I've read all the threads on this that I could find in the last hour with no prevail, and I only have about 4 days to fix this problem. When the car was first purchased, someone (Not me) decided it was a smart idea to jump start the car red to black. It blew out a fuseable link and what not. The link was replaced properly, and the car ran fine for months. After a while, the alternator appeared to die. The car was running off the battery. I replaced the alternator, and it worked perfectly. When I tried it again the next day though, it was back to running off the battery. So far, my only ideas are the volt meter (I don't even think there is one on this specific car), corroded wires, something being shorted, the aftermarket alarm system, or something that may have been damaged by the reverse polarity. It can't be fuses, since it worked again at first after replacing the alternator. Just to be safe I checked the fuseable link box and what not. I measured the voltage difference between ground and the power output of the alt while the car was off and got around 3V... That seems kind of strange to me. Also, how can you do a quick dead or alive test on an alternator? I want to check to make sure the old one was broken in the first place, to verify the problem.

 

It's not uncommon to buy a new part and have it fail almost immediately. Get a good charge on the battery and drive to a local parts store, most can test the alternator in the car. Also Nissan really likes the belts tight, with only a fraction of an inch deflection when tight, check your FSM, it might just be slipping.

 

Unfortunately the car is off the road right now so I can't drive it anywhere. I'll try tightening the belt, but I can see the alt is spinning and there is no change in voltage at the battery at all. If I can find a replacement alternator locally I'll give it another go. Are there any other alternators that are drop in fit and easy to find at a local parts store?

 

There are a lot of mods to upgrade the alternators on our cars. I found this thread, he has assembled other threads and trys to make sense of it all, overall a pretty good article with lots of good info.

Datsun Z Alternator Upgrade to GM CS130 Alternator

 

If it ran fine for months then I would check the fuse link again, just to make sure it's still good. I had a fuse link blow, replaced it and a year later I was having charging problems. Alt tested bad in the car, tested good on the bench tester. Turns out the fuse link broke loose. It's free to have it tested and they are stupid easy to take out and put back in (as are most anything on the L28), and if you bought it at an auto parts store then it should have a lifetime warranty... if not should be within a year warranty if it's only been 'months.'

 

So I didn't have all that much time today to work on the car, but I did take the original alternator to the auto store to verify that it was bad. To my surprise, it passed. I went back and pulled the fuseable link to verify it one more time, and it looks good to me. Just to double check there is nothing fancy to it, all I'm doing is a continuity check with an Ohm meter. Anyways, tomorrow I'm going to try tightening the belt, and maybe replacing the power and ground wires. If that doesn't fix the problem, I can only imagine the problem would have to be in the excitation circuitry. To verify, is it okay to give the lamp wire 12V while the car is running off the battery to see if it will start working?

 

Success!!! I don't know exactly what the problem ended up being, but I cleaned the electrical connections, replaced the belt (since I was already going to anyway), tightened the belt, and uhh, well I think that was it. Anyway, it runs! The brakes are pretty bad since they were rusted in place and all, and the engine isn't as smooth as it used to be, but that could just be stale gas since there was only 1/4 a gallon left in the tank. Thanks for the help guys!

 

If it has gone stale, I wouldn't run it through the injectors and filter, it will just clog things up. If you can't drain it, fill it up and use a cleaning additive to help purge out the bad, after you have a new fresh tank in the car go ahead and change the fuel filter.
Time is also a friend to corrosion, an enemy of many of the connectors exposed to moisture, particularly the engine bay. Cleaning the contacts, replace bad and broken connectors(fricfrac has a kit) Check out this thread I started on my 280zx, I did the connector upgrade, runs like a champ now no dodgy connectors held on with zip ties.
https://www.zdriver.com/forums/280zx...-worthy-39110/ page 2 covers the connector upgrade.