Bypassing bad diodes on failing alternator

Question

I'm troubleshooting my alternator and believe I have traced the issue to bad diodes on the diode plate. Here is the schematic of my alternator

So I've a couple of questions I don't understand.

What is the purpose of the three left diodes? Just for better protection against current flowing the wrong direction? I see the 3 stator wire outputs only go through one diode for each phase.

Secondly, this alternator has 4 posts in the back, as well as a 4 pin connector. In the schematic, N,F,E are on the connector. B is the output post. One pin on the connector is not used. So there are three posts not in use. I'm not sure this schematic matches exactly how the alternator is wired. here is a picture of the actual alternator:

The diode plate is actually two separate plates. The stators output to one set of diodes on the left. Then there are jumper wires over to the right plate that go through the other three diodes. All those diodes are failed as far as I can tell, and that plate has B+ on it.

Edit: added picture of diode plate. It Looks like you can buy diodes that look similar to these. I'm not sure I can remove mine? The spread pressed in?

My questions are:

1. Am i misreading the schematic and it actually represents the real alternator? I just don't get what those left diodes are in the schematic.

2. Looking at the real alternator, it appears the stator outputs are wired in parallel to diodes, one per output plate.

3. I'm thinking instead of using the current B+ post which has no output due to bad diodes, I can just move the wire that currently connects to B+ over to a post on the left plate. Not sure if that is a good idea or what might the ramifications be?

Thanks!

Answer 1

There are 2 ways a diode can fail, open or short. If the diode shorts you will drain your battery when the engine is off. If the diode goes open you will only lose some output and it won't be necessary to do anything.

If only one diode shorts, you can cut it and it's pair out and still get 2/3's output. If two different winding diodes fail, it's a lost cause. You alternator has three windings. There are a pair of diodes on each winding, one up, one down, so you get clipped dc output on both halves of the sine wave that winding puts out. Really, in your case, find someone who does complete rebuild on alternators. One that old should be rewound, get new brushes, bearings and a new regulator. Or get a rebuilt already with a shell that fits your application.

Answer 2

The alternator produces AC voltage that needs to be changed to DC. So, each phase or winding is constantly changing from negative to positive as the rotor spins. If we want to charge a battery, we need to catch all the positive pulses and send them to the B+ post, this is done by the 3 diodes on the right. We also need to catch all the negative pulses and send them to ground, that's handled by the diodes on the left.

The diodes you are looking at on the plate will be wired the same way, the bridging wires are doing the same thing as the diagram. They link up six diodes to 3 phases.

All SIX diodes are required for this circuit to work - there are no spares here. The diodes have all done the same amount of work and should all be replaced together. Pressed in and use high temp solder or spot weld to prevent failure under load. Good luck.

Answer 3

All 6 diodes form the full wave rectifier - if you remove 3 then half of the stator output will be lost. If you move the wire to the other post as you suggest you will let one stator winding discharge through the other stator winding so no useful output. Replace the diode pack as suggested.

Answer 4

Yes, you're right mikew, a full wave rectifier requires four diodes. But an alternator has three phases, each separated by 120 degrees, and due to the Star configuration, two phases work in series at any given time. So for each pair of windings in series there are four diodes in operation, so this is indeed a full wave rectifier.

Yes the diodes are pressed in, and they are not that hard to replace.

I have just tried to repair a more modern alternator where the diodes appear to be soldered onto the heat sink. All the auto electricians I have been to say that it cannot be repaired, but I haven't given up just yet!

Answer 5

These diodes are better tested under a 70% capacity load, not so reliably on a bench with an ohm meter. What was the symptom? A failed diode typically results in an AC ripple >0.5V at the battery terminal output.