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electrical problem?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 468ratman, Jan 15, 2003.

  1. 468ratman

    468ratman Registered Member

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    I've been trying to get this problem solved but it is not going too well. If you can help I'd be really thankful.

    I think it's the alternator but I am unsure and throwing parts at a problem can get expensive real quick.

    Basically, my truck runs like [censored] the more stuff I turn on. If I have just the truck running it works fine. Then when i turn on the heat and the headlights the truck misfires and sputters. Turning the lights and or heat off usually solves the problem right away but at 5 am and 0 degrees that's not really an option.

    I did go and have the battery and the alternator checked and the kid at the parts house said they were both good but I don't think he really knew what he was doing because the thing said 11.15 volts and he said it was OK.

    Another guy told me I could test my ALT by removing the negative battery cable while the truck was running and if it worked good then the ALT is good, and if not then it's not. Does that ring true?

    When I did that the truck ran but stalled right away when I turned on the lights or the heater.

    WTF? I don't know. Can you help? Thanks.

    89 350 TBI k5
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    You need over 12V to run the system really, and under a heavy load, it should not drop under 12V.

    The reality is the stock alts are fairly weak at low RPM, however, they should still put out 12V+ at idle, as long as there has not been any additional load introduced to the system from stock. (huge amp, etc)

    Sounds to me like a rebuild or replacement is in order.
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Was 11.5 volts the output of the atlernator when it was tested? That's way too low. /forums/images/graemlins/shocked.gif It should put out right around 13.8 volts. To test things, connect a voltmeter across the battery with the engine running. It should read 13.8 to 14.2 volts, depending on the temperature. Colder weather should result in the higher end of the range. Next, turn on the headlights. Does the voltage drop? Even the stock alternator should be able to keep the voltage pretty close to what it was before you turned on the lights. If the voltage drops by more than 1/2-volt or so, then the alternator isn't doing its job any more.

    One of our rigs is a '94 Suburban, with rear a/c and heat. At idle, with the headlights on, both blower motors on high speed, the wipers running and the radio on, the volt meter still reads over 13 volts. /forums/images/graemlins/cool.gif
     
  4. imiceman44

    imiceman44 1 ton status

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    Another thing to check is a short.
    I had a short in my truck and the alternator was a new alternator, and I thought it was bad then I noticed that even when I have everything off when I put the negative cable on the battery post it made a little spark, so I put a clamp meter on the line and there was a draw.
    I finally found the rear window wires were shaffed and hitting on the body and when that was fixed I got 14 volts.
    /forums/images/graemlins/thumb.gif
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Isn't your 'burb running a different alternator design? I'm sure it's rated at 80+ amps, the 70's trucks were lucky to get 50. (stock some years was 37) Of course the load was less, but power windows, door locks, rear ac/heat, power seats, better stereo and so on and so on, really up the need for a better alternator. Of course, alternator technology has improved as well.

    Some of the later stock alts are 120amps, and can be fitted to our trucks. /forums/images/graemlins/smile.gif
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Even my old junk would keep the voltage up at idle. But they only had the basics, light, heat, radio. /forums/images/graemlins/wink.gif The only ones I remember really dimming the lights at idle were really old and had mechanical voltage regulators. The alternators on those things were pretty weak. /forums/images/graemlins/frown.gif
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Mine still dips down noticeably at idle, it's "newer" (80's) and rated at 87 amps as I recall, with just a radio, lights, and heater. All I have to do is bring the idle up a bit to get it to put out enough so the lights don't dim with the fan on high, but I don't really feel like it. That's a crutch anyways.

    By the time the voltage gets to the fuse panel, its around 12.5 at idle under full load, and that was before and after a rebuild, with or without the overdrive pulley. Slightly higher at the alt. of course, but the headlights, fan, and gauge lights don't pay attention to that /forums/images/graemlins/smile.gif

    Since I never owned one of these things brand new, I'd be surprised if this is an inherent problem, but it is so prevalent compared to new vehicles, that I find it hard to believe it's just the way they were produced. Of course, back then, people still purchased manual transmission trucks, and drove the 4wd ones off-road, so the buyers may not have complained about slightly dimming lights. Now they complain when an auto transmission shifts firm.
     

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