Normal or Problem?

[Nomake Wan]

As I continue to drive my car around, I notice more and more little things that I never noticed before that may be due to aging.

Today, I was stuck behind a towtruck pulling somebody out of a ditch, so I put it in park. The revs went down to 750, and I thought I saw the headlights dim. I put her in neutral and pushed the throttle a bit, and they got brighter. Let off, down to 750, and now I've noticed that everything--panel and headlights--is dimming. And above 1000 RPM they're fine.

Normally I wouldn't be too worried... but this is without an audio system in it at all. Since I'm putting in an amp and another power outlet up front once the weather gets better, I'm wondering if there's going to be enough power to go around.

So, the important questions:

-Is this dimming at 750 RPM normal?
-When I put in my audio system, will it get a lot worse?
-If this isn't normal, what's the diagnosis? Battery or Alternator? I don't think either has been replaced, but the battery may have been.

Fortunately, I've still got a bit of money left over from Christmas, so if there's something wrong here, I can fix it.

Thanks!

(Heh, I've been asking a lot of questions lately...)

[dwreck30]

Mine does the same thing. 750 is right where it should be. I have a stereo in my car and it doesn't really affect it although it does put more strain on your alternator. Hope this helps.

[Brat4by4]

Completely normal. And you'll have plenty of power to go around without a sub. If you had a sub, you might want to add a stiffening capacitor which helps keep the sub amp from beating up on the alternator by asking for really fast & high loads.

[boostjunkie]

Completely normal. And you'll have plenty of power to go around without a sub. If you had a sub, you might want to add a stiffening capacitor which helps keep the sub amp from beating up on the alternator by asking for really fast & high loads.

Capacitors are band aids for insuficient alternator output. I should know, I tried to band-aid my system back in the day with two 1-farad capacitors (after the 3 .5-farad capacitors didn't work). The lights still dimmed the same as they did before the caps.

[Fishy]

My 94 turbo wagon acts exactly like you've described as well. I'm going to chaulk it up to a 'subaru thing' or something.

As for stereo load/dimming lights: Like boostjunkie says the caps are a band-aid fix. BUT in the past I've had pretty decent luck band-aiding a system that isn't pushing the alternator TOO far over the edge.

Example: I was running a rocklford 600.4a on two rockford punch DVCs and just a little below where my signal ran out of clarity the lights in my car would dim. I added a 1farad cap and I managed to get that last bit of clear signal out of it before audible distortion without dimming the lights. Of course your results may vary and I was only dealing with ~600 watts which isn't a monster system by any means. Just my 2 cents

Ultimately you're going to want: good strong grounds, thicker-than-you-think-you-need wire and an alternator than can give what you're trying to take from it

[Nomake Wan]

Well, I've got a 760W amplifier driving four 2-way speakers only, no sub. I've definitely got thick enough wire, and the grounds'll be worked on too. As for the alternator, we'll see when it's all hooked up, I guess.

[boostjunkie]

Also, how many miles are on your car? Might be time to change the alternator out anyways.

If you have a voltmeter see how much voltage your battery/alternator is putting out at idle. It should be in the vacinity of 14V. Less than that and your alternator is probably on its way out.

[Nomake Wan]

About 65k on the odometer. Verry low for a '90 Legacy.

[Tleg93]

Just a passing thought but maybe your ground connection is corroded. Try checking it out and if it looks a little weathered it wouldn't hurt to clean your contact surfaces. Cleaning up your battery connections may help too.

[simonasaur]

Mine used to do the same thing, although its a manual. I replaced the alternator, still didn't have a stereo, and it still did it.
Its possible that it is just a "subaru" thing.

[Nomake Wan]

The grounds near the strut towers look kinda... corroded. Yep, you put it well.

I'll be cleaning those connections once I begin the re-wiring process... but that doesn't happen until I get the fabrication I need done...

[Brat4by4]

[Napolean Dynamite Mode]

That is like... the most commonest thing EVER.

[/Napolean Dynamite Mode]

The alternator is like your engine, it puts out so much power at so many revolutions. It is usually spinning fast enough that it makes plenty and the power gets regulated. When you are idling, it is spinning very slow. Just like your engine, when spinning very slow then it can't possibly put out as much juice.

There is nothing wrong with cleaning your grounds, and adding an extra grounding system. These are all very good, but they will not help if the alternator just can't push that much power when idling so low. The true solution would be to get a larger crank pulley or a smaller alternator pulley to increase the RPM of the alternator at low idling speeds.