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.

According to the drawing in the FSM, the lines go to the common point.

However, you can easily verify this. Use a multimeter set to resistance/continuity. Place the lead in the common port on the common point, and place the other lead on the other side of the diode. You should need no resistance. Reverse the leads. You should see infinite resistance. If you see no resistance both ways, the diodes are bad anyway.

Thanks again. I didn't study the FSM close enough. Plus I think that I might be getting confused by the anode/cathode negative/positive nomenclature, and I looked at a weird web page, that appears to be backwards - http://www.technologystudent.com/elec1/diode1.htm

This one looks more correct - http://www.allaboutcircuits.com/vol_3/chpt_3/1.html

This one doesn't have enough detail of the diode - http://xkcd.com/814/

This one though, shows a nice picture - http://www.rockauto.com/catalog/moreinfo.php?pk=45765&cc=1209260

I picked up some Radio Shack silicon rectifier diodes, IN4005 - 600V peak voltage, 1A forward current, 30A max., reverse current 10 micro-amps. Anything higher in forward current was too big. They have the same form as the ones in the Rock Auto picture. I'll see what happens.

Zed Head,

In reading on the net it looks like the banded ends (cathode) of the diodes all go the same way whether the diodes are used for the alternator rectification to power the car or whether they are used just to power the voltage regulator as apparently some alternators use two different sizes. The diodes that power just the voltage regulator are smaller. Just orient the new one like the other two diodes as far as where the banded end is connected and you should be good.

Edited November 5, 201212 yr by Mikes Z car

I think that I got things back together in the right order. The circuits tested correctly with my meter. But I still don't have a good clue as to what the properties of the right diode would be though. There are hundreds of choices out there, and I don't really understand the specs.

I found that my little 12 watt and 20 watt soldering pens wouldn't generate enough heat to solder things back together (the wires are soldered to big flat plates in the brush assembly) so I went and bought a Weller 200/260 watt solder gun (D550PK) which was way overkill for the work, and not really the right tool. I managed to get the right combination of heat sinks to avoid destroying anything and still get good bonds. Learned and relearned a bunch about soldering, more new learning than relearning. The Weller gun broke as I was using it, the lights stopped working so I returned it. It was actually the second gun, the first one I got had the bulb glass broken and laying in the bottom of the case. I would feel bad for using the gun to get the job done and returning it but two poor quality items in one day was too many to stick with the the same product.

I did some more research and went to the local Fry's store and bought a Hakko FX-888 soldering station. Plus I browsed their selection of NTE diodes and the NTE catalog and made another guess about what might work if the ones in there don't, and picked up three more diodes.

I had a spare rebuilt Motorcraft alternator on the shelf so compared the L circuit resistances. The Motorcraft had 1,700 ohm through L to ground, the three diodes I just installed had 8,000. Could make for a dim charge light, but I did connect a battery through L to ground and the windings did magnetize. So, in theory, I think, it should work.

Also discovered that the local auto stores are getting weaker and weaker in knowledge and service. Baxters testing machine was broken and they had no plans to fix it, the guy at OReillys wasn't really sure what he was doing and said it failed but couldn't say why, and the local NAPA store said they didn't have the proper connector for the S and L plug so couldn't test it. OReillys machine may well be right by spec. and the 8000 ohms looks like a broken circuit to their machine. No way to over-ride it though, all they can do is plug it in, hit the test button and do what the machine says. I'm going to try Autozone tomorrow.

It's an adventure, but still looks promising. If anyone feels like throwing out some electronics knowledge just for fun, that would be great.

In retrospect, considering the failure mode, I wonder if I couldn't have just installed a single diode in the L wire to do the job of the ones that failed. Assuming that is the failure point.

Assuming the small diodes you have are for powering the voltage regulator (not the larger ones for powering the car) then I believe you need all three diodes instead of just one because the voltage out of the alternator is three phase and by using one diode you would have the voltage regulator turning on and off causing the alternator output to be turning on and off. Using three diodes lets you take advantage of the overlapping three phase current to give a much smoother voltage for the voltage regulator to work with. Hope that makes sense. With one diode you would get wild voltage swings coming out of the alternator because the voltage regulator would be responding to the wild voltage swings fed to it.

I'm starting to get it Mike. The diode trio has more functionality than I had thought. I would guess that the specs. for those three diodes are fairly important, so my Radio Shack diodes are unlikely to be long-lasting even if they do work. Still good to know that if I had the right diodes I could get them installed correctly. If the alternator windings are in good shape, rebuilding an alternator is not really very difficult, if you have the parts. I can source all of the parts individually except the diodes. Regulator, brushes, bearings and diodes are about it.

My expedition in to the world of cheap alternator repair is on pause. It must be one of the main diodes that's bad, they did look a little crusty. A fail on the lamp/diode part of the test at Autozone. From what I've read, people used to actually replace these diodes without too much trouble, three are pressed in to the case, and three more or are pressed/mounted on the diode plate. The FSM even says "replace if faulty". I'll probably take them out and test them when I get back in to it. There has to be some old stock out there in a warehouse somewhere.

To test the main diodes you have to desolder all of the connections to the stator and at least one to one of the diodes. Then pry the stator (the metal ring with all of the windings) out of the case to get two of the studs out. Pretty much deconstruct the whole thing,just like they show in the FSM. I did find one bad main diode. Now if I could just find a new set. Added pictures just because you don't see these that often. For anyone that's on their third or fourth try with an auto store rebuilt and looking for an alternative.

Edit - I guess that there are diodes out there - http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_x_5260086-P_x_x?cm_mmc=CSE-_-Shopping-_-VALUE3-_-VALUE4&srccode=cii_13736960&cpncode=32-189062079-2#utm_source=cse&utm_medium=shopping&utm_campaign=value3&utm_content=value4

Edited November 7, 201212 yr by Zed Head

I saw some more on Google Shopping & Amazon, too. There are some Standard Motor Product diodes that look similar. You may be able to get dimensions on their website.

Posting for my own amusement now but also to fill in - turns out the sub-diodes are available from good ol' Courtesy Parts. $10 each. Found the number on carpartsmanual, but the main diodes aren't shown.

http://www.carpartsmanual.com/datsunS30/DatsunZIndex/Electrical/Alternator/12V60A/tabid/1641/Default.aspx

http://www.courtesyparts.com/diode-p-281902.html

Edit - Thanks Steve. We posted at the same time. I'll bet Courtesy has them but I would also bet that they aren't cheap.

Edit 2 - I wonder if the part is called out on the classiczcar CD that I've heard about. I've not bought it, so don't really know what's on it.

Edited November 7, 201212 yr by Zed Head

$106 for a "DIODE ASSY". Probably the plate with the three positive diodes installed. At least I have a part number now.

(I found a new parts site also: nissan4u.com) - Part #19 - http://nissan4u.com/parts/280zx/us_s130/1978_7/type_2/engine_electrical/alternator_fitting_and_alternator/illustration_2/

You have to insert a dash before the letter in the part number for Courtesy's site to find it -

http://www.courtesyparts.com/diode-assy-p-281882.html

Edit - I think that this is the right number for 1978. Still $106. The above is for 1979. http://www.courtesyparts.com/diode-p-281880.html

Edited November 7, 201212 yr by Zed Head