Quote:
Originally Posted by Barberskum
I assume you mean to jump start the car then do the test?
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Yes ideally and a failed diode would show a hugely reduced alternator output <11v DC.
Alternatively, with the engine off you can disconnect the negative battery lead then the alternator live lead then use a multimeter to do a diode test both ways between the alternator b+ post and alternator casing.
On the diode test, you're looking for around 500-800 with red meter lead on casing and black meter lead on b+ and open circuit/ol/no reading with red meter lead on b+ and black meter lead on the casing.
If you get a reading both ways on the diode test, you have a failed diode.