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  •  » FYI Ford, Classic Mustang Tech Discussion
  •  » interior lights getting brighter, darker when motor is running...

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    5/19/2014 7:38 PM  #1

    Pony1965
    Unregistered

    interior lights getting brighter, darker when motor is running...

    I have a '65 fastback with a 289 2 barrel a/t... drivetrain is all original as is wiring harness.  When the car is started and gets warm, the interior lights often get brighter and dimmer when the car is idling.  When driving the car the same thing happens, whether accelerating or standing still... Voltage regulator is new, didn't help the issue, battery is new, alternator fairly new... the cycle of bright to dim is pretty regular and doesn't change under acceleration, or when revving motor in park or neutral... Any ideas what's causing this or how to fix this issue? Thanks!

     

    5/19/2014 7:41 PM  #2

    terry
    Senior 24-7 FYI Addict

    Offline

    From: vandalia, illinois
    Registered: 4/01/2013
    Posts: 1,850

    Re: interior lights getting brighter, darker when motor is running...

    one suggestion, check ALL your grounds.  Like battery to engine, engine to body.  Poor grounding can cause some strange things

     

    5/19/2014 9:43 PM  #3

    MS
    Forum Originator

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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 11,742

    Re: interior lights getting brighter, darker when motor is running...

    Mine did that exact same thing a few years ago.  I found a bad place in the big wire from the alternator to the solenoid harness.  Had to untape the harness to find it.  I think it was the big yellow wire but has been many years.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    5/20/2014 6:22 AM  #4

    wsinsle
    Senior 24-7 FYI Addict

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    From: Belle Vernon, PA
    Registered: 4/01/2013
    Posts: 1,664

    Re: interior lights getting brighter, darker when motor is running...

    Just elaborating on what Steve and Terry were saying. You can measure across the battery and from the alt. out put and see if the voltage is varing. This may tell if one of the main grounds have a bad connection. Check voltage at the fuse block to ground. Check fuses that feed the lghts, is it tight fit in the fuse holder. What lights are actually dimming, dash dome?, I think the dash lights are connected to the dimmer on the head light switch. Turn the dimmer back and forth, may help clean it up. Switch may be going bad.

    Last edited by wsinsle (5/21/2014 4:46 AM)

    If this forum can't fix it, it isn't broke.
     

    5/20/2014 7:50 PM  #5

    Pony1965
    Unregistered

    Re: interior lights getting brighter, darker when motor is running...

    All good suggestions to get me started... and in response to wsinsle, all the interior lights seem to get brighter/dimmer simultaneously... the dome, dash cluster lights, footwell lights under the dash and rear interior lights in the quarter sail panel trim area... 
     

       •Thread Starter
     

    5/20/2014 7:56 PM  #6

    R5CYA
    FYI FORD Member

    Offline

    From: San Bruno, Ca.
    Registered: 5/18/2014
    Posts: 57

    Re: interior lights getting brighter, darker when motor is running...

    sure sounds like the dimmer switch. did you crank it back and forth to see if it helped any?

     

    5/20/2014 8:26 PM  #7

    MarkinSC
    I FYI 24-7

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    Registered: 4/01/2013
    Posts: 444

    Re: interior lights getting brighter, darker when motor is running...

    Pony1965 wrote:

    I have a '65 fastback with a 289 2 barrel a/t... drivetrain is all original as is wiring harness.  When the car is started and gets warm, the interior lights often get brighter and dimmer when the car is idling.  When driving the car the same thing happens, whether accelerating or standing still... Voltage regulator is new, didn't help the issue, battery is new, alternator fairly new... the cycle of bright to dim is pretty regular and doesn't change under acceleration, or when revving motor in park or neutral... Any ideas what's causing this or how to fix this issue? Thanks!

    "alternator fairly new"........    sounds like a diode going bad in the alternator.   Take alt to autozone and have them test it for free.

     

    GET HER DONE BULLETBOB!!! ..
     

    5/20/2014 9:26 PM  #8

    jkordzi
    I FYI 24-7

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    From: Dallas, TX
    Registered: 12/28/2013
    Posts: 923

    Re: interior lights getting brighter, darker when motor is running...

    That seems to indicate to me that either your alternator is going south or you have a bad connection.   Can you place a volt meter across your battery at idle and when the engine is at about 2000 rpms and tell us the readings?  You should have at least 13.5 volts.  Do you have the main ground from your engine to the firewall hooked up? 

    Cheap, Fast, Good:  Pick Any Two
     

     

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