Lights dim when press brake

Posted by 98mitsubishigst, Jan 9, 2011
Electrical Tech - Wiring, turbo timers, electronic boost controllers, gauges, fuses, wiring harnesses, wire tucks, car audio, and all other electrical component discussions.

  1. 98mitsubishigst

    98mitsubishigst Proven Member

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    Pittsburgh, Pennsylvania
    So i noticed today that if im sitting at idle somewhere, if i turn my heater on to number two and have my lights on; my rpms will drop by like a 150.

    Also when i press the brake down, my headlights will dim along with my dash lights.

    It doesn't dim them alot but it doesnt enough to notice..

    The Alt is new, all new fuses, no system ether and nothing running straight off the battery... Do you think it could be my terminals of the battery?
    #1
  2. 95talongirl

    95talongirl Supporting Member

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    On a Jet Plane, Illinois
    Could be; or could be a bad ground. I had a similar problem except that the lights would dim an the coolant gauge would peg hot when stepping on the brake. Ended up running a new ground inside the car and that took care of it.
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  3. 98mitsubishigst

    98mitsubishigst Proven Member

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    hmmm. How many grounds does our 2gs have? and do you know if theres a section on here or a old post with each ground placement?
    #3
  4. spawn87

    spawn87 Proven Member

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    check alt and battery wires to c if they are loose. also when idleling turn on your dome light if its flickering your alt is bad.
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  5. adamsg

    adamsg Probationary Member

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    i had the same problem but my radio would turn off if i had too much stuff on. last place i checked was the terminals .. i could move the negative with my hand
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  6. FearlessNick

    FearlessNick Proven Member

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    Casa Grande, Arizona
    Bad ground some where. Or the break boosters use a lot of vacuum I notice. When I push on my breaks my idle swoops up and down. But its just trying to suck in a lot of vacuum. Maybe look in too getting a aftermarket break booster.
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  7. 98mitsubishigst

    98mitsubishigst Proven Member

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    Pittsburgh, Pennsylvania
    No dome light, sunroof.

    i can move my battery negative with just my hands.. i remember i forgot to tighten them last time i replaced battery when she died..

    think that could do it? i mean im gonna fix it tomorrow but since im on here ill just ask haha
    #7
  8. born2tune

    born2tune Proven Member

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    i partially agree on a bad ground, but if you find out thats not the problem then maybe you might have a bad "new" alternator or a worn alt. belt or both. causing it not to crank enough amps to power all of the above. test it!
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  9. Tyeler18

    Tyeler18 Proven Member

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    Casa Grande, Arizona
    Could be a bad "new" alt like ^^^ said. Also loose terminals will definitely do it. Get them tightened.
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  10. spawn87

    spawn87 Proven Member

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    sacramento, California
    well you still got the lights on your rear view mirror dont you? thats your "dome light"
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  11. 98mitsubishigst

    98mitsubishigst Proven Member

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    Pittsburgh, Pennsylvania
    ooo i getcha. ill check it tonight.
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  12. knochgoon24

    knochgoon24 DSM Wiseman

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    Rochester Hills, Michigan
    You'll always notice some dimming of the lights. Our factory alternators are really weak, 65 amp units (Canadian market: 75 amp). By the time you add up all the power consumption going on while your car is running, the sudden draw from brake lights and the drop in rpm usually associated with slowing down is enough to make the lights dim a bit. This can be more pronounced if you run an underdrive pulley.

    To minimize the affects, check your grounds and make sure you battery terminals are clean and the cables are on tight. Some people see some reduction in the dim by going to a better battery. It also helps to get an upgraded rewound alternator that puts out more amps. I saw a nice improvement by running a 140 amp unit. Our ground cables are now 12 years old at the youngest. I also saw some improvement by making new ground cables for all the existing ones. You may also want to check your cable coming off the alternator itself. It's actually two 8ga wires run in parallel. I replaced that too when I installed the higher output alternator.
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  13. jonbusa

    jonbusa Proven Member

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    I would definatly check into putn a bigger alt on it
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  14. Black Widow97

    Black Widow97 Supporting Member

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    Sioux Falls, South_Dakota
    Could be bad battery, bad ground wire, or the new alt would be bad as well.
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  15. Zdrummer15

    Zdrummer15 Proven Member

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    York, Pennsylvania
    This post sums it up perfectly. In a lot of cars you'll see some dimming of the headlights or interior lights when you are drawing a lot of power and do something else like push in the brake pedal or hit the window button. As long as it's not excessive and very noticeable, I wouldn't worry about it. Otherwise do what is mentioned in knochgoon24's post.
    #15

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