Sub-diodes - 1978 alternator internal voltage regulator

[Zed Head]

I picked up a $6 voltage regulator to see if I could fix my old 1978 alternator. In removing the VR, the "sub-diodes" came apart (I read the FSM after the fact, it suggests holding the lead with pliers as a heat sink to protect them :mad:). But I think that they might have been bad anyway since the other diodes checked out okay. Who knows. The alternator charged fine, by voltmeter readings, but the charge light went on and increased in intensity with RPM, which seems like a diode problem.

Would anyone know where anything about these tiny diodes and where a suitable replacement might be found? I don't really have anything to lose so any WAG is appreciated.

I've attached a couple of pictures, of the new VR and brush assembly with the three sub-diodes laying on top, and one of a single sub-diode.

Another possibility is the complete brush and VR assembly for ~$41 but I'm trying the super-cheap method first. Parts obtained from Rock Auto.

Thanks for looking.

[Mikes Z car]

Can you read any numbers at all on the diode with a strong magnifying glass or jewelers' loupe? Try typing in the numbers on the black part, I typed in 35-8100 and it came up as an automotive voltage regulator made in China though the website listed below has a minimum order of 10,000 of those gems.

http://www.enginesworld.com/alternator-regulator/4.html

Probably need a voltage rating of at least 20 volts, maybe a few amps. Need to know which end of the diode went to which wire, usually there is a band or other mark on one end.

[Zed Head]

There is a band on the diodes. My camera actually gave better magnification than any magnifying glass I have, but I don't see any numbers. I would guess that these three "sub"-didoes as they're called in the FSM are actually for the charge lamp circuit, seeing milliamps one way, through the lamp, to charge up the windings, then blocking the ~15 volts coming back. So your specs. look good to me.

Durability in the alternator is probably the key quality. I found a few alternator diodes that looked about right on some Asian manufacturing sites but you have to buy in lots of 1000.

I might be spinning my wheels...

[tamo3]

Isn't it faster and easier to pick up another alternator from junk yard to swap? I pick up '82 Maxima alternator and get rid of external regulator from 240z.

[SteveJ]

Isn't it faster and easier to pick up another alternator from junk yard to swap? I pick up '82 Maxima alternator and get rid of external regulator from 240z.

It depends. If he picks up a Maxima alternator that has diodes on their last leg, he hasn't gained much.

If his alternator has good windings and bad diodes, replacing the diodes would give him a serviceable alternator, and the process would give him a better understanding of the electronics technology used in the car that could help him repair other issues later.

[SteveJ]

I would have to take a look at the EE section of the FSM to have a guess about the diode functions. I'll let you know if I have a chance to go through it.

[Zed Head]

tamo3, I actually do have an 89 Maxima alternator in the garage, modified with the v-belt pulley swap, ready to install.

Yes, this is more of an exercise in figuring things out, plus I was disappointed when the alternator failed because it did put out more amps at low RPM than the previous "rebuilt" OReilly alternator. It was a good alternator, probably an original factory alt. from 1978, and seemed like it should still have some life left in it. The volt gauge read fine when it failed, but the charge light wasn't working correctly.

That would be cool if you would Steve, I've picked up just enough knowledge over the years to wonder, but not enough to really figure. I just tested the sub-diodes and realized that I wasn't even sure which way the diode symbol was indicating current flow, and blockage. I think that I did figure that one out though.

There's a diagram on Page EE-18. I'm still amazed at how much detail Nissan put in to the early FSMs. I tested the bracket that sub-diodes mount to and it is connected to the L terminal on the T plug (the bottom of the T, not the cross-bar).

[SteveJ]

Amazon has some 10A diodes here: http://www.amazon.com/Volt-Schottky-Diodes-Solar-Panels/dp/B0056RHMCG/ref=sr_1_17?ie=UTF8&qid=1351959697&sr=8-17&keywords=diode

I'm not sure of their physical size, though.

You may also want to consider getting a soldering heatsink. Here's one: http://www.amazon.com/Elenco-ST-23-Heat-Sink/dp/B0002LLWIQ/ref=sr_1_1?s=industrial&ie=UTF8&qid=1351961829&sr=1-1&keywords=heatsink+soldering

If you get it, you may want to use a small file to notch one side of the clamp to stay in place better.

I'm not sure of any specs on the existing diodes, but you found the right page in the EE section. Just point the end of the diodes with the line toward the common point. Also make sure you have a hot soldering iron to melt the solder quickly and minimize the amount of time you're applying heat to the diode.

[72 OJ]

Fwiw,,,http://www.ebay.com/itm/New-OEM-Nos-79-80-Datsun-Nissan-280ZX-Alternator-Brush-Assembly-23135-P3005-/280881780443?pt=Motors_Car_Truck_Parts_Accessories&hash=item4165dbd6db&vxp=mtr

[Zed Head]

Thanks for the links, I will definitely be using a heat sink when I put it back together.

Shouldn't the end with the line be away from the common point though? The common bracket leads back to the charge lamp, so having the line on that side would allow current to flow from the alternator to the charge lamp, if I understand things correctly. The line is the blocking side, correct? Current can flow through the diode toward the line but can't enter on the side with the line.

The tiny diodes in the 78 alt. do seem to have lines on the internal side, toward the windings.

[Zed Head]

Thanks 72, that definitely looks interesting for future consideration. I'm deep in to making this broken alternator work with my $6 VR and a few dollars worth of diodes if I can though. I'll probably end up with something as good as the typical auto store rebuilt. If it works, I might take it apart again and replace the cheap parts with higher quality.

Some of the VR's alone on Rockauto sell for $70 and the brush/VR assembly for $90, so the odds of good parts in a $50 rebuilt are low.

Edit - just noticed that the EBay brush assembly is still l missing the diodes. You can buy the VR, you can buy the brush assembly, but apparently the diodes never wear out.

Edited November 3, 2012 by Zed Head

[72 OJ]

Zed Head, just passing info on what I thought that it could work , good luck on the repair, these oldies some times require the extra care given by their caretakers , I at times feel lucky that most of my ''FrankeinZ'' drivetrain is current GM bits..............