Bad Diode? Charging indicator light on when vehicle is off

[mushmasher]

So last week I fixed the common issue with the alternator plug being rusted out. (It was the red wire that needed fixed)

After I fixed that I noticed that the charging indicator light is now dimly lit when the ignition is turned off. And it’s a slow battery drain. But the charging system does work when the engine is running (and the light goes off)

I’m guessing it’s a bad diode? At least the symptoms I'm having sounds like a bad diode.

Has anyone had this issue? And has anyone replaced the diode in the alternator? It looks like I can remove the back cover of the alternator without removing it from the vehicle, but I haven't tried that yet. I see a few different options for diodes on the SAV site. Not sure which one I need.

Cheers.

[rmel]

When you turn off the Battery switch does the Switch bypass relay stay ON (#53 in the schematic)
or does the dimly lit Alternator light turn off?

You could also try getting a voltmeter on the D+ terminal (Blue wire) as well as pin #61 on the regulator.
If one of (3) forward high current bridge diodes is shorted then you could see near 24V on D+, thus the question
on the bypass relay. With a partial short might see a lower voltage, not sufficient to latch the relay, but might
be enough to dimly light Alt light. With the IGN off, neither the Alt light or fuel gauge would be powered, but
the fuel guage could act as a leakage path to ground thus the possibility of a dimly lite Alt light and through a
560 ohm resistor so would be VERY dim. All depends upon what your reading at D+ and #61.

Just a couple of thoughts....

[mushmasher]

When you turn off the Battery switch does the Switch bypass relay stay ON (#53 in the schematic)
or does the dimly lit Alternator light turn off?

Thanks for the quick reply rmel. I haven't had time to check this until now. This may be the issue of the ground relay not working. I do hear a relay clicking but not sure it's the battery main switch. I noticed my manual ground discount switch behind the drivers seat does not seem to be working either. Look like I have to investigate more.

You could also try getting a voltmeter on the D+ terminal (Blue wire) as well as pin #61 on the regulator.

With the switch off I'm getting around 44 volts to D+

[mushmasher]

By the way. I'm looking for a way to test the relay (under the drivers seat area). I found some old threads that had a link to how to test it but the link is dead. Looks like if I just disconnect the ground leads to the relay this should disable it.

And another update I did find that my manual ground disconnect switch works. I recently installed a winch that went directly to the ground of the battery (my bad). This by passed the ground discount switch. So I've disconnected the winch for now.

[rmel]

How to test that relay -- there's a simple test.

Ordinarily, with the engine running, D+ is outputting 24V, which is connected to the Regulator AND the bypass
relay -- thus the relay closes in parallel with the bypass switch.

When the IGN switch is then turned off, the relay should remain closed as it provides a ground path for the battery
keeping everything powered and the engine running. Ya, seems like a odd feature. If the engine stops running after
opening the ground switch, the relay is either bad, or a faulty connection, or a bad alternator.

Then again if a particular diode is shorted it's possible that relay won't go off even after you turn off the IGN switch, If
D+ is positive enough due to a shorted diode. A leaky diode might be insufficient to do that but sufficient enough
to dimly lit the Alt lamp. Measuring D+ after the engine is off (and ground switch closed) would be a darn good indicator
of what's going on here -- it should read 0.0 volts.

[mushmasher]

Measuring D+ after the engine is off (and ground switch closed) would be a darn good indicator
of what's going on here -- it should read 0.0 volts.

WIth the engine off and ignition switch off D+ is reading 41 volts.

[rmel]

Do you mean 4.1 Volts ?

[mushmasher]

Sorry. Looks like that voltmeter went bad which sucks cause it was a nice one. Anyways checked with another voltmeter.

With engine off/key off.
D+ is 24.5v
Red wire is 25.5v
Power at battery is 25.8v

So D+ is getting power which is not good.

If I pull out the ground disconnect key then D+ and red wire are zero volts.


Looking through the manual. Looks like there are some different diodes in there?
exciting diodes
power diodes

Since I don't have the Bosch-Alternator-Tester they talk about in the manual looks like I can check the individual diodes by "Unsolder the stator winding from collecting points"?

What would you suggest next?

[rmel]

Well this result points to a shorted Diode. Note that D+ is about one diode drop below
B+ (Battery) -- that makes sense. There are 3 "Exciter" diodes, fancy name for the
Alternator output that drives the rotor field winding via the regulator. These diodes
are not your problem.

There are two sets of 3 high current diodes. They are diffent in now they connect to the
case of the diode. The set of 3 that likely are the problem are the ones that connect directly
to the Battery, sometimes refered to as the B+ diodes. They would have their cathodes
connected to the case then each pressed into a heatsink plate. The anode of these diodes
would then connect to the Exciter diode assembly. Neither SAV or puch.at makes it
obvious which of these diodes is which, but I am pretty sure it's 7121095522. I also don't
know how hard/easy it would be to press out these bad boys from the heatsink plate.

Oh, BTW that's enough voltage to keep the ground bypass relay ON so you would have more
than just the Alt light leaking down your batteries.

[mushmasher]

Ok. Found this in the manual. Looks like you are referring to #1 in this diagram.

pinz_alt.jpg (138.66 KiB) Viewed 6394 times

SAV sells the rear housing with the diodes pressed in? https://swissarmyvehicles.com/pinzgauer ... 25-884-208
But I think I will first try to remove the old diode and see how easy that is.

Can I remove this rear cooling housing without having to take the whole alternator out? Or does that just make things harder?

[rmel]

Yes, that drawing is showing the B+ diode and it's mounting/cooling plate #1.

The assembly from SAV would save you the effort of pressing out the old and
figuring out which ones are bad -- but, the leads of the B+ (and B-) Diodes are
soldered to the Exciter circuit board, which also connects to stator windings.
So there would be some de-soldering/soldering involved.

I have not repaired one of these 24V Bosch alternators before, so I don't know
exactly what issues you may encounter. Although Alt/Gen shops do this routinely.

If you just want this done with little hassle, a shop familiar with Porch 911
alternators should be able to repair this. They look identical. A complete
overhaul might be a good thing with new brushes and bearings.

[mushmasher]

Thanks Ron. I'll dig into this further and report back. Thanks for the help so far.

[Stumbler]

I know this is an older thread ... but there's a super simple solution. Take the alternator out and take it to your local alternator shop. They charge about $100 to $130 to rebuild, degrease/paint the entire thing, and bench test. I have 1 that I had rebuilt and then put on the shelf. It looks like new and puts out almost 40 amps. (They rewound it well) Just sayin', for $130 you can ensure you don't have blown diodes or broken wires.