Alternator test, AC Volts.

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seanrx
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Alternator test, AC Volts.

Post by seanrx »

Hey all.

My C5 battery is draining overnight, so seeing the various draw tests on AxleAddict I used my meter for the 'AC on battery' test. Easy one first!

A reading of 0.26 AC volts. Which translates as a bad diode and so a new a'nator. It should read 0

But is this a good accurate test?

When car is running, DC volts is about 13.5 and with various switches on, up to 14.6 max.

Note, the only test I've done tonight as battery has drained to 12v in just 45 minutes.
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xantia_v6
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Re: Alternator test, AC Volts.

Post by xantia_v6 »

It is not a good test. For one thing, there will be significant ripple (at least what you have measured) if the alternator is trying to charge a flat battery.

You can detect a faulty diode by examining the waveform with an oscilloscope, but a digital multimeter is not the best tool for that job.
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white exec
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Re: Alternator test, AC Volts.

Post by white exec »

Ignore that a.c. measurement. Output of an alternator will be d.c., and the figure you got (which is almost zero) is probably just an indication of the 'ripple' on the d.c. output.

The other two figures suggest the alternator is ok, and are the voltages to be expected.

To pin down overnight discharging (a high residual current that shouldn't be there), you would need to connect your test meter (set to Amps first) in series with one of the battery cables, to see what the drain current looks like, then one by one pull individual fuses to narrow down which circuit is remaining on.

An acceptable residual current is in the region of 50mA at most. 75-100mA and more is enough to be a problem.
Popular culprits include boot lights not extinguishing, as well as newly-installed extras, audio, etc..

Others here will be able to advise on C5 peculiarities, and procedure for disconnecting/reconnecting the battery.
seanrx
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Posts: 95
Joined: 02 Feb 2017, 13:11
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Re: Alternator test, AC Volts.

Post by seanrx »

Thanks for replies, I knew I'd need to double check this test method. The inline draw test will be next

If i do the fuse volt drop test, I should lower the suspension? To stop it's sensors while I'm in the cab?