I suspect a bad diode on the alternator in my truck. Have never seen the insides of this particular alternator, so have some questions before I dismantle it. Any suggestions on how to check for a bad diode while on the truck? It appears to charge just fine.

Can the diodes or diode set (rectifier?) be changed? Did a search in Genos and Rock Auto and they do not list diodes.

Are the diodes internal and need to split the two halves to see them? Or are they inside that black plastic gizmo attached to the back of the alternator? Can the diodes be isolated and checked individually? Am guessing that there are at least 4 diodes. Where can I buy the diode/rectifier for this particular alternator? I already have new bearings and brushes ready to go in.

Or should I just bite the bullet and install a rebuilt OE alternator from Rock Auto? Any comments about the Duralast alternators from Autozone?

All comments and suggestions will be appreciated.

 

You made me chase you down.
1. Most,if not all automotive alternators are 3 phase, and use a full wave bridge rectifier, which is 6 diodes.
2. I don't think a bad diode would whine. Smoke maybe, but not whine. Bearings whine, and most bearings supplied for alternator use are close to junk right out of the box. If you put one in a regular electric motor, it would sound like the motor is completely shot right away. (Been there, done that, got the T shirt) If I go to the bother of replacing bearings in an alternator, I spend a couple of extra bucks and get an electric motor grade bearing. Theoretically the noisy bearings will last just as long, but I find that hard to believe.

The way to find a bad diode is to execute a full load, full field test and measure the output current compared to specification. 1 bad diode will reduce output by 33%. Often under such a test, they'll literally let the smoke out, making them easy to find.

 

Did that on purpose to give you something to do.

John, thanks for the info. Will do more digging on both the alternator and the airbag problem.

From new, this alternator has whined under load. In cold weather the few times that the grid heaters come on, I can hear the alternator whine from both inside and outside the truck. As soon as the grid heaters switch off, the whine vanishes.

Just did a voltage test and the more load that I apply, the higher the voltage. With nothing switched on the voltage is 14.42. As I apply load I watch the voltage climb and with just A/C it is 14.47V. Lights plus A/C it is 14.51. The only time it drops off if I switch everything on including the compressors and then it will drop to 13.89. Did not hear any whine today. The whine is only evident at first start of the day and goes away after about 20 minutes of driving.