Battery Switch Wiring - JeepForum.com
 
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post #1 of 8 Old 07-02-2021, 07:08 PM Thread Starter
JeepinGeo
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Battery Switch Wiring

Hey everyone. I would like some insight as to wiring my battery switch. The switch I have is a Fastronix rated for 300A continuous and 1000A intermittent, with an indexing pin to keep the switch from spinning. It has 4 posts, two of which are 1/2" studs, the other two posts are smaller for an ignition cutoff. The switch is setup for positive side, but I was hoping to connect it to the ground side to keep my wiring simpler (and safer). If I wire the switch to the ground side and the Jeep is already off, it cuts power to everything and it won't start. The problem: If the Jeep is already running, however, then it really only cuts power to the accessories, starter, etc., and not the actual engine, given that the alternator will still backfeed and run the distributor. I suppose the only way to kill the engine itself at that point is to wire the distributor feed (+) to a separate switch. But then that will require two switches to do one job. This is because, I assume, that the switch is internally rated for a single pole (uninsulated) and will not control both a negative lead off the two larger studs and a positive lead off the two small studs. Does anyone have any realistic suggestions? I can wire the switch to the positive terminal but it is not ideal for my setup. Thanks.

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post #2 of 8 Old 07-02-2021, 07:18 PM Thread Starter
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Well I think I just answered my own question. The switch can in fact be used as a Dual Pole configuration. So this would mean I can control the main circuit as a ground and the smaller circuit as a hot for the distributor, correct?
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post #3 of 8 Old 07-02-2021, 07:55 PM
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These types of cut off switches work fine on a Jeep to isolate the battery when standing for long periods. If you just wire it to the Battery positive or negative cables, the engine will still run if already running. IT IS A BATTERY ISOLATOR SWITCH!!!!

It is not ideal to isolate the battery whilst running, it can occassionally lead to spikes in the alternator rectifier and damage it. Hence it should only be used as a battery isolator.

I do have one Jeep with it wired into the negative cable, rather than the positive and note:

- It works fine but I never use it to turn off the engine, that is what the key is for.
- It is also not very practical on a modern jeep because they often have radios with memories. A better install is to put it on the positive lead, and connect a small wire with a small fuse from the in side of the switch to the radio memory wire. the small drain will be hard to notice and no danger of the jeep catching fire from it.
- If you have other positive wires connected at the batery terminal, they should be connected at the solenoid instead. At the battery terminal they are subject to movement and often fray.


The model you have has an additional feature of a second smaller cut off switch incorporated but rated at 20 Amperes and specifically designed for the ignition circuit. Yes, you could use this for the ignition circuit and hence create a kill switch, but this is designed for a race car where a fried alternator is the least of their concerns.

It is not recommended for cutting the feed to the distributor. Unless you have an all in one HEI style or points, the distributor does not have an electrical feed but this may refer to more than that.

By connecting to the ignition circuit, rather than the 'Distributor" you can cut the ignition module, alternator field terminal and some other circuits all at the same time. Just cutting the feed to the ignition module, which is simple enough, may not be as effective as cutting all the ignition circuits.

How you connect it depends to an extent on where you mount it. I have mine on the instrument panel near the ignition switch, with a removeable key that is chained to the ignition switch, which means it gets turned off every time to isolate the battery. You could put it on the outside, like in a race car, but that lets strangers turn your CJ off.

If you have it in the instrument panel area you could snip the outgoing red with tracer wire at th ebase o the steering column (from the ignition switch to the coil, module, alternator etc) and wire it up and through the switch. (there is a second one that goes to the fuse box, make sure you intercept the correct one) This will isolate the battery at the same time as cutting the module and alternator.

I would still not recommend cutting the engine in this way, there is a chance of alternator damage.

BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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post #4 of 8 Old 07-03-2021, 10:03 AM Thread Starter
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Thanks for the reply Bagus.

I am currently running an HEI coil in cap. I am planning on mounting this switch (no key) in-dash or under the dash against the firewall (per a custom bracket). Use is mainly for ease of working on the Jeep without causing arcs, but also for safety in the event something goes haywire on the trail - which can, but shouldnt happen. Ultimately, the switch only performs 100% effectively when the vehicle is already off, given that it can't start at all unless the switch allows power. Now if the vehicle is running, that's a different story. I know you explained some of it, but curious how you would wire it to shut down the engine as well withOUT burning up the alternator. Is there any way to do that while still using the ground side? I was just planning on using the two smaller studs to run the distributor feed through; Cut the switch - distributor has no power - engine stops. I understand there will still be some backfed power from the alternator, which in theory would dead end right at the stud terminal on the switch.

As for radio memory, I'll just wire a separate fused always hot and an always ground wire and that will be that, no trouble.

Also, since you will likely see this tomorrow, Happy Independence Day!
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post #5 of 8 Old 07-04-2021, 07:44 AM
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Are you planning to run your neg cable from the battery into the cab to the switch then back out to the engine ground? Why not simply put the switch under the hood next to the battery with a high amp fuse or circuit breaker to protect the system? You have protection against a massive short, the ability to turn off the power, and your cables remain inside the engine compartment.



Automotive wiring is common ground. While I think an interrupter switch on the neg side is a better idea safety wise due to you routing the cables in and out of the cab, I think you are going to have issues with attempting to single wire ground your radio without isolating it from the chassis ground.

Paperwork will ruin any military force.
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post #6 of 8 Old 07-05-2021, 02:20 AM
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i agree with Jeepdaddy, if you provide a ground for the radio it will somewhat defeat the switch. You have a good chance all ground will now go through the radio.

The point about wiring the ignition circuit through the switch is that it will disconnect the HEI as well as the field tab on the alternator at the same instant. There will be no feedback or spike etc. If you look at your wiring diagram you will see where both items should be fed from
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BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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post #7 of 8 Old 08-02-2021, 04:02 PM Thread Starter
JeepinGeo
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As always, you guys have awesome replies. Thanks Jeepdaddy and Bagus. A couple things - I decided I'm not gonna fool with wiring an always grounded circuit to the radio. In the event I really do have to turn off the master switch (i.e. fire or heavily involved electrical repairs), losing my radio memory is the least of my concerns. That covers that potential ground path issue one of yall mentioned.
The other pertains to the location of the battery switch. I could put it inside the engine bay, however, I want immediate access to it in the case of an emergency. Trying to raise the hood would only take longer - not sure how much, but it's certainly something else I don't want to worry about. The location of the new custom fuse box is actually where the old heater box was. Since the heater was removed, the aluminum fuse box actually tucks up behind the dash under the cowl. This means the distance between the battery in the engine bay and the new fuse box is very short. 2 feet max. It is not an issue running the negative ground cable from the battery through the firewall to the switch (which is located in the top of the fuse box and is not easily visible) and then back out to the engine block/frame.
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post #8 of 8 Old 08-02-2021, 05:39 PM
John Strenk
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You could always wire in a relay that opens when it looses ground.

Run the switched contacts between the wire running from the alternator.
Run the relay coil from the (+) of the battery and the other side of the coil to your ground switch.

That way, when the switch is opened up, you loose the battery ground. and the feed from the alternator that is keeping every thing running.

ground-cut-off-switch.jpg
Pretty soon we'll have enough relays for a YJ!!!

[edit: No, darn it, that won't work. Need plan "B". Alternator still has ground... ]


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