you should locate it to the left side of the ecus.
    its the one with a fuse on top.
    There is also another relay next to it (climate/kickdown?) irc. Dont get mixed up with the ovp relay which is on the rhs

    you should be able to feel if the relay working.
     
    Last edited: Sep 2, 2013
  4. boomtings

    boomtings Active Member

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  5. OP
    OP
    Phileas Flash

    Phileas Flash New Member

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    Portsmouth
    Car:
    1994 E280
    Thanks for the tips chaps, I'll be getting my hands dirty again on Saturday to try and get some progress on this one. I'll let you know what I find.
     
  6. tippandy

    tippandy Active Member

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    st albans
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    If the aerial is wired correctly then turning off the radio should stop it from activating when you turn on the ignition.
     
    optimusprime and spike60 like this.
  7. OP
    OP
    Phileas Flash

    Phileas Flash New Member

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    Car:
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    The ariel isn't wired up right, it comes up with the ignition and down when you turn the key off. A small issue for another day!

    Progress update...
    I disconnected the ariel and turned the key, and hear the fuel pump prime and run for a second or two after I stop cranking the engine.

    Seems to point to a lack of spark, which I intend to check tomorrow.

    The trigger for this whole problem was me shipping off the engine harness to a wiring specialist to be rewired. He's since double checked his work, and I'm sure I've reconnected it up right, so it seems something else has gone awry.

    Any ideas on where to look ignition/spark wise?

    Thanks,
    Phileas.
     
  8. grober

    grober MB Master

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    If you have an aftermarket alarm is possibly the immobiliser stage that has gone faulty. It normally isolates the solenoid coils of the fuel pump relay and OVP relay. As it's aftermarket it won't be covered by the factory wiring diagram. The fuel pump relay is 2 stage the first primes the fuel system the second requires an engine running signal to continue to pump fuel. Just because you hear stage one doesn't necessarily mean the FPR is OK. You need to examine the small PCB with a small magnifier to pick up dry joints. Just go over the PCB contacts with a soldering iron to "remelt" them. If you are not comfortable doing this a local TV repair man would do it for you for the price of a pint.
     
  9. OP
    OP
    Phileas Flash

    Phileas Flash New Member

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    You can open up the green fuel pump relay? I'm happy running an iron over the joint, no problem. The immobiliser is a scorpion Merc approved one from new and the fob seems to be working fine. I've been locking it with the key recently since it's not running to keep the battery from draining.
    Thanks again.
     
  10. neilrr

    neilrr MB Enthusiast

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    Sometimes a bad loom can blow your ECU, which in the case of a W124 E280 is an HFM unit that I believe is coded to the car.
     
  11. OP
    OP
    Phileas Flash

    Phileas Flash New Member

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    Car:
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    Hi again.
    Fixed! I don't know how which makes it feel like a rather hollow victory. I had a look at the fuel pump relay. The fuse was ok, and I heard it operating with the antenna disconnected so I decided to check for spark. I put an inline light on cylinder one and it flashed, and for the first time in months, I heard it try to fire. So after some time I have it running and it's now nice and smooth. I just don't know why. Presumably a connection somewhere got knocked when I removed the harness originally, and now I've put it back.
    Thanks for all the suggestions guys.
     
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  12. DaveDacombe

    DaveDacombe New Member

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    Hampshire UK
    Car:
    W124 m103 300e
    Hi, trying to solve starting issue on my W124 M103 300e. Starts every crank but only runs for a couple of seconds. Fuel pumps both run and system primes no problem, but i am now wondering if the engine running signal is missing. How/where can I check for this. As the engine only runs for such a short burst do I need any specialist meters/oscilliscope?
     
  13. grober

    grober MB Master

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    IIRC the engine running signal came from a hall sensor on the rev counter on early cars and later from the EZL ignition unit via the flywheel crank position sensor. Will try to dig out some pin idens and voltages to look for.
     
  14. DaveDacombe

    DaveDacombe New Member

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    Thanks,
    Rev counter is def. working and shows engine picks up after cranking and runs up to about 1500 rpm before engine stops.
    Flywheel position sensor is new replaced this summer along with plugs, leads, dist cap, rotor and coil.
    Dave
     
  15. Will

    Will Hardcore MB Enthusiast

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    Could be the fuel pump relay. They energise the fuel pump(s) to prime when the ignition key is first turned to pressurise the system, then switch off after a second or two. Once the engine is cranked over/running the fuel pump relay should energise the pump(s) again. I have had relay faults that did exactly that. Easiest thing to do for diagnosis is link the pins out to enable the fuel pump(s) to run continuously - 30 & 87 IIRC (should be marked underneath, opposite each other)

    HTH :thumb:

    Will
     
  16. DaveDacombe

    DaveDacombe New Member

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    Location:
    Hampshire UK
    Car:
    W124 m103 300e
    Brand new fuel pump relay fitted. But made no difference. As the car runss for such a short time I cannot tell if the pumps re-energise so am going to try with the pins linked.
     
  17. DaveDacombe

    DaveDacombe New Member

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    Sep 9, 2019
    Location:
    Hampshire UK
    Car:
    W124 m103 300e
    Update - with fuel pump running constantly car does not fire up. Fuel Pump relay in place car runs for a couple of seconds every time cranked. With the wiring plug disconnected from the cold start injector NOTHING. Would I be wrong to assume the car only runs briefly because the only fuel its getting is from the cold start injector?
     
  18. optimusprime

    optimusprime MB Enthusiast

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    Have you change the distributor top and the rotor along with ht leads and spark plugs . Check you fuses in the fuse box - give them a turn for now. Look at the wires on to the coil for corrosion .And look at all the vacuum pipes for splits .
     
  19. neilrr

    neilrr MB Enthusiast

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    Stop guessing & take it to someone who knows these cars before you throw any more parts & money at it.
     
  20. optimusprime

    optimusprime MB Enthusiast

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    Neilrr i thought that the forum was hear to help other members to try and save a little cash . The things i tell Flash about is general maintenance if they are not kept up to scratch, then problems will happen in due course . If every one took their car in to the garage for the simplest thing it will cost the earth. Along with knowing very little about their own car,, and how it works .And second hand parts wont be needed anymore reason being that the garage will only fit new parts ..
     

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