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Headlights Dim When Brakes Applied


CanadaCraig
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Hi Guys!! :)

Still nothing to update re acceleration 'problem'. I think I'm hoping 'it' will just fix itself. But I WILL update everyone as soon as there's something to update. [i might ask someone to drive my car to see what HE thinks!!

Now... to the question.

Whenever I apply the brakes - the headlights dim but almost immediately become bright again. I often wonder if any oncoming traffic think I'm flashing my lights at them - but I'm not sure how obvious it is to those facing my car. What could be causing this? [or is just a typical Lexus 'thing'?] Apparently the battery was replaced just a month before I bought the car - which means it's now about 14 or 15 months old.

Thanks,

Craig!! :)

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Remove the battery connections, clean and burnish, wirebrush, the connections and the battery posts, repeat each spring or as required.

When you step on the brakes the engine is at idle or is rapidly headed in that direction. So the alternator output will start to "fall off" accordingly, AND you just added a fairly large electrical load via all those brake light bulbs.

If the battery connections were good, and the battery fully charged, as well it might still be, the battery would instantly take up the slack, fill in the "gap", and you wouldn't see (as much of) a dimming and then recovery as the voltage regulator ramps up the alternator output.

And a small note of caution..

Some of what you describe is normal, SOP.

In order to maintain a good level of battery charge the alternator output is often a volt or 2 above the battery charge level, so as the engine goes to idle and the charge rate declines, and the brake light come on, you will often see the headlights dim slightly and then recover as the voltage regulator reacts to the lower system voltage as a result of the alternator reduction in RPM.

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Hi SKperformance and wwest!! :)

I appreciate the responses.

I like how you - wwest - explained 'it' in such a way that it made perfect sense. Just maybe - my alternator is ok afterall. Once the snow storm passes - I'll take your advice - SKperformance - and at least check all the connections. [and replace if necessary]

Thanks again,

Craig!! :)

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What you described is exactly what happens to my ES when I have the headlights on. Only for a second the lights dim and afterwards they are back to normal. I too wonder if other drivers think I'm "flashing" them. I asked my service advisor about this he said that it's normal because of added load caused by the brakelights (like what wwest said), and he also informed me that the alternator was replaced in 2002 or 2003 (forgot actually). Checking the wiring and battery posts is a good idea though.

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I have the same problem. In my case, the battery needs replaced (It barely turns the car over) . I am replacing the battery in the morning. If it is not the root cause and the problem still exists, I will post again with other potential sources of the problem that I discover from my car.

As a side note, the car is a 97 LS400. It replaced a 91 LS400 I totaled over the holidays.

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I have the same problem. In my case, the battery needs replaced (It barely turns the car over) .  I am replacing the battery in the morning. If it is not the root cause and the problem still exists, I will post again with other potential sources of the problem that I discover from my car.

As a side note, the car is a 97 LS400. It replaced a 91 LS400 I totaled over the holidays.

Test the battery before having it replaced...make sure you're not taking the shotgun approach.

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I have the same problem. In my case, the battery needs replaced (It barely turns the car over) .  I am replacing the battery in the morning. If it is not the root cause and the problem still exists, I will post again with other potential sources of the problem that I discover from my car.

As a side note, the car is a 97 LS400. It replaced a 91 LS400 I totaled over the holidays.

Test the battery before having it replaced...make sure you're not taking the shotgun approach.

Good sugguestion. Had the battery tested before progressing any further. The handheld device at Autozone indicated a good battery. The cells appears to be full and ok. Upon further visual inspection however there was a good size buldge at the base of one side of the battery. I believed it to be suspect enough for me and just replaced the battery. My car carries my two year old and I don't take any risks of inconvenience with her. No dimming problems noted after replacement.

HTH

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I just replace the Battery in my Lexus. I went to Costco and the only one they had of that size was a deep cycle marine battery. I had measured the size and found one that was slightly narrower but had 900 CCA (cold cranking amps) as opposed the the 650CCA that was in the car. I only replaced it because it was dead at the airport after a wicked cold weekend in Newark and my wife had to get it jumped.

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