Battery / Headlight Question

[spta97]

What does it mean when at idle your headlights and instrument lights are not at full brightness, then when you hit the gas they light up to full brightness?

This happens on my Explorer, and worse on my sisters car. Is this a sign of a bad battery or alternator?

Thanks..

 

[ExplorerDMB]

Well, more than likely alternator problems. Do you have a hard time starting? Slow to start? You may be running off your battery a little which isn't exactly great. I would hook up a AMP meter to the alternator wire that goes to the battery. Also, if you can't do that - get a multimeter and measure the DC current at the battery while running. Shouldn't be lower than 13.5V (14.5V normally)

-Drew

 

[spta97]

Well, more than likely alternator problems. Do you have a hard time starting? Slow to start? You may be running off your battery a little which isn't exactly great. I would hook up a AMP meter to the alternator wire that goes to the battery. Also, if you can't do that - get a multimeter and measure the DC current at the battery while running. Shouldn't be lower than 13.5V (14.5V normally)

-Drew

The Explorer always takes a few seconds to start, my sister's car starts right up. I'll try the multi-meter thing...thanks.

 

[aldive]

get a multimeter and measure the DC current at the battery while running. Shouldn't be lower than 13.5V (14.5V normally)

-Drew

I believe he means voltage, not current.

 

[aldive]

Do you have any heavy electrical users such as an electric fan, a powerful sound system?

 

[spta97]

On my truck I do have a small amp (200 watts) on my sisters nothing. This is also happening when the stereo is off on my truck. When I pull into the driveway at night and the headlights are on the garage door they are dimmer than full bright. Then when you hit the gas to rev the engine they brighten right up. This happens when stopped at a light as well.

On my truck it is not as pronounced as on her car. I was thinking battery problem because I figured the headlights are running off of the battery power when the alternator is not spooled up to speed. When the engine is revved the alt is running at full power so the battery is fully juiced and acting as a go between. At least this is my understanding.

edit: it has been cold when I noticed this. Not sure if the same is true in the warm weather.

 

[exploded99]

spta97, you are correct about the alternator not putting out enough juice as idle to run the headlights and everything else. As a battery gets old, it often will not put out the juice it did when new, and you get noticable headlight dimming at idle since it does not compensate as well. Usually you will also start getting quite a bit of corrosion on the battery terminals beacause the battery does a lot of discharging / charging as it gets weaker. I replaced the battery on my 99 last year for those reasons - all is fine now.

So, if you are getting normal voltage from the alternator, figure on doing a battery before too long. Also, the more cranking amps a battery puts out, the better it does at keeping those lights bright.

 

[spta97]

Thanks. I had some problems with my other car and did not get a chance to look at it. However my battery is fairly new (Diehard) and I keep the felt washers and spray protectant on the terminals. The dimming does not happen as bad on my truck as my sisters car but I will measure the alt when I get a chance.