Alt. Light Glows until Engine Revved?
 

I just noticed an odd quirk with my car. When I start it from cold in the morning, everything works perfectly. After driving it, turning it off, and letting it sit for a while, when I start the engine again, the alternator light glows until I rev the engine slightly.

I'm wondering if there's a specific RPM in which the alternator is excited? Other ideas?

Hi Mike,

By no means am I an expert, but when I recently got my alternator replaced, I had the same problem. It turns out that my battery was not receiving a charge. A day later, guess what, the darn car shut off on me on the Garden State Parkway over Labor Day Weekend.....what a bummer. Anyhow, I was able to get it recharged, and then drove it right back to my wrench, where he was very sorry, and fixed it properly.

In short, what I am saying is that in all liklihood, you've got something wrong. If you have a voltmeter, you can properly measure the output, and whether the battery is getting juice. I'm sure that another P-head will give the proper readings, and I'm sure they will let you know what the light means exactly. It all depends on when it dims (or something like that). Usually, someone will be able to tell what the problem will be just by giving a better description as to when it flickers/dims/brightens. Examples are poor grounding, or bad part in general.

You might want to check old posts as well, as this subject has been covered frequently. Have fun

regards,

Get a volt meter...battery should read 12.5 volts across the terminals with engine off.....less than that, needs a charge, bad battery or charging circuit....could be bad grounds.

13.5 at idle 14+ at 2500 rpms.....any more than 14.5 the voltage regulator is suspect. Anything less, the alternator is suspect.

I agree with the above advise...get it checked asap. Earlier this year my alt light started to glow and it would stop when the engine revved....I assumed that the battery was charging at the higher rpms....wrong. Two days after I noticed the light glowing at low RPMs I had a dead battery and could not start the car...lucky I was near home on a side street. I needed to replace my alternator.

low idle
 

My 911sc has a 3.6 which i think has alower idle, anyway does you car have a really low idle, it will light if that

To answer your first question ... yes, there is a minimum, or 'zero-Ampere' rpm level, so idle speed could affect whether the lamp is on or not.

Second, if it <b>ONLY</b> happens after a heat soak, there is another reason it may be happening ... <b>ALL ALTERNATORS PRODUCE LOWER VOLTAGE WHEN VERY, VERY HOT,</b> so it is understandable after a good heat soak!

There may not be an actual problem with the alternator, but it won't hurt to check the details: battery Voltage, both after sitting overnight, and immediately after a long drive of 30 minutes or more.

Thanks everyone. The alternator is a brand new Valeo unit that I just installed, and my wiring harness is also a nearly new factory part.

I'll check the voltage at the battery as recommended.

One other thing to check is the tension of the belt. If it is incorrect or slipping, the alternator will not charge correctly.

I've replaced two alternators with rebuilt units but since you said you bought a new unit, then other problems I could mention such as sticking brushes should not be the problem.

I'm clear over at the other end of the scale on this one. I am not concerned at all. I have seen many cars behave this way, that were working just fine. As Warren says, a hot alternator will make slightly less electricity, but it may just be an idle speed thing. Your warm car may start and idle at lower RPM than the same car cold. Generally speaking, alternator lights will glow until the alternator reaches a certain speed, then the light will go off and stay off. Again, no special Porsche experience, this is just the case with cars generally.

Mike,
One of my club friends had a similar situation to yours. The alt light would glow dimly at times. He thought this meant trouble with the alternator.
Conicidently, he just had the lamp replaced at a shop.

(Now you probably know the lamp actually completes a circuit path for an excitation current for the alternator field windings, right?)

Anyway, it seems the lamp that was installed was a different wattage than it was supposed to be (according to spec). When he replaced it with the proper one, the lamp only glowed when it was supposed to, ie: when the alt was not charging (as in motor "off".)

So, you might want to check that lamp and see if the right one is installed.