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Ammeter Discharge When Headlights On

Discussion in 'Electrics' started by SydneyRoverP6B, Jul 15, 2017.

  1. SydneyRoverP6B

    SydneyRoverP6B Well-Known Member Staff Member

    I noticed the other day that when I switched on my headlights, there was a discharge on my Rover's ammeter. It self corrected after maybe one minute or so. Without the headlights on, the ammeter needle would sit at the null position whilst driving, but I suspected something was in need of attention. So today I switched on the headlights again, and again a discharge, except this time there was no self correction.

    So, I surmised that one of the brushes was likely in need of replacement, and upon inspection, that is exactly what I found. The brush closest to the bridge rectifier makes contact with the slip ring at a distance from the centre, whereas the brush closest to the voltage regulator, makes contact with the slip ring at its centre. It is the former brush that was in need of replacement.

    The voltage regulator in on the left, the bridge rectifier on the right. The two brushes reside within the brush box, atop the slip ring.

    The shorter brush is 7mm in length, thus in need of replacement. The stated minimum allowable length for replacement is 8mm. The longer brush is 12.5mm in length. When new, the brushes are 13.0mm in length.

    I use Cargo brand replacements. The voltage regulator of the same brand has proven to be extremely reliable.

    After replacing the outer brush, subsequent testing showed a successful outcome. The culprit was indeed the worn brush!

  2. GRTV8

    GRTV8 Well-Known Member

    Good thread thanks Ron.
    SydneyRoverP6B likes this.
  3. SydneyRoverP6B

    SydneyRoverP6B Well-Known Member Staff Member

    Gee thanks, pleasure Gerald :)

  4. ButterFingers

    ButterFingers Active Member

    interesting post Ron, are you talking about the actual regulator inside the alternator that you worked on?
  5. SydneyRoverP6B

    SydneyRoverP6B Well-Known Member Staff Member

    Hi Peter,

    I did no work on the voltage regulator, merely identifying it. It was a carbon brush which is entirely separate that was the problem.

    If there is ever a problem with the voltage regulator, being a sealed unit, just fit another.

  6. ButterFingers

    ButterFingers Active Member

    thanks Ron for the reply.
    So are you talking about the alternator and a part inside or a different unit mounted elsewhere?
  7. SydneyRoverP6B

    SydneyRoverP6B Well-Known Member Staff Member

    Pleasure Peter :)

    Yes, the alternator that is in my Rover, and one of the carbon brushes within. It is quite easy to replace them when needed, no special tools required.

  8. ButterFingers

    ButterFingers Active Member

    Thanks Ron, now I understand your post...........cheers
  9. mike rigg

    mike rigg Member

    Hi Ron
    Interesting article. Got me thinking. My ammeter never seems to show a positive charge but putting a meter across the battery terminals with the engine running I'm getting 14.3 volts. Can the brushes affect the amp reading even if the voltage output looks good.

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