Thinking a bad alternator, but don't have the christmas tree effect

Discussion in '80-Series Tech' started by jasonb75, Dec 8, 2010.

  1. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    My 91 with 159k, is having problems with the charging circuit. I can drive all day without lights or heater on, but when I turn the lights, front, and rear heater on, my voltage gauge drops and the truck eventually dies.

    I have had an alternator go bad on an 85 4runner, and had the Christmas tree effect on the dash.

    Has anyone ever had an alternator go bad without the dash lights all lighting up? The only light that stays on is the check engine light, but that stays on all the time. Brake, A/T temp, nor any other lights stay on.

    Probably unrelated, I had to smack my started the other day because I had a stuck plunger, but I don't see how that would mess with the charging system.

    One other thing I did try (I know it isn't a good thing to do) was removing the positive wire off the battery terminal and the truck died. In my limited knowledge, that was a good indication that the alternator was bad.

    Do you all think that the alternator is bad like I do?

    Thanks, Jason
  2. rockytop

    rockytop

    Messages:
    44
    Location:
    Texas Hill Country
    Jason,
    Do you own a multi meter? You need to check the voltage output of the alternator whith engine running. should be about 14 volts. Then you can check the amp draw on the system when you turn on lights, etc. If the alterterator is charging, then you probably have something wrong with the light system, like a short and are pulling too many amps. Also check condition and age of battery.
  3. Eicca

    Eicca Professional Noob

    Messages:
    1,732
    Location:
    Rexburg, ID
    IIRC, the 91-92s don't do the Christmas tree thing. At least mine didn't when alternator #3 completely seized...
  4. 96bruzer

    96bruzer

    Messages:
    117
    Location:
    Southern California
    Had same thing happen to me before. First issue was the red battery light in my dash was lit when my alternator went bad, replaced it and was fixed. Second issue was just like you describe and my battery was bad.
  5. 96Cruiser13

    96Cruiser13

    Messages:
    1,459
    Location:
    Burlington NC
    sounds alot more like alternator than battery, but get a multimeter and check with lights off, then lights on
  6. arndog123

    arndog123

    Messages:
    3,320
    Location:
    Everett, WA
    If it's the alternator it probably just needs new brushes.
  7. landtank

    landtank SILVER Star

    Messages:
    17,251
    Location:
    Groveland MA
    I worked on an 80 that when you turned on the radio the engine would die. I replaced both the positive and negative cables and it was fine. The actual copper wire was pretty much done and wouldn't pass amps.

    good luck
  8. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    The battery is a brand spanking new 850cca battery, and when I put that in, I replaced all the cables and terminals just as a preventative measure.

    I am an electronics technician, and all my electrical tools are are work, I will bring my DMM home from work and check everything out tonight.

    Does my 91 have a battery idiot light, or just the gauge? If it has the light, I have never seen it come on. If it does, it may ne time to take the dash apart again and change some bulbs.

    Thanks everyone for the help, this website is a godsend.
  9. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    Never mind about the battery light, I went out to my truck and saw it. With the key on it is lit, but goes off when it starts.
  10. mikgrail3

    mikgrail3

    Messages:
    251
    Location:
    Arizona
    Mine did when my alternator was bad. I believe it happens after the battery starts to lose its charge below the 12v. When my Christmas tree lights happened, if I recall correctly, The battery was reading in the 11's.

    To the OP. Your alt might be bad, but still maintaining enough of a charge to keep the battery charged to the 12.3v it needs, but not enough to get to the 14v needed to maintain lights and such. Test with a meter for the output of your alt. Autozone will test for free as well.

    And as a reminder to all, get an OEM alt as a replacement. I went through 2 AZ warrantied alternators before finally going OEM.
  11. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    Ok, this seems weird, it seems to be working properly now. Battery voltage with the truck off = 12.5 volts

    battery voltage with the truck running with everything off 13.7 volts

    battery voltage with everything on - headlights, front heater, rear heater, driving lights, front and rear wipers, and radio = 12.5 volts at idle, but when revving to approximately 1k rpm goes up to 13.7 volts and gets stable there. back to idle and back to 12.5 volts.

    voltage at the alternator - from large wire with the rubber cap to the alternator frame = 14.07 volts.

    from what I am reading here, everything seems to be doing ok now. maybe me smacking the hell out of the alternator with a 1 1/8" combination wrench out of frustration did something?

    I may just buy a now OEM alternator and have as a spare in case something happens while out of town. Toyota doesn't charge a core do they? I figure not because they don't sell reman.

  12. poppy walnut

    poppy walnut

    Messages:
    259
    Location:
    KY
    Then you should have known better than load dump like you did.

    Regardless, am I reading that right? You have a near 1.5v voltage drop from the ALT to the batt?
  13. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    Not 1.5v the voltage drop is .37 volts. Alternator is reading 14.07 and at the battery while running it is 13.7.
  14. poppy walnut

    poppy walnut

    Messages:
    259
    Location:
    KY
    So is the alt output 14.07V and with nothing on it's 13.7V at batt. Then it's 14.07V output and everything on it's 12.5V at batt?

    What is your idle speed?
  15. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    Idle speed is dead on 750 rpm with a/c off.

    That is correct poppy, 14.07 at alternator at idle with everything on, and the battery shows 12.5v on the meter, until rpm's come up then goes to a max of 13.7. With nothing on it shows 13.7v on the battery no matter what engine speed is. Load tested the battery with an old school battery tester, and held the switch for 15 seconds and showed no drop in voltage on the meter.
  16. jonheld

    jonheld

    Messages:
    4,961
    Location:
    New Jersey
    With the alternator in a charge cycle you should see approx 14.4 across the terminals of the battery with the engine running at idle. Either your alternator isn't putting out what it should, or your cables are faulty.

    You stated that your alt is putting out 14 at idle at the output lug. That is low IMO.

    Its also possible you have a shorted cell in your battery dragging down the system. Yes I know you said your battery was new, but stuff breaks all the time. Have it load tested or you can do a simple load test with a meter. Without the motor running monitor the voltage on the battery as you turn on all your accessories. Your voltage will drop into the low 11s or possibly 10 and change. With all accessories switched off, it should recover fairly quickly to over 12.5.
  17. rhyary

    rhyary SILVER Star

    Messages:
    1,403
    Location:
    Albany, NY
    Unless the 91 is different than 96.
    Voltage when the car is running should be 14.x after start and it should settled down to 13.x when idling. You should see no 12.x when the truck is running.
    You should see 12.x only when engine is stopped. With the engine stopped 13.x should settled into 12.x after awhile. If nothing running with new battery it should take few hours to drop from 13.x to 12.x.

    New battery should settled into 12.6 overnight or 12.5V with no accessories.

    Mine settled into 12.3V with number of gadgets running.

    Hope this helps.
  18. jasonb75

    jasonb75

    Messages:
    47
    Location:
    Pensacola, FL
    When it is idling, without anything on, it shows 13.7. The only time it shows 12.5 is with everything on, bump the idle just a hair and back to 13.7 nearly instantly.
  19. rhyary

    rhyary SILVER Star

    Messages:
    1,403
    Location:
    Albany, NY
    Sounds normal for a stock alternator.

    I upgraded mine to the 150amp alternator using photoman bracket. Now it is solid 13.7/13.8 no matter the idle or what accessories are on.

    Rami

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