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I race the car quite often in the summer and I talked to one of the tech's at the track today and he said if the battery is in the trunk then you need a shut off switch. No problem I said, he replied that many people put the switches in incorrectly. If you have an alternator, which I do, then the alternator must cutout when the switch is activated. How do you go about doing this? I always thought if the rpm was below 1000 (idle) the alternator wouldn't keep the motor running without some help from the battery?
 

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If you're running the standard Ford Alternator and regulator, It's pretty easy to cut off the power to the alternator. Look at the regulator and find the terminal marked "I" for Ignition. Route this wire through your master cuttoff switch and the alternator will cut off when the master switch is turned off. You can also gain a few extra horses by cutting power to the alternator during a run with a heavy duty toggle switch in this same wire.

Dave J

So many Mustangs, so little time!
 

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In the past we would mount a 12V NO solenoid controlled by the shutoff switch in the wire going to the I terminal of the regulator...that obviates the need for running a heavy guage wire to the trunk....IIRC we used 16ga for the control circuit with an interior toggle switch to cut out the alternator on the run...
The master cutoff in the taillight panel still was the overriding control for the circuit....

I just use two large deep cycle marine cranking batteries and they usually last me 3 days or so of racing...no alternator required....but my car is built strictly for racing...

Pat
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So, If I've got this right, I run the wire that normally goes to "I" on the regulator to the back of the car through the on/off switch then to the regulator? I just want to make sure I do it right the first time.
 

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Yes, just imagine that master cutoff as an ignition switch you can't control....it and the track staff are the final arbiter of whether you go or stay...

The "I" wire comes from the ignition switch and allows the alternator to be excited so it can produce current to charge the battery...

Or at least that's what I learned back in auto shop in the dark ages...*G*

Remember to run the same guage wire (I'd recommend one guage larger because of the long run) back to the master switch as is used in the wire you remove from the regulator. Unless you want to shut off the master switch every time you stop the car, I'd suggest running a wire from the "I" wire to the switch and another back to the regulator....if you just run one from the master switch to the regulator and you leave the switch on, you'll likely encounter a dead battery after a while...
Or maybe I'm full of it......*G* ...that's why I used the solenoid, ran the wires as described above and never found out the alternative...of course that was a long time ago; maybe they wouldn't allow a solenoid now...I don't know...

Pat
http://albums.photopoint.com/j/View?u=1570936&a=11937754&p=42910787.jpg
 

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IIRC, we used a 15A SPST toggle switch....

There are more options out there now but a good basic toggle switch always works...let the wire guage govern the amperage rating of your switch....15A will work with anything up and including 14 guage wire....20A to 12ga...etc...

We used a solenoid so the switch just took the minimal load necessary to trigger the coil...

Pat
http://albums.photopoint.com/j/View?u=1570936&a=11937754&p=42910787.jpg
 

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As I understand it, if you disconnect the battery when the alternator is still spinning, the current produced by the alternator will have no where to go and could fry some of your more sensitive electronics.

I have a cut off switch that provides a circuit that routes that current through a resistor when the switch is off.

I want to move the battery to the trunk too, but I haven't figured out how to do it yet with out running a ton of wires from the engine compartment to the truck thus reducing the performance advantage of moving the battery (how much does 13' of 1ga wire weigh?).

'65 fb restomod
 

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I cases where the cutoff switch comes into play on the drag strip, fried electronics would be the last of your worries! A race official will only "shut you down" in case of a serious risk to your health, like a fuel leak or some broken suspension part. The outside shut off switch is just a safety device and shouldn't be used unless the situation calls for it. So don't worry about your electronics.

As to the weight advantage of moving the battery vs. the extra weight of the cable, it more an issue of where the weight is located rather than anything else. The weight of the cable is more or less distributed evenly between the front and rear, but moving the battery reduces the weight on the front (for better weight transfer) and increases it on the rear (better traction).

Dave J

So many Mustangs, so little time!
 
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