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Projects Wiring Alt issue? Help

Discussion in 'Traditional Hot Rods' started by porkchop4464, Aug 20, 2016.

  1. porkchop4464
    Joined: Jan 20, 2009
    Posts: 870


    Anyone ever have this? Last night, took the car out for an illegal spin around the block (have reg and insured, just no inspection), I noticed that when the car is idling, back in driveway after drive, the rear tail lights brighten and dim with the lope of the idle. Also, when the car is off, the brake lights and tail lights(night lights) are distinctly different, but once running, it is hard to tell the difference? Alt charging into system?I bought one of the Speedway rip- off diode wires, just so I didn't have to run a parking light in the alt line, but the lights seems to be surging with the alt? Am I getting charge through system? Ground issue? Help
    Last edited: Aug 20, 2016
  2. porkchop4464
    Joined: Jan 20, 2009
    Posts: 870


    Oh, new alt, with 13.5 volts on gauge. New bat as well (5 months old).
  3. 13.5 volts is a little low, my GM 3-wire runs at 14.1 all the time. Best guess, the regulator is bad or you have a bad ground. Make sure the engine has a good ground to the body or chassis, I like to do both.
  4. what type of bulb? diode? standard 1157? I can see the lights would "follow" the lope of the idle but should still be a difference between taillight and brake light. Some of the cheaper LED bulbs don't have much difference. Try different bulbs

  5. garyf
    Joined: Aug 11, 2006
    Posts: 217


    What kind of alternator in what kinda car ? Possible causes-- pulley diameter, idle too low, loose belt, body ground.
    Cosmo49 likes this.
  6. porkchop4464
    Joined: Jan 20, 2009
    Posts: 870


    Thanks gang, will check grounds. A good place to start
    firstinsteele likes this.
  7. I've had bad bulbs do all sorts of funky things to me and they're often overlooked. But do the grounds first. What type of alternator is it?
    34toddster likes this.
  8. trollst
    Joined: Jan 27, 2012
    Posts: 1,900


    Battery probably not fully charged, drive it more, longer periods, sounds to me like the battery is a little low, sitting in the shop, short runs, stuff that doesn't allow the battery to be fully charged. I think you're worrying too much, at 13.5 volts it's charging, give it time to catch up and settle in.
  9. 325w
    Joined: Feb 18, 2008
    Posts: 5,451


    He says it showing 13.5 volts. Should be charged.
  10. SLAMIT
    Joined: Sep 9, 2002
    Posts: 930


    325w I think he is referring to the charging voltage at idle not the current battery state of charge while engine is off. A low battery will do funky things, and I think that trollst is right on. let it fully charge and see what happens. 13.5 is a touch low and poor connections on both the positive and negative side will also give you fits. recheck all your conections and make sure they are tight and very clean. Have Fun!
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 28,164


    It should be charging around 14.2 to keep up with the lights, run the engine and still put a charge back in the engine. If the battery is all the way up to snuff charge wise the 13.5 may be all that is demanded though.
    I'm with the line of thinking that either the engine idle speed is a bit low to fully work the alternator or the pulley setup isn't spinning the alternator fast enough at an idle = smaller diameter drive pulley on the crank than on the crank of what the alternator was intended
  12. 14.1 to 14.5 is the range I like, mine charges at 14.1 and the battery is always up on the charge.

    I would swap in a known-to-be-good battery before going too far with things.

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