Why are my headlights dimming when I hit the brakes? - Subaru Outback - Subaru Outback Forums

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Old 09-09-2009, 02:05 PM   #1 (permalink)
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Default Why are my headlights dimming when I hit the brakes?

Has anyone else had the experience of dimming headlights when the brakes are applied? I have a 1998 OBW. It happens every time I hit the brakes, even softly, and it is really quite a dramatic dimming effect. What in the world even causes this? I mean the brakes are hydraulic--why are they sucking a massive amount of power from the electrical system when I use them? Wierd.... Thanks in advance,

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Old 09-09-2009, 03:01 PM   #2 (permalink)
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It's a known thing, almost a They All Do That. Snoop around your electrical connections and look for things like corroded terminals, including grounding connections on the body.

The brakes should just draw enough power to activate the brake lights, it wouldn't matter how hard you apply them. Make sure those lights are working OK and there's no goofy incorrect connection like an incorrectly-installed bulb, which can sometimes happen.
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Old 09-10-2009, 05:38 AM   #3 (permalink)
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It's the brake lights causing the problem, not the brakes themselves.

It's possible the two lighting systems share a common ground or other distribution point. The positive sides should be fed from separate fuses, so from the fuse block on they should be independant.

It's possible your battery or charging system is the culprit. If you can put a voltmeter on the battery terminals and watch the voltage while someone applies the brakes, you should see very little change in voltage (engine running).

Put on one of the overhead dome lights, and see if they dim when you apply the brakes.
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Old 09-10-2009, 08:18 AM   #4 (permalink)
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I have the same problem but it has been like this for 2 years now and the car runs fine. I thought it was something really bad, but it just seems to be normal. I even changed the battery (cause it died on me) and even with the new battery, it still did it. I checked the voltage across the battery terminals and it was to spec.

Other things that I notice as well are that when I turn my headlights on, you can hear a slight change in the rythem of the engine, as if there was a loss of power to the spark plugs or something... I'm still getting great gas mileage though ever since I changed out the Knock Sensor (28 highway / 21 city), so it's running ok.

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Old 09-10-2009, 08:54 AM   #5 (permalink)
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Now, if you have ABS, when the ABS works there is a dramatic increase in load on the electrical system.

You'd probably notice the pulsing in the brake pedal, but you could pull out the ABS fuse and see if the dimming decreases, just to make sure there isn't an ABS problem. Unlikely, but possible.


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Old 09-10-2009, 09:19 AM   #6 (permalink)
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Clean the battery posts even if they don't look like they need it. When lights dim under a slight increase in electrical load it is a sign of amp starvation.

Corroded lamp terminals could also be the culprit, but not likely in my experience.
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Old 09-10-2009, 09:52 AM   #7 (permalink)
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i had a friend who found the continuum transfunctioner to be faulty

it was a $15 part i believe
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Old 09-10-2009, 01:30 PM   #8 (permalink)
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Quote:
Originally posted by Hooray
i had a friend who found the continuum transfunctioner to be faulty

it was a $15 part i believe

Are you sure it isn't just a misadjusted frammis on the bloviator valve? (I believe you need a left-handed Allen wrench for that)
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Old 09-10-2009, 03:43 PM   #9 (permalink)
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There was a video supposedly from a big electronics firm like RCA or GE or some other well known name, a guy in a suit talking about their latest product. It was so full of made up parts and technology, I don't know how the guy got through it with a straight face. Funny stuff if you're in the business.

[edit] Here it is:
http://www.youtube.com/watch?v=RXJKdh1KZ0w
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Old 09-11-2009, 07:49 AM   #10 (permalink)
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check batter terminals, and grounds
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