Bad News... Good News... Bad News

[Patstuff28]

I bought a 1981 Z28 a few years back that had an old aftermarket stereo and separate amplifier in it. You could hear a whine in the stereo when the engine was running, but the unit played fine when the engine was off. My buddy a mechanic said it was a diode in the alternator. Sure enough, the noise went away with a rebuilt one.

 

[Gary S]

Test repeated…
Car idling, headlights on, fan on high, stereo on…

Same result with the multi-meter.

The VAC setting is confusing to me as there’s no numbers on the scale.

How do you read the VAC scale on this old non-digital multi-meter?
(Previously, I've only used it to measure resistance and 12 DC tests.)

AC and DC share the same scale on your voltmeter so the reading in your picture suggests a reading of 28vac. However, you probably can't get a meanful reading here as you don't have a true AC waveform on an alternator. For the AC of an alternator to be converted to DC, it first has to be rectified. (The diodes do that). Then, it will still have most of the AC ripple in it until it is filtered. An alternator has no filter. On a car, the battery acts as a filter so you get true DC at the battery.

I don't think this test means much. An oscilloscope would give you more information. A bad diode would show up there as missing rectified cycles of the AC.

 

[Twisted_Metal]

Thanks for the technical explanation of the meter AND the electronics, Gary.
I had no idea why it would measure that much voltage if it's supposed to be "Zero".

The alternator seems to be working and it was most likely just a "bad" battery.

If the thing won't take a charge with a trickle charger the alternator probably wasn't going to charge it either.
That one never seemed to be a great battery, even when it was new. (550 CCA, supposedly)

Thanks for all the input, guys!

I should go research that A/C pusher fan thread again...

 

[balloonpilot]

Test repeated…
Car idling, headlights on, fan on high, stereo on…

Same result with the multi-meter.

The VAC setting is confusing to me as there’s no numbers on the scale.

How do you read the VAC scale on this old non-digital multi-meter?
(Previously, I've only used it to measure resistance and 12 DC tests.)

It has been a while, but I read that as 28 ACV.

I am not sure what battery maint unit you use but the one I have on my RV is a smart unit and is plugged in 24/7 365 and is only unplugged when towing (although the truck is charging it then) and is the same battery that was in it when I bought it in 2012. Regular duty battery and was working great and holding charge as of last weekend.

Also, I would check voltage draw when everything is turned off. It is suprising what can be drawing power that you never thought of.

Good luck.

 

[Twisted_Metal]

It seems a new battery cured the issue.
The car sat for 2 weeks without being on a charger, cranked over quickly and fired up in 2 seconds.

The Volt gauge actually varies now... It used to sit at the 14V mark all the time when the car was running.
The battery tested "good" @ 12V but it just wasn't taking a charge any more.