Voltage unstable

Discussion in 'General Motoring' started by yourpete, Dec 15, 2006.

  1. yourpete

    yourpete Guest

    During night driving I have noticed that the head lights vary in
    brightness. They brighten up at about 2500 to 3000 revs and dim when
    the revs drop. But when in say 4th or 5th gear at about 1500
    revs(Diesel model) and I lift off the accrelator the lights
    brightenup?. Strange. This will voltage varation only happens AFTER
    the engine has warmed up. The brightness varation is also noted if you
    over-ride the interior lights. My garage has replaced the control unit
    but no change. Any suggestions. The battery and alternator have been
    FULLY checked out and found to be OK.
    yourpete, Dec 15, 2006
  2. It would help to know what year and model.

    Michael Pardee, Dec 15, 2006
  3. yourpete

    yourpete Guest

    Accord 2.2 diesel 4 door saloon. June 2004

    yourpete, Dec 15, 2006
  4. OK, lets look at what causes a visible variation: change in voltage. Since
    you say the alternator and battery have tested OK, then its likely a
    mechanical rather than an electrical problem.

    What causes an alternator to vary voltage? A slipping belt is the usual
    reason. A bad ground is another.
    Have a good look at all the alternator mounting brackets. Is anything loose?
    eg the main pivot bolt on to the block. Check all the engine block to
    chassis ground links.

    Is the crank pulley loose or is a two part balancer type? Could be the
    pulley is separating and moving the belt around causing slipage or lateral
    movement in the alternator.

    Stewart DIBBS, Dec 15, 2006
  5. yourpete

    yourpete Guest

    I have thought of this route BUT this problem ONLY happens after the
    engine has warmed up. This leads me towards an electrical problem and
    your thoughts about a poor earth will be checked. If a poor earth
    should this not have shown up when the battery and alternator were
    The unit that controls electrics has just been replaced with no change
    to the problem
    yourpete, Dec 16, 2006
  6. I had this same symptom when I swapped in a JDM B18C into a USDM 96 Civic.
    The problem was the fact that the USDM car had an Electical Load Detector in
    the under-hood fuse box and the JDM ECU does not use one so it was confusing
    the JDM ECU and it was "telling" the alternator to stop charging when I would
    press on the accelerator. I remover the ELD and all is fine now. It could be
    that your ELD (if you have one) is bad, or the connection to it in the under-
    hood fuse box needs cleaned.
    GingerGSR via CarKB.com, Dec 16, 2006
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