Self Exciting Alternator

[bchesley]

Can the alternators on or CUCV's self excite themselves if they are spun at a high RPM. I know this sounds weird, but let me explain what happened yesterday. I have been tracing down an issue with the exciter wire on Gen 2 over the past month so with no gen 2 light at start up. Yesterday I was finishing up a guage install and started the truck to check the new installation. I let it warm up to make sure the temp worked and then decided to rev the engine a little to see if the oil guage reacted well. All of a sudden, the volt guage went from at the green and yellow line to just below the green red line.

Prior to this incident gen 2 never put out voltage. I got my volt meter out to double check and I had 28 volts at both the batteries with 14 volts at the front battery. Gen 1 has always worked, and gen 2 was tested to be a good. What could be happening here?

 

[doghead]

They will not "self exite", but they do need a minimum rpm to get the regulator to function. Often, when a regulator is going bad, they need an increase in rpm, to get the 'cut in" voltage activated.

 

[donalloy1]

They need at least 12 volts input and at least 250 RPM to excite is my understanding. Am I getting warm doghead?

 

[doghead]

Your cheating, your kids are reading the TM's to you!

 

[bchesley]

I have a spair that was freshly rebuilt that I am going to swap out, but I am still stumped how it started charging. I still dont have 12v with the key on at that alternator. I have been trying to figure out how to get it there so it will charge. I have traced is to the diode, but that is another thread.

 

[ryan77]

Did you check the gen light bulbs?

 

[supermechanic]

They will self-excite.
The rotor in the alternator will retain a small amount of residual magnetism, sometimes it is enough to excite.
Problem is the high rpm needed to get this phenomenon to manifest itself.
Anyone with a Delco system can test this themselves, just remove the voltage on the number 1 connector (flat plug), then whack the throttle. The alternator will usually come in.

 

[bchesley]

Thanks super! That is exactly what is happening. I dont have a tach so I don't know exactly when it comes on but I am not over revving it. Maybe 2500 tops. Glad to know that I don't have a crazy gremlin either. Still need to pull the dash and check the diode. That is my last hope before a rewire of that circuit. Thanks.