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Dimming headlights??


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14 replies to this topic

#1 fnlyfnd

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Posted 25 April 2006 - 11:26 AM

I have never noticed this before a couple weeks ago. My headlights dim when I hit the brakes. I started to notice this after i had my alternator bolt problem, so I figured it wad the tension. I now have the correct tension on it with a new bolt (that isn't broken). My lights still dim when I hit the brakes, dash lights too.

I checked the connections at the alternator and they seem to be fine, I also checked the batt. terminals and they are tight also. Could it be a loose or corroded ground somewhere? battery? dying alternator? incorrect tension??

#2 Subarian

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Posted 25 April 2006 - 12:01 PM

I would hunt for a ground that's loose or corroded.

#3 vic/se

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Posted 25 April 2006 - 03:28 PM

I have never noticed this before a couple weeks ago. My headlights dim when I hit the brakes. I started to notice this after i had my alternator bolt problem, so I figured it wad the tension. I now have the correct tension on it with a new bolt (that isn't broken). My lights still dim when I hit the brakes, dash lights too.

I checked the connections at the alternator and they seem to be fine, I also checked the batt. terminals and they are tight also. Could it be a loose or corroded ground somewhere? battery? dying alternator? incorrect tension??


D'ont worry it's normal a slight change in tension!

#4 OB99W

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Posted 25 April 2006 - 04:21 PM

D'ont worry it's normal a slight change in tension!

Why do I suspect that when fnlyfnd says "tension", the reference concerns a force applied to the alternator drive belt, but vic/se is likely referring to "voltage"? (Please understand, I'm not being critical, just making an observation about language usage.)

#5 Rodney7286

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Posted 25 April 2006 - 04:34 PM

hmmmm....my 99 OBW does the same thing, i went to 100w highs and 80w lowbeams...and 80w fog lights
i didn't notice if it did it before the new bulbs....i don't worry about it, i just don't hit the brakes :grin:

#6 ron917

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Posted 25 April 2006 - 06:29 PM

My '99 OBW also does it, especially when the fan is on high, the seat heater is on, and the rear defroster is on. I figure the 7 year old, original equipment battery is getting a bit weak.

#7 daehttub2000

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Posted 26 April 2006 - 04:17 PM

Your battery is probably a bit low.

I used to notice that on my 95 Legacy. I've since been more careful about keeping the battery cells topped off with distilled water (yes, "maintenance-free" Subie batteries are NOT "maintenance-free" (my first battery went bone dry and died). I also picked up a trickle charger to keep my battery topped off from time to time and especially in the winter when it gets cold and dark. Now it doesn't dim the lights even when I have to jump start my less fortunate friends...

#8 fnlyfnd

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Posted 27 April 2006 - 07:19 PM

Forgive me if I am wrong, but I thought the alternator runs all operations when the car is running. Essentially all the battery is used for is cranking the engine.

#9 OB99W

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Posted 27 April 2006 - 08:13 PM

Forgive me if I am wrong, but I thought the alternator runs all operations when the car is running. Essentially all the battery is used for is cranking the engine.

The battery does more than provide power for cranking the engine. In an automotive environment it acts much like a large capacitor. At idle and low speeds, the charging system isn't capable of providing a lot of current. Therefore, if the demand is high (headlights, seat warmers, window degoggers, etc. on) at low engine speeds, the battery is called upon to "fill in". The other capacitor-like function the battery performs is to filter the output of the alternator. An alternator is an AC generator with diodes that rectify the AC and "convert" it to DC; however, the DC from the alternator pulsates, and the battery serves to "smooth" the voltage.

The amount of current drawn by the brake lights isn't insignificant, and that additional load when the headlights are already on is often enough to require the battery to supply some power. If the battery isn't up to it, or if there's excessive resistance in connections due to corrosion, etc., the lights might dim. So, even if the battery terminal clamps are tight, there may be some voltage drop that could be minimized if the connections were cleaned. Or, as others have already mentioned, the battery may not be fully charged. If the alternator belt was loose for long enough, it may take a bit of running before it brings the battery up to full charge. Another possibility is that the battery has some problems, and isn't able to take a full charge or deliver full output.

By the way, there's no "free lunch". When the alternator is providing current (either to run things or to charge the battery), the energy comes from someplace, and that place is the engine. The greater the current required from the alternator, the more of a load it puts on the engine. That actually slows the engine somewhat; at road speeds you may not notice it, but at idle and low speeds it can be more easily detected. Naturally, if the engine slows, so does the alternator, enough to drop the alternator output a bit; when the brake lights come on this happens to some degree, and could be another reason for some dimming.

