Alternator? What Do I Have?

[sasto]

Re: Alternator? What Do I Have?

I found the problem to be the oil sender. I jumped the 2 wires going to it and the volt meter reads 13V now.

Question I now have is the sender NC which would open when oil pressure comes up? What would happen if I leave the jumper on it for a few hours rinning?

 

[sasto]

Re: Alternator? What Do I Have?

Hey Bubba,

I'm thinking the oil pressure switch in NC. When I fire up the engine 12V goes thru the switch to excite the alternator. Once the oil pressure raises the switch becomes open which would cut off the 12V to the alternator. I'm assuming the alternator doesn't need constant voltage to it.

What do you think? I am at a loss of what oil pressure switch to purchase. I'm thinking it would be an alarm switch. Without the jumper across the switch at start up, the alternator doesn't charge. If I remove the jumper while running the alternator continues to charge.

Thanks!

 

[zbnutcase]

Re: Alternator? What Do I Have?

Those Prestolite alts dont have any "screens" as the brushes are sealed in their own pocket and no sparks can escape.

 

[sasto]

Re: Alternator? What Do I Have?

Sasto, I am at a complete loss on this. (The wiring diagram you have simply makes no sense.) I pulled up some wiring diagrams from about the same years and in every case there is an external voltage regulator. Wait, let me look at something..... TADA, here is the wiring diagram that is specific to the 1972 - 75 LM 318 marine alternator. Note, it has a voltage regulator.

My alternator has a built in regulator, Bubba. I'm fairly sure the wiring diagram I showed fits my set-up. Ign supplies voltage to the volt meter send which is pigtailed from there thru the 25A breaker mounted on my engine. From there it goes thru an oil pressure switch, not a sending unit, then to the field connection on my alternator.

With a built in regulator is this oil pressure switch NO or NC? Is there resistance in the switch to reduce voltage to the field connection? Does my alternator need constant voltage to the alternator field, or is it momentary to get the thing to charge?

I can't believe Bubba is stumped!

 

[sasto]

Re: Alternator? What Do I Have?

Thanks Bubba, your right, the schematic is wrong!

I just figured it out..with the help of a local captain friend.....It is a single circuit, no ground, that eventually goes to a NO oil pressure switch. Once the oil pressure builds up to the rated pounds on the switch, this completes the circuit to excite the alternator. The reason I could excite the alternator by reving her up was because the alternator becomes self-excited around 6500 RPM's. This explains why I could rev the engine to about 4000 RPM and the alternator would function. The oil pressure switch was put in place to prevent the 12V being directed to the alternator field with the ignition switch left on.

I appreciate the imput you gave me!

 

[sasto]

Re: Alternator? What Do I Have?

Thought I was beginning to lose it.

Nah....You still got it, Bubba. Thanks again for your effort.