Electrical question

[mlaw7]

As I was driving home this evening, my brake warning light suddenly came on and at the same time the red (charging?) light on my voltmeter came on. The voltmeter reading also had dropped to around 12v, which is considerably lower than its normal level.

The car still ran fine, no brake or other mechanical issues. The lights may have been a little dimmer but I can't say for sure. If it makes any difference, this happened as I was slowing to a stop.

When I got home, I checked the fuses - all look good. Didn't see any loose or bad connections/wires under the hood. The fusible links also seem to be ok. I also tried restarting the car - no problem but the symptoms still persist.

I've had no indications at all of any electrical issues prior to this.

Any thoughts on what could be happening? Alternator? Voltage regulator? Battery?

This is a 1978 280Z.

Thanks!!

[EScanlon]

Have your alternator checked.

E

[Zs-ondabrain]

is the pulley belt still there? I know it sounds dumb but you said you checked all the electrical but did ya look any further than that?

When I lost a belt on the freeway, I got all the same signs as you. weaker lights, warning lights, etc.

[FastWoman]

If your volt meter is reading only 12V with the engine running and the alternator actually spinning, then you either have a bad connection to the alternator, or your alternator is bad.

With your engine stopped, measure the voltage at the output post of the alternator. If you get a 12'ish V reading there, then the connection between the alternator and the battery is good. There's also a smaller voltage reference connection that should get 12V. I forget whether the engine needs to run for that.

Anyway, alternator tests are quick, easy, and free at most auto parts stores.

Your battery is fine, and your voltage regulator is internal to the alternator on a '78.

I have no idea about the brake warning light. I would think that would be a separate problem.

Edited January 14, 2011 by FastWoman

[mlaw7]

Had the alternator checked this morning and the test showed bad diodes. Not sure if it's the original unit but it does look pretty old. The battery, which is about 1 1/2 years old, didn't test very well either. Evidently it experienced a fair amount of power drain due to the alternator going bad.

Interestingly, after five minutes on the road on the way to getting this checked, both the brake warning light and charging light on the voltmeter went out. However, I did notice the voltmeter fluctuating as I drove.

I replaced the alternator and she's now back to normal. I think I'll give the battery a day or so with the new alternator to see if there's any improvement. If not, I'll get a new battery.

As to the brake warning light, evidently the variance in voltage somehow affected the circuit. The brakes are fine.

Anyway, problem solved.

Thank you for your input!! What a great group of people on this forum!!

[FastWoman]

Well, being drained is pretty hard on a battery and does sock some of the life out of it, but as long as you can start your engine, that's all you really need of the battery.

Anyway, glad you got the problem tracked down!