Alternator Question
Thread Starter
Racer
Joined: Nov 2003
Posts: 430
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From: Central KY
Alternator Question
There have been several recent threads on alternator/voltage/battery issues. A little online research revealed a discussion regarding how to diagnosis defective alternator diodes. The discussion stated that measuring the AC voltage across the battery with the engine running would indicate defective diodes. No AC voltage would indicate all diodes were working.
Can anyone confirm that this is a viable method?
Can anyone confirm that this is a viable method?
Race Director
Joined: Oct 2005
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From: Camp Verde AZ
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Well, that confuses me.
if your bat voltage is only registering about 12.6 or so, You have a charging issue, if the voltage is 14.5-15 or about, your alt is working.
BUT,
you can still have a bad diode which will limit current output, (IIRC), and the only way to verify is test each diode individually, which isn't hard, but you need to remove the rectifier bridge from the alternator, which again, isn't hard, but you need to observe the instructions for reassembly getting the brushes onto the slip rings.
Back in my poor days about 40 years ago, I woudl press out a bad diode from the bridge,and press new one in its place for a few cents vs $35 or so for a rebuilt alt.
Doug
if your bat voltage is only registering about 12.6 or so, You have a charging issue, if the voltage is 14.5-15 or about, your alt is working.
BUT,
you can still have a bad diode which will limit current output, (IIRC), and the only way to verify is test each diode individually, which isn't hard, but you need to remove the rectifier bridge from the alternator, which again, isn't hard, but you need to observe the instructions for reassembly getting the brushes onto the slip rings.
Back in my poor days about 40 years ago, I woudl press out a bad diode from the bridge,and press new one in its place for a few cents vs $35 or so for a rebuilt alt.
Doug
Safety Car
Joined: Mar 2009
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Whether the technique would work depends on whether any bad diode is shorted or open. An open diode won't conduct anything, so there'd be no AC across the battery from an open. That said, even 6 good diodes will still leave some measuable AC ripple at the battery. They conduct one way, but the AC from the alternator is variable voltage. The voltage variations will still make it through the diodes in the conducting direction. A shorted diode will bleed a lot of AC to the battery. In general it's not a reliable test. Better to disconnect em and check em with an ohm meter.
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It is very rare for a semiconductor (diode in this) case to short. They almost always open and usually caused by heat.
This test procedure works well:
http://www.erareplicas.com/427man/wi...or/trouble.htm
This test procedure works well:
http://www.erareplicas.com/427man/wi...or/trouble.htm
