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Thread: alternator test procedure

  1. #1
    Regular Member simon7257's Avatar
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    Default alternator test procedure

    i thought this might help after reading all the other posts about alternator faults
    (this is not vauxhall specific)

    this is how i test every alternator job i do, i have a tool called an amp clamp which is brilliant and a must for any mechanic
    available on ebay for £26.30, item number 370482639139

    1) check battery voltage, 12.2v is 50% charged and the minimum voltage to be running tests

    2) start vehicle, battery voltage should not drop below 9 volts and using the amp clamp check the amp draw when cranking, a basic rule is a petrol starter draws 150 amps and a diesel is 300 amps

    3) with engine running check battery voltage 13.5v - 14.5v is good but be warned some new vehicles will charge up to 15 volts and check amps

    4) load the alternator by switching on heated rear screen, blowers to max speed and headlights

    5) check battery voltage, will drop slightly above 13.5v is good, compare to amp reading eg 14volts 30 amps is a good reading 12.8volts and -20amps is alternator failure and discharging your battery

    6) volt drop engine earth by putting multimeter on battery negative (black lead) and to engine block (red lead) 0 volts is good if you get a reading above 0.5volts could be loose or broken engine earth strap

    7) volt drop battery positive to alternator b+ terminal, again 0volts is good and above could be loose or broken cable or connection

    8) check you have battery voltage to ind wire on alternator by back probing multiplug whilst connected

    i hope this helps solve any problems

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    Regular Member alcutler's Avatar
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    Vehicle : vectra c 1.9cdti 120 estate

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    Looks a handy piece of kit - this would make a nice little How2 with a few pictures to show the meter connected and doing the checks!

  3. #3
    Regular Member simon7257's Avatar
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    Leave it with me and I will do a write up in a few days with pics, it can be used for so many checks I'd be lost without mine

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    Regular Member alcutler's Avatar
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    Quote Originally Posted by simon7257 View Post
    Leave it with me and I will do a write up in a few days with pics, it can be used for so many checks I'd be lost without mine
    Thanks for that - shame the clamp meter comes in pink! but think I will get one (bit of a toolaholic) - what's the advantage over a normal digital multimeter other than the clamp facility.

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    Regular Member simon7257's Avatar
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    The one I have doesn't have the multimeter on it, if you just use the amp clamp then it's uses are endless, one use is if you have a flat battery when you jump start your car you can put this round the battery cables (usually earth cause its easier) you can see what condition the battery is, rather than just looking at 14 volts for example you can see if your getting high amps lets say 50 amps you know that your battery is excepting the charge whereas if you see 14 volts and say 5amps then it's probably not going to recharge, and like in my op if you load the alternator and see negative amps the alternator is failing, you can also use it to check things like fuel pump draw, I have 2 bits of wire on a fuse holder, pull the fuel pump fuse insert the two wires and the the fuse in your made up fuse holder put your amp clamp around 1 of the wires and bingo 5 amps is a normal (petrol) amp draw less is out of fuel more is seized pump,you can do things like fuel pump draw with a multimeter but the internal fuse on a multimeter is about 15amps an amp clamp will read up to 400amps

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    Regular Member alcutler's Avatar
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    Thanks for that - sounds good - just been looking on Ebay and there are quite a few to choose from - but I will be getting one. Look forward to seeing the How2

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    Thanks for this, I'm trying to track down an intermittent charging fault currently. Where is the multiplug that you refer to? Which pin do I probe? All I've got for reference is Haynes and they're useless for wiring.

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