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Tow bar electrics

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by Wingnuts, Nov 3, 2012.

  1. Wingnuts

    Wingnuts Guest

    As part of a very long investigation into why my car drains its battery in three days of unuse I have found only one anomaly.
    Have noticed that there is 220mv wrt earth (towbar socket earth and directly to chassis) on each of the tow bar socket pins with the ignition and sidelights off.
    I have fitted many tow bars in the past never seen this before.
    Do you think it is just a characteristic of the Jaeger Trailer Module?
    This particualar bar is a genuine Mercedes approved fitted at a dealer with a certificate.
    No doubt a red herring but will leave it disconnected for now as part of the elimination process.
    Thanks
     
  2. television

    television Always remembered RIP

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    The best way is to test with an fuse in line ammeter
     
  3. OP
    Wingnuts

    Wingnuts Guest

    Yes thanks, probably shouldnt go over old ground but I have covered virtually everything on the drain fault but noticed the odd tow bar electric voltage today.

    New MB battery no change still flat after three days.
    Connected DVM and monitored alternator while driving - no problem 14V including when fully loaded with air con etc

    Checked current with Maplin fuse type checker and series ammeter for the four bigger fuses one shows 20ma - remainder 0.00ma. (doors not locked but closed & lights off)

    Overall current with all doors closed and interior lamps off is 40ma after aprox 8 min (1.5- 3A with door open or things switched on)

    Checked for AC voltage on the battery leads with engine running nothing seen indicating diodes OK.

    All seems fine but still the battery goes down in three days.
     
  4. television

    television Always remembered RIP

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    If the total is no more than 40ma when closed down with meter in series with the battery and the battery goes down perhaps a faulty battery, most cars pull 55ma when locked
     
  5. OP
    Wingnuts

    Wingnuts Guest

    Thanks for reply - new battery from MB and it behaves exactly like the old one which was only replaced for the same reason.
    No problem when running, battery does not fail then only when car is unused.
    Battery shows about 12.5v after turning engine off.
     
  6. television

    television Always remembered RIP

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    In that case the drain is more than you say,,, it has to be
     
  7. OP
    Wingnuts

    Wingnuts Guest

    Yes you are right but... I think the fault must reoccur after Ive measured the currents a real gremlin.
    Thanks for your help
     
  8. television

    television Always remembered RIP

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    If you think it is the tow bar wiring, disconnect it for now and see, but I just cannot see that, this is where a clamp meter come into play, so easy to use
     
  9. OP
    Wingnuts

    Wingnuts Guest

    I have a V & I clamp meter but I have also measured the battery current with a DVM in series for accuracy.
     
  10. television

    television Always remembered RIP

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    Very few clamp meters can read DC m/a
     
  11. OP
    Wingnuts

    Wingnuts Guest

    Well not to a great accuracy. My clamp meter (£100 plus) on the DC volts scale (clamp) reads to one decimal point so if it shows for example 0.0amps I am confident it is less than 100ma certainly accurate enough to show the 2Amps or whatever I am looking for in the case of a three day battery drain.
    It is also backed up by my DVM in series with the battery reading aprox 45ma
    Doors shut, lights off but alarm not set.
    Really unless you are in a position to have your meter calibrated every three months at a cal company we have to rely on what we see.
    Going to go over the whole thing again now.
    Thanks for all your help.
     
  12. Roxie

    Roxie Senior Member

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    I have known cases when after the battery is disconnected the current drain does not return until the vehicle has been driven again.

    so my advice would be to use your clamp as close as possible to the battery without disturbing or disconnecting any thing.
     
  13. OP
    Wingnuts

    Wingnuts Guest

    Thanks for that will look again from that view.
    Pity the battery is in such an awkward place on theW169. Hve heard of new owners who couldnt find it. It is posiible to get my clamp on even if you do have to be a contortionist to see the dial.
     

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