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Electrical help

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ANTMAN, Jan 13, 2003.


    ANTMAN Registered Member

    Jan 27, 2001
    Likes Received:
    Raleigh NC
    My 82 GMC Jimmy began showing discharge whenever heater fan is on high. also begins to show discharge when lights are turned on. Now it does not do this all the time and will sometimes fluctuate from discharge back to charge. replaced the alternator and tested with my multimeter and it shows it charging at 14-14.5 volts until i turn on either the lights or the heater with fan on high or both on and read 11.5-12 volts. guage in dash shows discharge at this time . Any ideas would be appreciated.
  2. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

    Feb 17, 2000
    Likes Received:
    Start checking for ground problems. On GM trucks there is a ground strap off the rear of the passenger side head to the fire wall. Yeah there is a pig tal at the battery to the coresuport but your pulling threse several poor connections so GM put that ground strap to give the tub the best possible connection back to the block. The ground off the battery goes to the block also.
    Blower is a major draw and as old as that truck is my bet is the engine has been out and that strap was left off. The gage ground to the tub so it it got resistance back to NEG then it would show a poor reading. A quick way to further invesitgate if this is the problem is at night whith the headlights on flip on the blower. they ligts will dim a little but if your down below 12v on that gage the lights will dim a LOT if the voltages is below 12 at the lights. The lights ground the the coresuport so they have a good path back to NEG at the battery.
    Your test with the multi meter also tends to lend credit to this. Also at idle a lot of times the draw will exceed the output of the alternator. Bring it up to about 900 RPM and check the voltage with the meter.
  3. dyeager535

    dyeager535 1 ton status Premium Member

    Dec 13, 2000
    Likes Received:
    Roy WA
    While I agree that voltage is too low, what RPM are you turning, what additional electrical accessories are you running, and have you tested voltage with the lights on at an RPM above idle?

    With a 550-700RPM idle, I would consider anything above 12V to be "normal" for a stock alternator. They don't do well at all at low RPM's and some load.

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