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Alternator coming 'on-line' issue

Discussion in 'A4/S4 forum(B5 Chassis)' started by PAULF, Oct 8, 2012.

  1. PAULF
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    PAULF Active Member

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    [Oct 8, 2012]
    I'm now back to electronauting Mrs PaulFs' Avant.

    The problem is the alternator sometimes takes a long while to come on-line.

    On my saloon, I start the car, and the voltmeter drops to 10-12 volts or so. Once started, the battery recovers, and the voltage goes up to 12-13 volts. After a brief wait (less than a minute) the generator comes on-line, and the voltage goes up to 14-ish volts - and a 'click' is heard.

    On hers, after start the battery recovers, but the voltage can take an age to come up, although it is above the 13 volts. It is worse in the cold, being almost unnoticeable in the summer. If this is in the winter, putting lights, heated seats and blower on cause the voltmeter reading to drop, and the voltage actually does - the lights dim!
    When it does eventually come on-line, the 'click' is heard, and there is no further issue
    I have changed dashboards (for another reason) so it's not the gauge, and I've changed the alternator for another (used) unit, so think it unlikely to be that. I also changed the battery for a giant sized unit - still the same! What brings the alternator on-line - I assume there is no dedicated GCU, so assume a relay control somewhere?

    I haven't looked into the wiring yet - as once again, I have SVG Viewer issues!!

    Has anybody else had this fault?

    It's not the end of the world, but with the cold, dark Scottish winter lurking :cold: I would like to sort this for her.


    Paul
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  3. adamss24
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    adamss24 Well-Known Member

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    [Oct 9, 2012]
    That click you hear is the glow plug relay...
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  4. PAULF
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    PAULF Active Member

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    [Oct 9, 2012]
    Thanks Chris,

    I assumed the click was some kind of GCU relay
    Could it be that the glow plug relay is sticking a bit and causing the voltage to be dragged down? (I also have a new Audi CTS fitted)
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  5. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Oct 9, 2012]
    I've noticed something similar on our Avant. Obviously i dont have glow plugs, so after starting the voltage does come straight up to 14v, however apply some electrical load (heated rear screen, blowers, lights) and the guage will drop back from 14. More stuff you turn on the more it drops, and on a cold winter morning with everything on you can get it down as far as 13v.

    The saloon however doesnt do that, it stays happily at 14v with a normal electrical load.

    Whats really odd, is that it does it regardless of the engine RPM.

    Its fair to suggest a large electrical load with the engine idling would cause the voltage to dip, the alternator can only put out 20-30A at idle, but once you raise the revs the alternator catches up as its output climbs to its rated 90A or whatever.

    However on the avant, the voltage drop stays when you rev the engine. This leads me to thing its a wiring issue like a faulty earth strap, or a faulty lead between the alternator and the battery, but i must admit i've not actually tried to see whats causing it yet.

    My first plan is to connect a jump lead between the engine block and the battery negative terminal. This will rule out the earth straps
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  6. PAULF
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    PAULF Active Member

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    [Oct 9, 2012]
    I've tried the jump leads when I was looking into my dash fault. I also did a resistance check.

    I assumed it was the alternator dying - so fitted another. It is strange, because as you say the revs don't make a difference.

    It really seems to be that there is a controller not switching it online fully - maybe a dirty contactor/relay?

    Even more strange is that your Avant has the fault, and not your saloon - the same as us!
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  7. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Oct 9, 2012]
    Get a multimeter onto the positive terminal of the alternator with the engine running, and see if the alternators actually putting out 14v.

    If it is, but theres only 13v at the battery, then either the live cable between the alternator and starter, or the starter to battery cable isnt large enough.


    On the 1.8T theres no controller or relay, the alternator is just wired directly to the battery (via the starter wiring).
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  8. PAULF
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    PAULF Active Member

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    [Oct 9, 2012]
    I'll try that over the weekend - Mrs PaulF is at work in it today, and I'm back to work tomorrow, so it'll be saturday before I can look at it.
    I'll be fun trying to do that on a 2.5 with it running!

    If there is no control system on your 1.8T, I very much doubt that I have anything.
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  9. murran
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    murran Active Member

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    [Oct 10, 2012]
    the only "control" is the blue exciter wire on the back of the alternator. this comes from the instrument cluster, in the olden days to the battery light on the dash and goes live when running to extinguish the light. id check the continuity on this wire. cus if its broken (as on my neighbours skoda felicia a few months ago) it simply wont charge but the light wont be on either! but a high resistance might cause poor charging? without the light being on?

    as this wire goes thru the plugs into the car under the box where the engine ecu lives. that'd be my first port of call. looking for corroded pins in there.
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  10. PAULF
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    PAULF Active Member

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    [Oct 11, 2012]
    Thanks Murran,

    I could be an exciter issue - although as it does charge, the wire won't be broken. High resistance is a good possibility.

    I'll have to look into the circuit from the dash and get the meter out. A straight resistance check end to end should be fairly straight forward............
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