Master cutoff switch

racecar77

Pro Stocker
Aug 11, 2007
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Crete,IL
I know this has been asked before, but I need some advice. I have my master disconnect switch wired to the ground side instead of the pos. side. It works but won't pass tech because engine keeps running after master switch is turned off because alternator keeps it running. I don't want to fabricate new battery cables or run alt. wire back to trunk. I'm thinking of installing another cutoff type switch in the car that just breaks the circuit in the alt. output wire. (one-wire alt.). This way I can shut the alt. off in tech and leave it off for a ten second pass and the engine should then shut off when master is turned off, correct? Thanks for any help, I appreciate it, Lee
 

Goob

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Jun 6, 2003
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If you install the master switch with the alternator cut-off, you still have to run the wire to the rear.
You could do a relay on the alternator wire, but you'd need something with 60-100 amp capacity, and with your ground side break switch arrangement, that may not work as it should either.
 

LS6 Tommy

MalibuRacing Junkie
May 15, 2004
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Breaking the negative side is not legal for tech no matter what else you do. It's also a recipe for a good electrical fire in the event of an electrical problem. Never switch the ground side of a DC circuit or the neutral side of a 110 VAC circuit. Even with the switch off, all the loads are still energized if the ground shorts to the chassis.

Tommy
 

racecar77

Pro Stocker
Thread starter
Aug 11, 2007
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Tommy and Goob, I understand what both of you are saying. I know the right way to do it is on the pos. side. What I was going to do for now is use a 250amp master switch in the dash, breaking the alt. output wire. Wouldn't that essentially be the same as not running an alternator at all? I only want to shut the alt. off going through tech and for a 10 second pass. We didn't run alternators at all on our oval track cars on any races under 100 laps and never had a problem running the MSD and electric fans. Thank you for your replies, Lee
 

Goob

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Jun 6, 2003
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Go ahead and re-invent the wheel while you are at it.... :mrgreen:
 

LS6 Tommy

MalibuRacing Junkie
May 15, 2004
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No disrespect intended, but I think you missed the point- a cutoff switch on the alternator output alone will not pass, even with the switch you already have on the negative side of the battery. You must disconnect ALL positive sources of electricty- at the battery & the alternator. Breaking the negative at the battery & the positive at the alternator still allows the positive wire from the battery to all the accessories to stay energized, even with your switches open. If the negative battery cable grounds upsteam of the disconnect switch, you have no way of shutting down the power.


Tommy
 

grizwald

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Feb 21, 2008
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will running a wire from the post on the alt to the positive battery post keep the alt from running the engine once the switch is off?
 

Goob

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Jun 6, 2003
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grizwald said:
will running a wire from the post on the alt to the positive battery post keep the alt from running the engine once the switch is off?
Big maybe......in this negative switched deal, for normal negative earth systems, yes.
 

racecar77

Pro Stocker
Thread starter
Aug 11, 2007
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Tommy, I appreciate the info and your courteous way of presenting it to me. I respect your knowledge and you help a lot of people on here. Thank you. As I stated in my post previously, I understand the proper way to do it, but apparently Goob thinks I'm an idiot or something, based on his rude reply. Goob, I have read many of your posts on this forum and I'm well aware that you are very knowledgeable also. But truthfully, I don't care if you discovered the theory of relativity with Einstein, you were out of line with your comment. I have never been rude or degrading to anyone on this site for any reason. I'm virtually certain that this forum is here to help people that need information and advice and it can be done without insulting people. I wasn't second guessing or doubting you, I was looking for an easy way out of my problem temporarily. I'm not stupid and don't like being talked to as such.
 

Goob

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Jun 6, 2003
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Indianapolis
I know the right way to do it is on the pos. side. What I was going to do for now is use a 250amp master switch in the dash, breaking the alt. output wire. Wouldn't that essentially be the same as not running an alternator at all? I only want to shut the alt. off going through tech and for a 10 second pass.

So you know the right way, the legal way, yet you are looking here for validation and rationalization to do otherwise. ???
Sorry. [-(
 

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