Why do lights dim when I press the brake?

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Dec 16, 2006
East Coast
Once in a while, my lights dim a bit when I press the brake pedal. The engine is running when the car does this. I notice this at parking lot speeds when the car is first started. Is this a sign of a weak battery or alternator? The car is a '99 Nissan Maxima w/ 131k miles with the original alternator.
Maybe with time corrosion on the grounds and wires builds up that causes this, making the wires not provide the factory spec resistance?
You probablly have a bad ground wire somewhere, or a weak battery. Have your electrical system tested and/or the ground wires checked. (Dads a mechanic)...
luckily you don't have a Corvette-the lights dim occasionally when idling--without stepping on the brake. It's included in the price of admission Steve
Odds are it's the battery. The battery is a Nissan battery that is about 5 yrs. old and has approximately 70k miles of use on it so far.
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5 years on a Genuine Nissan/Infiniti battery is overdue for replacement based on Maryland climate- I get almost exactly 42 months out of those batteries{with 27 years experience driving Nissan/Infiniti produts in Maryland}
Normal at idle, the alternator can't keep up, especially if you're running the electric radiator fan/AC. When the alt can't keep up the system voltage drops until the battery can start supplying it... which, chemically, has to be a couple volts. Stick it in neutral and rev to 2500 RPM then tap the brake, I bet the lights won't flicker much if at all!
I think you've nailed it! I assume there is nothing to worry about. Perhaps the battery is on its way out. I do service the battery once in a while by adding distilled water; making sure the plates are covered.
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GM is my experiences are known for bad grounds and [censored] alternators... I would check these first... Have your mechanic also do a Load Test on your battery and/or charging system if he is properly equipped.
I'll probably do a load test at AutoZone for free. The car in question is a Nissan which was 100% assembled in Japan. The alternator is most likely a Hitachi unit.
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I have observed something similar. When road conditions are icy, if I am driving at night and have to brake, but the road is slippery so ABS kicks in, I notice the headlights dim a little while the ABS pulses. I don't think it is anything worth worrying over. You may still want to replace that battery, though.
My old '94 Corsica has done this ever since I put a cheap Advance Auto Parts reman alternator on it six years ago. Never caused a problem otherwise. I replaced the battery last year- made no difference. I think a new alternator would probably eliminate the problem, but I just don't care.
 Originally Posted By: Spartuss
I'll probably do a load test at AutoZone for free. The car in question is a Nissan which was 100% assembled in Japan. The alternator is most likely a Hitachi unit.
Sorry I thought you said it was a GM... Then if it's a Nissan I would check the Battery or Alternator... More likely the Battery. Sorry bud..
There is an extra load from the brake lights. Not unsubstantial, either. Most cars are pretty good about driving the alternator at idle , but I can always tell a drop or change with the brakes on vs. off. I use an underdrive pulley on my crankshaft, so this is more noticeable on my ZX2. One more odd thing to check is the power brake booster - it may be leaking. To check it, clamp off the vacuum line to it, and test it by depressing the brake. If clamping fixes it, you have a problem there. The bottom line is that some drop is very expected and normal.
I agree with testing the battery and also the charging system. Many auto parts store will do this for free.
It might be nothing and can't be fixed. My Camaro did this intermittently as well, maybe 1 out of 3 times when I hit the brakes at night. Within a year of noticing it, both the battery and alternator needed to be replaced. I figured it would stop after that...nope still happened anyway. So, I stopped worrying about it. The brake booster and everything else was fine with the car as well.
Every GM I've ever owned has done this. If it starts doing it excessively, time to check the alternator.
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