This is not stereo related, but I am curious about my electrical system. I have had no problems with my cars eletrical system yet.

Last night I was cruising down a country road with my highbeams on. There is a stop sign so I Push in the cluth and coast a little bit before shifting into fourth. As the engine speed drops to idle, the head lights dim, and them dim a fair bit right as the car reaches idle speed (it almost seems as though the engine slows down too mcuh). Then the rpms settle in a little bit higher and no problems.

I tried this again with everything off - no stereo, fan defrost etc. The only thing on is the lights. The lights still dim right as the rpms bottom out.

Is this normal? or does it indicate a future battery or alternator problem? Is it normal for my engine to dip too low as it returns to idle then settle in at a comfortable rpm?

I would not say it is 'normal' but I would say it is 'typical' for a car with some miles on it.

In my car when stopped with the lights on it definitely idles rougher than with them off. At idle the alternator puts out less and the drag from it affects the motor more - especially on a 4cyl which has little torque at idle anyway.

I wouldn't be concerned. And IMO any battery that is 3+ years old could be due for replacement any time - be it a week or 3 more years.

I think the idle isn't set right due to the age of the car, so the rpms fall too law when you push the clutch in and your alt isn't able to produce power at that low of RPM. If the alt charges your batt and powers everything running down the road it should still be fine.

It might be a voltage-regulation issue with your alternator, or it might be completely normal.

Alternators put out more juice as they spin faster. That's one thing to watch out for with these "high output" alternators even... it might be a 150 amp alternator, but it might only be able to support, say, 75 amps worth of current draw at idle (got to shop carefully!).
And your stock alternator isn't really any different.

When you exceed the current capabilities of your alternator, the extra current is simply drawn from your (at that point) fully charged battery. It's absolutely no big deal, but when that happens, your car's voltage drops down to the 12v level of the battery (from the 14.4v level of the alternator, the moment earlier).

And since lights are brighter at 14.4v than they are at 12v, your lights dim when this happens.
That's not at all an issue as long as your battery still has more time to recharge than it is being drawn from... or in other words, as long as your battery's voltage isn't continuously falling, things like alternator upgrades can't be said to be 'necessary'.

Since you said this wasn't a stereo related thing, wasn't related to booming the system, it might just be a faulty (or maybe even normal) voltage regulator in your alternator..
It seems like your voltage is dropping at idle. That could be completely normal - I've definitely had cars like that, but none that were brand new that were like that. Could be age.

Honestly, I wouldn't give it much thought if it isn't causing a problem.

I didn't think it was much to worry about. The car is a '97 tercel with 110,000KM on it. And the dimming is only for a 1/4 - 1/2 second. Right when it hits idle they dim but restore to full intensity while still at idle. It is just when the engine moves from 2500RPM to idle that the dimming quickly occurs then its gone. Also I, really have to watch closely to see it happen with the low-beams on. High-beams it is easy to see.
Thanks for the replies. I appreciate it [img]graemlins/thumb.gif[/img]

Your car probably doesn't (but I'd check), but if your alternator has an external voltage regulator, you could replace it pretty easily.

I'm not sure what it would cost to have an internal one replaced, but it would involve removing and reinstalling your alternator which is a bit involved.