An alternator diode test.

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seanrx
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An alternator diode test.

Post by seanrx »

Hi.

I want to test alternator diodes with this procedure that I'm not sure if correct because I've just made it up. Great.

My low load battery, when refit to car and systems boot up, the volts drop to 12.4 and car won't start.

Is a faulty alt diode draining it?

I want to do this test, engine will NOT be running.

Remove alternator positive cable.

Refit battery and wait for systems switch on.

Test volts, if 12.4, just battery fault.

If higher volts, because alternator is not connected.

So is the above test possible? Will my electrics fry! Or just a waste of time?

Note, my 'C5 all systems off' post has all the battery test woes.
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white exec
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Re: An alternator diode test.

Post by white exec »

Forget that.

Fit new battery, as already decided.
Start engine, and check voltage at the battery with engine running.
Should be approx. 14.4v at a fast idle.
If that sort of figure, then the alternator is working ok.

If you need to, you can check your quiescent (residual drain) current out of the battery. Should be 30mA max, or thereabouts.
seanrx
Donor 2021
Posts: 95
Joined: 02 Feb 2017, 13:11
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Re: An alternator diode test.

Post by seanrx »

Chris.
Thanks for your quick reply, I can now buy a battery at end of day.