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03-11-2010, 09:44 AM
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CC Member
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Join Date: May 2009
Location: American Fork,
Ut
Cobra Make, Engine: 66 Cobra
Posts: 930
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Not Ranked
Positive vs. Negative Power Shut Off
I am getting close to putting my body on for the last time and am debating about putting in a power shut off switch. This is the time to do it. Anyway, I have read some threads on the subject that addressed types of switches and locations, but none really got in to the issue of putting it on the positive vs. negative side. I have read over on the FFR forum where a few guys changed over to the negative side to solve some issues for them.
I am thinking of putting it behind the passenger seat for easy access. Since my battery is in the trunk I can easily run the ground wire to it and then bolt the ground wire off the switch to the frame on an unused roll bar pad mount. It would seem that the ground wire switch would be the more logical and would avoid all the other issues with the positive set up. Am I missing something?
Wayne
Last edited by Wbulk; 03-11-2010 at 09:46 AM..
Reason: correction
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03-11-2010, 10:30 AM
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Senior CC Premier Member
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Join Date: Nov 2004
Location: SoCal,
CA
Cobra Make, Engine: CSX #4xxx with CSX 482; David Kee Toploader
Posts: 3,574
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I used the Taylor Vertex battery cut-off switch connected to the negative side. Taylor Vertex strongly recommends using the negative.
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All that's stopping you now Son, is blind-raging fear.......
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03-11-2010, 10:43 AM
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Senior Club Cobra Member
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Join Date: Nov 2006
Location: Charleston,
WV
Cobra Make, Engine: SPF 2592, Shelby alum. 527
Posts: 325
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Not Ranked
I know this topic has been discussed before, and I remember a concern is that if the switch is intended to be a safety shutoff the engine can continue to run on alternator power even if the ground (or positive) lead to the battery is disconnected.
If the switch is intended to be only a security device, installing the switch in either lead from the battery will work.
Rodger
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03-11-2010, 11:53 AM
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CC Member
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Join Date: May 2009
Location: American Fork,
Ut
Cobra Make, Engine: 66 Cobra
Posts: 930
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I have a diode on my exciter wire to the alternator. Yes, that is an issue with the positive shut off, but I'm not sure that is the case with a negative shut off. I guess that's the point, will not using the negative for the switch shut everything down? No ground current can't flow.
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03-11-2010, 01:29 PM
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CC Member
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Join Date: Nov 1999
Location: Pentwater,
Mi
Cobra Make, Engine: Professional Cobra & Streetrod Builder
Posts: 5,352
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Not Ranked
Normally a negative "break" is used for EFI's/PCM's-ECM controlled cars. (Probably why so many FFR guys go the negative route.
Usually the normal....Carb'd motors will shut down the hot side.
Last edited by Double Venom; 03-15-2010 at 12:56 PM..
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03-11-2010, 02:06 PM
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CC Member
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Join Date: Feb 2006
Location: Alexander,
AR
Cobra Make, Engine: B&B 427 Stroked Windsor TKO 600 w/3.50 posi 9"
Posts: 789
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Not Ranked
There is absolutely no difference from an electrical standpoint.
If you want to run a bypass for Alarm or Radio backup power or some other reason, then use the positive so that you will have the frame ground if needed.
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03-11-2010, 02:30 PM
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CC Member
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Join Date: Sep 2002
Cobra Make, Engine: SPF#0760
Posts: 3,405
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Interesting that this topic should be brought up again. I was just discussing the issue of cut-off switches with another person a few days ago.
I believe you will find that most cars will continue to run after flipping off a power cut-off switch, weather positive or negative cable installed.
WARNING: IF YOU DECIDE TO TEST THIS STATEMENT, BE ADVISED THAT IF YOU DISCONNECT THE BATTERY WHILE AN ALTERNATOR EQUIPPED CAR IS RUNNING, THE ALTERNATOR AND IGNITION BOXES CAN BE DAMAGED BY A VOLTAGE SPIKE.
This spike can be avoided by giving the alternator output a safe path to a ground when the switch is turned off. What happens on most cars with cut-off switches installed is that car will continue to run on the alternator output voltage. I have found but not yet tested, a cut-off switch that looks to eliminate this serious issue. Pegasus Auto Racing Supplies sells a switch (#4430 Master Battery Switch) which I think solves this problem. The way the instructions are written (by Pegasus) is a bit confusing but after talking to their tech today it all makes sense now. This looks to be a good upgrade for everyone that might need to stop a running engine in an emergency and wants to minimize collateral damage to the electrics. For those who use the cut-off as a security device, there is no need to change it out.
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03-11-2010, 03:34 PM
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CC Member
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Join Date: Dec 2001
Location: San Diego,
CA
Cobra Make, Engine:
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Wire it to the possitive side of the battery and wire in the power source for your ignition (MSD) or which ever on the back side of the switch. That way when you pull the key the ignition is disabled and the car will shut down.
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03-11-2010, 03:53 PM
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Join Date: Oct 2003
Posts: 4,078
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NHRA requires positive side kill.
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Chas.
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03-12-2010, 06:11 PM
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Senior CC Premier Member
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Join Date: Nov 2004
Location: SoCal,
CA
Cobra Make, Engine: CSX #4xxx with CSX 482; David Kee Toploader
Posts: 3,574
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Here's the link to Taylor's website with cut-off switches. According to the site, their 4-Post Switch complies with NHRA requirements by having the ability to cut off the battery, alternator magneto etc. I guess if you really want to kill it, and don't mind the extra wiring, this would be the way to go.
For me, I'm good with just cutting the negative side and taking they key with me. I don't stray too far from the car.... 
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All that's stopping you now Son, is blind-raging fear.......
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03-12-2010, 06:14 PM
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Senior CC Premier Member
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Join Date: Nov 2004
Location: SoCal,
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Cobra Make, Engine: CSX #4xxx with CSX 482; David Kee Toploader
Posts: 3,574
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__________________
All that's stopping you now Son, is blind-raging fear.......
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