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Alternator / charging help?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Woodman89, Dec 20, 2000.

  1. Woodman89

    Woodman89 Registered Member

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    Whenever I leave my truck overnight the battery dies. I can disconnect it and hook it up the next day and the truck will fire up. I have tested the battery and it is good (it is fairly new). I am pretty sure that the alternator is charging it because the battery never dies if I keep diconnecting it when it isnt running. I assume there must be a short somewhere that is draining my battery when its parked. I have a decent meter, so can I hook it up somewhere to see if there is voltage getting drawn from the battery when it is off? My neighbor told me that the alternator may have a faulty resistor (???) that is draining the battery.

    Also, what should the meter read as output from the alternator when the engine is running? The battery reads 12.6VDC with the engine off and 13.4 running.
     
  2. fknfbm53559

    fknfbm53559 Registered Member

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    if its running 13.4 volts while the truck is running your alternator is fine. to me it soulds like you have a direct short somewhere that is draining the battery. ive heard of a few ways to test the alternators but i never mess with it if i suspect its bad i just replace it. but the first way ive heard is start the truck up and disconnect the negative cable from the battery, if the truck stays running the alternator is good. ive never tried this method. and the second way ive heard is to run the truck and stick something metal to the back of the alternator, if it acts like a magnet and sucks the wrench or whatever to it then the alternator is fine. check around your starter for shorts. on a k-5 the frams is really close to the starter and its very possible that the cable running from your battery could be touching the frame. its happened to me before and its not a pretty site. if thats not it check everyother major electrical spot. also make sure the battery is grounded good. this is just my 2 cents
     
  3. fknfbm53559

    fknfbm53559 Registered Member

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    oh yea and also my blazer would run about 9-10 volts when it was in motion, i could never figure out what was causing it until one day when the fuel pump went out. i crawled under the truck just to make sure it wasnt a clogged filter. i reached up under the frame rails to feell for the filter when all ov a sudden something shocked the living piss out of me. further investigation led to the wire that runs to my brake lights. someone had spliced it and it was a real shotty job. good thing i found it because that short was arcing off my frame and fuel lines :O i have no clue whether that was a factory boo boo or the carelessness of the dumbass that owned the truck before me. just make sure you check that or else it could lead to disaster and one rather toasty blazer! MMM anyone up for a roasted 350?
     
  4. Burt4x4

    Burt4x4 3/4 ton status

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    I think the disconecting the negitive cable idea only works with old points style distributers...
    I would start removing fuses to see if you can eliminate the causes. Thease can be tuff to find so good luck!

    72K5[​IMG]Led Zeppelin[​IMG]Rock ON![​IMG]
    http://www.burt4x4.coloradok5.com
     
  5. HarryH3

    HarryH3 1 ton status Author

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    I've never verified this personally, but I've heard that some alternators will fry if you try the "disconnect the battery cable while the engine's running" trick. That could be a $90 mistake. [​IMG]

    Best way to test the alternator is to connect a voltmeter to the battery. With the engine running, turn on the headlights, then the heater fan, wipers, etc. Watch the voltage as you turn each item on (and leave on the other stuff that you've already turned on). The voltage should stay up around 13.4-13.8 volts. You might have to kick up the idle just a little bit, to make sure the alternator's turning fast enough as you get everything on. If the alternator can't keep the voltage up as the load increases, then it's time to look for a new one (after you make sure the belt is tight and not glazed!)

    <font color=black>HarryH3 - '75 K5</font color=black>
    http://ThunderTruck.ColoradoK5.com
     
  6. Thunder

    Thunder 3/4 ton status

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    You have a short somewhere. A firend had a similar problem as you. I told him to go out to his truck at night and check it out. He found that the glove box light was stuck on. If its not a light on here's more.
    You said you have a good meeter so:
    With the ignition off. disconnect the pos+ cable from the battery and leave the neg- cable connected. Put the red+ voltmeter probe on the battery terminal and the black- probe on the pos battery cable end(with + cable completely disconnected from batt). If you have a short you will get a voltage reading between them. If getting a voltage reading, start pulling fuses. when the voltage drops to 0 you found the circut where the short is.
    If you suspect the alternator, completly disconnect it. If you still get a voltage reading the alt is ok and the short is somewhere else.
    Also if you have a clock or raido that has a hot lead you may need to disconnect it or pull the fuse because they may draw a small ammount of power and will give bad readings when looking for a short.
    Check wireing that is hot all the time like brake lights headlights, Starter and solenoid, Towing wireing, ect.
    Shorts can sometimes be a bear to find. Good Luck!
     
  7. Channing74

    Channing74 Registered Member

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    I work at a shop and thumpers is the best explanation on how to go about finding that short. We have had problems in the past with f**d alternator resistors shorting out but not to many chevys.

    Bigger Is Better
    Channing74
     
  8. HarryH3

    HarryH3 1 ton status Author

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    Since it's an '89 with TBI, the main power to the ECM comes off of the power stud mounted to the firewall. There's a plug in the line that supplies constant power to the ECM. You'll need to disconnect that before testing as well.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
     

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