lights brighten when accelerating and dim at idle

Discussion in 'Interior/Electronics' started by 1975-RN23, Jun 24, 2018.

  1. 1975-RN23

    1975-RN23 Newbie

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    so had a new problem pop up while i was test driving my truck, all my interior lights and head light get a lot brighter when im driving, and when im at idle the get super dime. i have every thing stock right now, since then i have replaced the voltage regulator, but had little to no change. I put a voltmeter on the battery and it read 12.3 at idle with out load, turned on all the light with little change to the voltage. when i used the carb linkage to accelerate the engine the volt meter read up to 15.3 volts. a buddy of mine thought replacing the alternator might fix it.
     
  2. Tuhd The Troll

    Tuhd The Troll Veteran

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    well, my '82 has this problem, but I don't think it gets to 15.3. It just idles too damn low for the aftermarket GM alternator to start juicing the battery. I did a swap.

    If everything is stock, either your voltage regulator is going out, the alternator is going out, or one of those plus the battery has a bad cell.

    Not an easy to diagnose situation, but with some time, troubleshooting, and documentation of your findings, you can figure it out
     
  3. jetas

    jetas Grand Toyotaholic

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    As stated above its probably the alt/regulator/battery taking a dump. Im going to say regulator. It shouldnt rise to 15v.

    Dont have more input since I have a 100A GM 3 wire alt on mine.
     
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  4. Clay

    Clay Member

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    They both are right. it's likely a combination of or the alternator/regulator. If you have an oily engine bay, taking the alternator apart and thoroughly cleaning it may save you some bucks. #1 killer of alternators is oil in my experience.
     
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  5. 1975-RN23

    1975-RN23 Newbie

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    I have replaced the voltage regulator without a change. Got someone else to apply power while i read the volt meter. It got up to about 17 volts. Found out the battery isn't fully charged not sure if that would could be the over charge problem. i have a new alternator on the way. Also checked all the wire from the alternator, didn't find any breaks in the line or bad connections. Thinking it might be that the voltage regulator is do its job but the alternator is pushing to much power, and its able to get the volts lower for some reason
     
  6. Clay

    Clay Member

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    Best of luck with the new alt. Though if that last bit is true, the new voltage regulator may now be bad.
     
  7. 1975-RN23

    1975-RN23 Newbie

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    The new voltage regulator did nothing to the voltage. New alternator comes in tomorrow. That's the last thing it could be as far as i can think of.
     
  8. Tuhd The Troll

    Tuhd The Troll Veteran

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    If your battery is spiking to 17 volts, that says to me it's likely a combination of bad internal cell, possibly bad ground(s), and/or the regulating diodes inside the alt itself.

    Last two times I had this problem it was a bad cell in the battery, which would cause the alt to jump up production to get power in. I confirmed this by putting it on a charger without the engine running, and it would still spike while on charge. Usually took a couple minutes to spike while on slower charge, and almost instant voltage spike on high charge.
     
  9. 1975-RN23

    1975-RN23 Newbie

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    So i have replaced the alternator. I tried the new alt with the old reg. At idle with head lights on it will change at 13.4, when i trottle it to 2000rpm i get it to 14.8-15.3 at 3700 rpm its at 15.8 which is to high from my under standing. I then tried new reg with the new alt and at idle its 12.4 with light on, it will get up to 12.9 at around 3700 but doesn't exceed 13.0 which is now to low. Had the battery tested and its good. Any idas?
     
  10. jetas

    jetas Grand Toyotaholic

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    How are all the wires?
     
  11. 1975-RN23

    1975-RN23 Newbie

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    They are all good didn't see any brakes or bad connections.
     
  12. jetas

    jetas Grand Toyotaholic

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    Wonder if it could be a grounding issue
     
  13. 1975-RN23

    1975-RN23 Newbie

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    We checked all the grounds. I could add some. What would be a good place to add one or two?
     
  14. YOguyDA

    YOguyDA Addict

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    While taking your readings with your multimeter, maybe you could utilize some jumper cables as temporary grounds?....to see if there is any effect.

    Try body to block, block to frame, frame to body, negative terminal on battery to block/frame/body, etc. Might tell you if a ground is weak, just by reclamping instead of physically bolting new ones in place?

    Also, if you have an external regulator, does that ground to the body? Could throw clamping that into the equation, lol.
     
    Last edited: Jun 30, 2018
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  15. 1975-RN23

    1975-RN23 Newbie

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    I haven't had a chance to add more grounds, but i have been driving it the past few days with no problems. I got another regulator to try too
     

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