Alternator diode

[DonDDR]

Wifes car was dead (no crank) this morning. Jump it and it fires right up. She is out of town so I send her to Autozone. Battery is fine, alt charging, but test guy says the diode is bad. IIRC or not, seems to me GM alternators with bad diodes would charge but drain the batt down if left sitting for several hours. Any input much appreciated.

 

[rubydist]

the diodes generally fail open, which means that the alternator output is effectively reduced, and with it being dark more of the time now and lights on more, etc. the battery often never gets fully recharged, so eventually it gets low.

You can buy a new regulator, but its almost as much as a rebuilt alternator, so unless you like surgery on alternators, you are better off to get a decent alternator. I have only had decent luck with the NAPA lifetime warranty ones - all the others seem to die a couple months after the warranty expires...

 

[Storm-Chaser]

What the diodes do, is explained here:

.... A less common alternator problem is a bad diode. These control the flow of electricity from the alternator,....