Of course, it's possible that the alternator just isn't putting out enough. If that's the case, it might not charge the battery fully or provide enough output to keep lights at full brilliance if the demand is high enough.

#10 Olnick

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Posted 28 April 2006 - 12:34 AM

Nice explanation OB99W. Thanks.

#11 nipper

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Posted 28 April 2006 - 09:35 AM

The battery does more than provide power for cranking the engine. In an automotive environment it acts much like a large capacitor. At idle and low speeds, the charging system isn't capable of providing a lot of current. Therefore, if the demand is high (headlights, seat warmers, window degoggers, etc. on) at low engine speeds, the battery is called upon to "fill in". The other capacitor-like function the battery performs is to filter the output of the alternator. An alternator is an AC generator with diodes that rectify the AC and "convert" it to DC; however, the DC from the alternator pulsates, and the battery serves to "smooth" the voltage.

The amount of current drawn by the brake lights isn't insignificant, and that additional load when the headlights are already on is often enough to require the battery to supply some power. If the battery isn't up to it, or if there's excessive resistance in connections due to corrosion, etc., the lights might dim. So, even if the battery terminal clamps are tight, there may be some voltage drop that could be minimized if the connections were cleaned. Or, as others have already mentioned, the battery may not be fully charged. If the alternator belt was loose for long enough, it may take a bit of running before it brings the battery up to full charge. Another possibility is that the battery has some problems, and isn't able to take a full charge or deliver full output.

By the way, there's no "free lunch". When the alternator is providing current (either to run things or to charge the battery), the energy comes from someplace, and that place is the engine. The greater the current required from the alternator, the more of a load it puts on the engine. That actually slows the engine somewhat; at road speeds you may not notice it, but at idle and low speeds it can be more easily detected. Naturally, if the engine slows, so does the alternator, enough to drop the alternator output a bit; when the brake lights come on this happens to some degree, and could be another reason for some dimming.

Of course, it's possible that the alternator just isn't putting out enough. If that's the case, it might not charge the battery fully or provide enough output to keep lights at full brilliance if the demand is high enough.


What he said with one other point. If you have a car that is heavily laden with electronics (like power glove box doors ... dont laugh its only a matter of time). ac on or all the winter heaters on and your sitting in traffic for a long time and have not moved, put the car in neutral and give it a little gas for a few minutes every so often. The altenator starts making full power about 1000 rpm. It is possible to kill the battery while the car is idling in traffice for the reasons stated above.
What you may think are cars pulled over due to overheating is becoming more and more are dead batteries.

Altenator 101
http://www.1stconnec.../alternator.htm

nipper

#12 fnlyfnd

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Posted 28 April 2006 - 10:45 AM

thanks for the info guys

#13 Tiny Clark

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Posted 28 April 2006 - 03:32 PM

Cars with anti-locks brakes draw power for that system when the brakes are applied. Ever notice how your lights dim when the system chatters the brakes?

The reason the brake idiot light comes on when the alternator isn't putting out is because the battery voltage has dropped below about 10.5 volts.

It's nothing to worry about when sitting at idle.

#14 Wayne Boncyk

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Posted 28 April 2006 - 04:42 PM

Hey, it didn't dawn on me until just now to pull your profile to look at the year and model that you're driving! If this is related to your '96 OBW, you might not be aware that there was a factory recall of late '95 thru mid '96 Legacies (including Outbacks) due to a poorly designed alternator. I know, 'cause I had mine fail BEFORE the recall was made and I had to go through the hassle of submitting my receipts for the replacement in order to get reimbursed. As it turns out, the diode circuit in these offeding units wasn't properly designed, and they were prone to heat failure. Since it was a heat-related failure, often during testing (when the units didn't get as hot as they did during normal road use) the problem couldn't be seen. The symptoms came on gradually -- first a lower than usual voltage output at idle, followed by no voltage output at idle. Eventually they even fell below spec voltage at higher RPMs, and that is when we all started seeing things like the dimming headlights, radios that would mysteriously quit for a while and then come back on, etc. Finally the units just failed outright and eventually the battery couldn't supply enough charge to keep the car running. I'll never forget when and where that happened to me - as I tried to make a left-hand turn across 3 lanes of opposing traffic!

Bottom line, if you have one of those few remaining vehicles with an original alternator from this period, check the recalls and see if your VIN is within those vehicles affected. You may be due for a new alternator, at Subaru of America's expense!

#15 fnlyfnd

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Posted 28 April 2006 - 07:04 PM

the alternator has been serviced for the recall, had a carfax to make sure.




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