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Thread: Bad Alternator?

  1. 1st July 2013, 09:28 AM #1
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    Chomby
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    Bad Alternator?

    Yesterday I drove around 100km in my 110 with my lights and air con on. I noticed as I was leaving that my vdo tacho was not working properly and just sat at 500rpm. After returning home and leaving the vehicle sit for a few hours it would not start again (sounded like not enough voltage to kick the starter over.

    I left it on the charger overnight and it started fine in the morning. I then took it to the auto leccy to put a multimeter on it and with no lights on or fan on it was reading over 14 volts. As I was driving home I noticed my tacho was working now, I turned on the lights and fan and the tacho dropped when each of these were turned on.

    My guess is that the drop in the tachometer's level might indicate an alternator on its way out? Or is this normal for the needle to drop. Note that the engine revs did not drop only the tacho.
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  2. 1st July 2013, 10:49 AM #2
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    bee utey
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    Sounds more like the alternator drive belt is slipping, 500rpm indicated engine rpm is aboout the cut in speed of an alternator. What kind of engine/belt drive is yours? It sounds like you have a vee belt 3.5 V8 setup as the alternator is the last item to be driven. Pre 1999 the tacho runs off the alternator.
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  3. 1st July 2013, 06:01 PM #3
    ian4002000
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    Have a look at the alternator pulley if it is very shiny there is a good chance the belt is slipping and tightening the belt will fix it,
    If not sure get voltmeter and put on battery start engine and hold revs up a bit , it should read 14 volts give or take a little, then turn one electrical item on a time, if the voltage goes down with each item being turned on , the alternator has an output problem
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  4. 1st July 2013, 07:39 PM #4
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    flagg
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    I had this problem when I had an 40Amp alt. My A/C would draw at least 35 (much more with both running) so if I had my lights on as well I would flatten the battery as the alt couldn't charge fast enough. It took me an embarrassingly long time to work it out
    Hercules: 1986 110 Isuzu 3.9 (4BD1-T)
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  5. 1st July 2013, 09:03 PM #5
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    Flipper
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    Your battery is on the way out.!!!
    Do not be fooled by what the alternator voltage shows as this is telling you the battery is failing.
    I owned a battery shop for some time and have seen it all. Not saying I have all the answered but from what you have stated I will garrentee the battrery is stuffed.
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