Prestolite alternator???

[SycloneDriver]

I'm working on a 1980 Catalina 38. The boat is in rough shape. We been working mostly on getting the motor to start reliably. After a new filter head, ignition switch and a relay, we have that task completed. Now we are replacing the gauges. I swapped the ammeter for a volt meter, so I need the rewire the alternator output. Upon removing the alternator I discovered that the output cable was not connected to the output post, it was instead connected to what I think is the voltage regulator sense post. I'm not sure the ignition exciter lead was even connected, it fell out of the crimp connection at some point.

On the way home I stopped by 2 automotive stores to have it tested. The first one refused to test it because it was off of a boat. I probably test a thousand alternators when I worked at an automotive parts house and I never turned anyone away without at least trying. The second one tried but he seemed very confused and even forgot to connect the output lead at first. I think the boat was charging last year.

After cleaning the alternator up I've made a few observations.

1. It has an extra diode plate connected to the output. After a little research it seems to be called a isolation diode.
2. I don't see a stator output. No wonder the tach doesn't work.
3. I believe the post with red wire is connected to the output of the rectifier. If the is in fact true then the pervious ower bypassed the isolation diodes by connecting the output cable to this stud.

Why use an isolation diode? It seems like it would lower the out voltage and do little more.

 

[SycloneDriver]

I fiddled around with the alternator. I connected it to a battery and spun it with a impact gun and a socket.
If i connected to the output and the exciter lead I could get a little output, 2 to 4 amps @ 13.2 volts, remember I'm not spinning it very fast. If I bypassed the isolation diodes the alternator drew about 2.5 amps from the battery when it wasn't spinning but put out power when spun. So it seems that with this regulator/alternator the isolation diode is required to prevent the alternator from drawing power when not in use.

I plan to test it again on the boat before I add a stator tap.

 

[SycloneDriver]

I reinstalled the alternator and corrected some of the wiring, The alternator works, the voltage is a little low @ 13.3, but it works. I'll add a stator tap and see if I can get the tach working. I'll take some pictures, maybe someone will find them helpful.

 

[Rodsails]

Diode was probably so it didn't back feed if you're using more than one battery. helping keep the starter battery up on voltage.

 

[SycloneDriver]

Diode was probably so it didn't back feed if you're using more than one battery. helping keep the starter battery up on voltage.

That's a good guess but with only one output post on the alternator it's not possible. This alternator was probably designed in the mid 60's. It was probably easier/cheaper to add the diode to the output than to engineer a regulator that wouldn't draw power when the alternator wasn't spinning. The transistor was invented, not in use, in 1947 and was still new when this regulator was designed. That's my theory.