Master and Battery Cutoff Switches

[4.6 litre]

Gentlemen,

I have a 2014 Roadster and a Gen 1 Coyote Engine. My electrical diagram for both do not include the location for a master or a battery cutoff switch however my kit did include a cutoff switch. Can you tell me what is the purpose of a master/battery cutoff switch and where should it be installed in my electrical system.

Thanks,

[egchewy79]

I have my battery pos cable running to one side of the switch, and all my other Pos cables on the other side (starter, etc). I've seen others run the cut off to their ground cable.
I also have a bypass circuit with a small amperage resettable fuse (10 or 20A, can't remember) that allows my EFI and clock to continue to work even if the cut off switch key is removed. If the car is attempted to be started with the key out, the started will pull a high amperage and shut everything down.
Some use the cut off to conserve the battery when storing the car for the winter. Some use it for theft deterrent. And it's a safety requirement for some racing applications I believe.
I'm using it mostly for theft deterrent. I also have a "kill switch" wired to the ground circuit for my inertia switch. Since the fuel pump is wired into the relay that is also grounded by the inertia switch, turning off this switch will stop my fuel pump. Again, mostly for theft deterrence but also a safety feature in the event that I'm in an accident, but the inertia switch doesn't flip off and there's fuel spraying everywhere.

[4.6 litre]

Thanks for your response.

My plan is to put the cutoff switch between the battery and the starter. I will put a 10 amp auto resettable circuit breaker across the cutoff terminals. When the cutoff switch is open all the Keep Alive Circuits will be active. Should anyone try to start the roadster, the circuit breaker will trip and reset itself after 10 seconds. I think this should work. Any comments?

[AC Bill]

My plan is to put the cutoff switch between the battery and the starter.

Is the starter where all of your other 12V+ power is run from?
The norm is to be able to cut off all power, to all parts of the car. This offers a safety aspect, if in a crash, or if there is a short, in addition to a theft deterrent. The keep alive is usually for maintaining computer (ECM) memory, and/or for a clock, both when parked temporarily, or for long term storage. Personally, I chose to use the switch on the ground cable, between the battery and the chassis ground point. Unless the battery was jarred loose, and the ground terminal touched the chassis, cutting off the ground cuts power completely to everything. I have a fused, keep alive wire, for the ECM, which would also act as a theft deterrent. Any large draw will blow the fuse.

The method you describe, using a circuit breaker, should work fine for a theft deterrent.

[4.6 litre]

Is the starter where all of your other 12V+ power is run from?

Yes. If you look at the schematic below, if I put the (open) cutoff switch, paralleled with a low amp circuit breaker, between the +battery terminal and the starter, all current will be interrupted between the battery and the starter except for a small amount needed to supply the Ron Francis "Radio Memory" circuit for my clock and the Coyote "HAAT" circuit for my computer through the paralleled fuse.

Due to my lack of knowledge, I am not in a position to comment on whether to put the cutoff switch on the + or - battery post.

Wiring Schematics -- Sheet 03.jpg

[Al_C]

I'll offer another opinion. Granted, my car is a Gen II coyote, but I believe the Gen I also has a HAAT (hot at all times) requirement. I, as have others, located my master cutoff below the dash on the forward wall over the transmission tunnel.

Input to the master cutoff is from the pos terminal on the battery. The output side of the cutoff goes two places: the starter, and the RF harness main port. Additionally, there is a wire off the INPUT side of the master cutoff that goes to the coyote 250 A fuse, then branches in two directions, the PDDB (power distribution) and to the cable labeled C500. The C500 cable provides the HAAT connection for the coyote PCM.

I hope this helps!

[AC Bill]

As it is currently wired, you may have to isolate the clock memory and the ECM, so power doesn't back feed from the keep alive circuit, into the existing wiring. This would happen when the keep alive circuit is connected to the same terminal on the clock, as is currently powering it via your main harness. Same with the ECM. Not sure if that would be a concern or not? Just keep that in mind if your working on the car, that even with the master switch turned off, there is still live current in the harness.
I suppose preventing back feed might be accomplished by using diodes on the wires currently feeding both of them 12V+ . Diodes would allow power to feed to them from your main harness when your master switch is on, but not allow power to back feed into the main harness, via the keep alive, when the switch is off.. Know what I mean?

If it is possible for your set up, you could also just run independent power wires, one to the clock and one to the ECM directly from your keep alive, and remove the existing wires to them from your harness. I'm not familiar with the amp draw on your Coyote computer when running, so that might not work

I only needed a keep alive for my ECM, (no clock), and by connecting the ECM's ground terminal directly to my keep alive, (being that my master switch controls the negative side of the battery circuit), and there was no other circuit involved, so no chance of back feed. My ECM is always grounded via my keep alive, regardless of the master switch being on, or off.

[4.6 litre]

As it is currently wired, you may have to isolate the clock memory and the ECM, so power doesn't back feed from the keep alive circuit, into the existing wiring. This would happen when the keep alive circuit is connected to the same terminal on the clock, as is currently powering it via your main harness. Same with the ECM. Not sure if that would be a concern or not? Just keep that in mind if your working on the car, that even with the master switch turned off, there is still live current in the harness.

Thanks for your comments. I appreciate your insight.