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How do you check alternator output with a DMM?

Discussion in 'The Garage' started by Can Can, Jul 26, 2004.

  1. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    I haven't driven the K5 that much this summer and if I leave it for a week or two the battery is dead when I go to start it up. Chances are the battery is toast(it's almost 8 years old), but I thought this might be a good chance to double check the alternator output and such.

    Where do I put the leads for my multimeter to check how much juice my alternator is putting out? Anyhting else I should look for while I'm checking it?

    Also, how do I tell if the battery isn't holding charge or if there's a draw on the system somewhere?

    Thanks, brothers!!! /forums/images/graemlins/thumb.gif
     
  2. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Red lead to positive battery terminal and the black lead to negative battery terminal and you will be reading voltage that's currently in the battery, now start the truck and it will be reading what the alternator is charging at. /forums/images/graemlins/thumb.gif
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Re: How do you check alternator output with a DMM? *DELETED*

    Post deleted by HarryH3
     
  4. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Okay, here's some figures to consider:

    Battery after the truck was boosted and running for 5 minutes- 12.6

    Reading at the battery posts at idle- 14.3

    Reading at the battery posts with lights on and heater on full- 13.3

    I thought I might add that when the alternator was under load it was whining a little bit. Without load it sounded fine.

    So is my alternator fawked too? Or is the dying battery making the alternator work harder than it has to?
     
  5. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    I would say the battery as well. Even if the alternator isn't 100%, it is working. You know your SOL when the voltage doesn't change between off and running. You either have a draw or the battery just can't hold a charge for long. Since the battery is so old I am leaning towards it, but on trucks like ours it could be a draw as well /forums/images/graemlins/dunno.gif
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    To check for a current drain or short curcuit,remove the negative battery terminal and put the test light between the battery cable and battery terminal,if it lights up,something is drawing current from the battery.Then you have to start pulling fuses or disconnecting the wires to the suspected curcuit--once you have found the drain the light will go out.Be aware if you have a clock it may make the light flash once in a while,and your dome light will show up as a drain if the doors are open. Also,I notice on my trucks the HEI distributor will make the light barely glow,until I pull the pink BAT + wire off,I dont know if its just my trucks,but they all do it,doesnt seem to kill the battery though! /forums/images/graemlins/dunno.gif Bad diodes in the altenator can do the same thing too,allow the battery to discharge when vehicle is shut off,and still charge some,at a reduced output,like when one or two of the 3 diodes croak in the altenator. But I'd say your 8 year old battery has a lot to do with your symptoms. /forums/images/graemlins/crazy.gif
     
  7. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Good info, brother. Thanks for the reply!!! /forums/images/graemlins/thumb.gif
     
  8. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Another thing to remember is that an alternators purpose is not to charge a battery but to maintain the charge that is in the battery. If you have a weak or dead battery and jump start the vehicle then let the alternator try to charge the battery that is making the alternator work very hard and could easily kill the alternator.

    What does this all mean you ask? You need to charge the battery fully with a SLOW charge before starting the vehicle and trying to test anything.

    As a rule after about 3 years a battery starts to die and by 5 years is almost ready for battery heaven. I would get a new quality battery then start checking for any shorts or draws on the circuit and go from there. /forums/images/graemlins/thumb.gif
     

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