alternator - Page 5

SGSST109E · 05-14-2025, 06:29 AM

Those magic alternators that charge and discharge. I had one last year. Tom's solenoid solves the issue.

SST159 · 05-14-2025, 09:33 AM

Brilliant idea Tom!!
As usual thanks for sharing your knowledge.

Jack Zimmerman · 05-14-2025, 10:45 AM

An alternative to the relay to shut the power off to the alternator when in the trailer or in storage is to use a Cole Hersee P/N 75912 double pole battery disconnect switch. The main battery cable gets wired to one pair of terminals and the alternator field cable to the battery goes through the other pair.

goinbroke2 · 05-14-2025, 11:08 AM

I run master switches in everything, motorhome, bracket car, antique, Stocker. If there is a problem, I can turn the power off at the batt. (or if it's sitting for long periods)

Also, if an alternator is drawing power while not in use, you have a diode gone. You can unhook the cable or put a master switch on it, but that is a band aid to the real problem. Further, if one of the three diodes is gone and you have a batt light, you will see it flicker or come on at idle and go out when you add throttle. This also means the alternator is not putting out as much amps as it used to with three diodes.

I've never heard, and I'm trying to figure why, cutting the field wire would hurt an alternator. That is how it works, the regulator turns the charging on and off by regulating the power going to the field. It's how I test an alternator, spin it and add power to the field wire, if it charges, the problem is something else. (ford with FSAI wiring to the external regulator is mostly what I deal with)

Not saying anyone is wrong, I just don't understand how it could be a problem.

Tom Goldman · 05-14-2025, 11:22 AM

Quote:

Originally Posted by Jack Zimmerman

An alternative to the relay to shut the power off to the alternator when in the trailer or in storage is to use a Cole Hersee P/N 75912 double pole battery disconnect switch. The main battery cable gets wired to one pair of terminals and the alternator field cable to the battery goes through the other pair.

Jack, I used to use those on all my builds ,but in recent years I and several of my customers experienced switch failures . .that prompted my switch to the relay which has been very reliable. The 75912 switch is US made now [previously it had been being made in Mexico] and is rated at 750 amps intermittent load . Some of the 4 post type switches that that are being sold by Jegs ,Summit and others have smaller gauge secondary terminal posts are only rated for a 30 amp load and will not survive the output from the alternator.

cad · 05-29-2025, 10:13 AM

Just spoke with tech support @ Tuff Stuff Performance which is one of the primary sources for the high output @ low rpm CS121 alternators.

He said it is unfortunately common that standard race car wiring and shutting off the master cutoff will zap the regulator.

I purchased another 7937ST and it is working fantastic.

Next time I have to go through a full tech, I will be removing the alternator belt.

Clark

L.Fite · 05-29-2025, 04:46 PM

QUOTE:
Not saying anyone is wrong, I just don't understand how it could be a problem.

I don't think the problem is actually the "alternator" as it is the "regulator" that is getting zapped...

If the alternator is putting out a high amperage and the switch is turned off, it sends a spike through the regulator which is a bunch of made in china electronics, diodes/SCR's and such.
Electronic circuits don't like sudden changes in volts/amps.

Back in the day the external regulators were relays and ceramic wound resistors, and while not bullet proof, could handle a lot more in the way of hard use.

05-14-2025, 06:29 AM	#41
SGSST109E Member Join Date: Jun 2007 Location: NJ Posts: 351 Likes: 62 Liked 126 Times in 61 Posts	Re: alternator Those magic alternators that charge and discharge. I had one last year. Tom's solenoid solves the issue.

05-14-2025, 09:33 AM	#42
SST159 Member Join Date: Jul 2005 Posts: 315 Likes: 5 Liked 20 Times in 16 Posts	Re: alternator Brilliant idea Tom!! As usual thanks for sharing your knowledge. __________________ Drew Wonderly SST 159

05-14-2025, 10:45 AM	#43
Jack Zimmerman Junior Member Join Date: Dec 2001 Location: Clemmons, NC Posts: 35 Likes: 0 Liked 11 Times in 6 Posts	Re: alternator An alternative to the relay to shut the power off to the alternator when in the trailer or in storage is to use a Cole Hersee P/N 75912 double pole battery disconnect switch. The main battery cable gets wired to one pair of terminals and the alternator field cable to the battery goes through the other pair.

05-14-2025, 11:08 AM	#44
goinbroke2 Senior Member Join Date: Mar 2002 Location: NS CANADA Posts: 885 Likes: 1,570 Liked 384 Times in 149 Posts	Re: alternator I run master switches in everything, motorhome, bracket car, antique, Stocker. If there is a problem, I can turn the power off at the batt. (or if it's sitting for long periods) Also, if an alternator is drawing power while not in use, you have a diode gone. You can unhook the cable or put a master switch on it, but that is a band aid to the real problem. Further, if one of the three diodes is gone and you have a batt light, you will see it flicker or come on at idle and go out when you add throttle. This also means the alternator is not putting out as much amps as it used to with three diodes. I've never heard, and I'm trying to figure why, cutting the field wire would hurt an alternator. That is how it works, the regulator turns the charging on and off by regulating the power going to the field. It's how I test an alternator, spin it and add power to the field wire, if it charges, the problem is something else. (ford with FSAI wiring to the external regulator is mostly what I deal with) Not saying anyone is wrong, I just don't understand how it could be a problem. __________________ Jim Miller 103 U/SA

05-29-2025, 10:13 AM	#46
cad Senior Member Join Date: Apr 2009 Location: Omaha, NE Posts: 702 Likes: 202 Liked 91 Times in 51 Posts	Re: alternator Just spoke with tech support @ Tuff Stuff Performance which is one of the primary sources for the high output @ low rpm CS121 alternators. He said it is unfortunately common that standard race car wiring and shutting off the master cutoff will zap the regulator. I purchased another 7937ST and it is working fantastic. Next time I have to go through a full tech, I will be removing the alternator belt. Clark Last edited by cad; 05-29-2025 at 01:20 PM.

05-29-2025, 04:46 PM	#47
L.Fite Senior Member Join Date: May 2020 Posts: 513 Likes: 544 Liked 494 Times in 241 Posts	Re: alternator QUOTE: Not saying anyone is wrong, I just don't understand how it could be a problem. I don't think the problem is actually the "alternator" as it is the "regulator" that is getting zapped... If the alternator is putting out a high amperage and the switch is turned off, it sends a spike through the regulator which is a bunch of made in china electronics, diodes/SCR's and such. Electronic circuits don't like sudden changes in volts/amps. Back in the day the external regulators were relays and ceramic wound resistors, and while not bullet proof, could handle a lot more in the way of hard use.