W123 300TD glowplug relay weirdness

Discussion in 'Engine' started by SamCritch, Nov 17, 2008.

  1. SamCritch

    SamCritch New Member

    Messages:
    4
    Joined:
    May 17, 2006
    Hi,

    The other day I took my 1984 300TD to one of those manual carwash places here in the Netherlands - one of those coin-in-the-slot places and you get a couple of minutes of soapy brush and high-pressure hose (held at a good distance of course).

    Straight after the wash, the car started perfectly, but when I next started it a few days later, I got only faint dashboard lights, no glowplug light, and either a strange clicking or a whirring from the glowplug relay. When I remove the big plug on the relay, I get full dashboard lights when I turn on the ignition. The headlights etc seem to work okay.

    I was lucky enough to find a great tutorial here, so I got out my tester and tested everything:

    1. Main battery at 12.4V - is this enough?
    2. All sockets in the main glowplug relay connector showing 0.6 Ohm resistance, which seems fine.
    3. Voltage coming into the glowplug relay = 12.4V
    4. Voltage on both sides of the strip fuse at top of relay = 12.4V
    5. Voltage on each of the 5 terminals on the relay during glowplug activation = 12.2V

    I can't quite work out the problem here. The voltage seems fine, the glowplugs are fine (as far as I can tell). I was expecting a dodgy relay, but that actually seems to be fine, providing 12.2V to the glowplugs when required. The only thing I would expect is that the starter would crank the engine (it doesn't), but if the starter's the issue, then why are the dashboard lights dim only until I remove the connector from the relay?

    Any ideas? I guess the next step's to replace the relay, but whilst it's quick, it also costs a lot if that turns out not to be the cause of the problem.

    Thanks in advance for any help you can give!

    Sam
     
  2. compress ignite

    compress ignite Hardcore MB Enthusiast

    Messages:
    208
    Joined:
    Jun 4, 2006
    Location:
    32 degrees North by 81 degrees West
    Car:
    '90 300D 2.5
    H2O and Electrical connections = No Fun

    First,Let me say, the natural inclination is to point the finger of blame at the
    "Self Car Wash" and it's overindulgence in Aqua.
    This may not be the case/cause.

    Are you saying that if you unplug the Pre-Glow Relay the Starter will Work?

    Even then I would not condemn the GP relay out of hand...The Starter
    deserves your excellent,methodical diagnostic attention...To eliminate it
    as a Co-Conspirator (Or the Cause).
     
  3. grober

    grober MB Club Veteran

    Messages:
    26,506
    Joined:
    Jun 22, 2003
    Location:
    Perth, Scotland
    Car:
    W204 C200CDI Estate
    How old is your battery?;)
     
  4. OP
    OP
    SamCritch

    SamCritch New Member

    Messages:
    4
    Joined:
    May 17, 2006
    Thanks compress_ignite and grober.

    I don't know how old the battery is, and I've had the car about 2 1/2 years. now. In the last few hours (since I posted) I've read a couple of web pages about how car batteries are not just about voltage, but until now the car has always started pretty much instantly.

    The starter doesn't crank the engine whether the relay is plugged in or not. Should it crank the engine whatever happens?

    Thanks,

    Sam
     
  5. grober

    grober MB Club Veteran

    Messages:
    26,506
    Joined:
    Jun 22, 2003
    Location:
    Perth, Scotland
    Car:
    W204 C200CDI Estate
    Battery voltage can be deceptive though it can be an indication of charge as long as you take ambient temperature into account. Couple of possibilities but try a set of jump leads from a friends car battery first. If things go back to normal its your battery. Then check your engine earth strap is making good contact with the car chassis.
     

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