Wire to automatically switch battery isolator on when running - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 05-16-2015
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Wire to automatically switch battery isolator on when running

I'm planning on hooking up my second battery tomorrow, but I haven't been able to find anywhere the wire that they might have used to have it connect automatically. I will put a switch for a manual override, but still want it to connect automatically any time the engine is running. I was thinking of tying into the signal wire for the alternator, but don't want to damage anythign by doing so. RonD, help me out here.
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Old 05-16-2015
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Second battery is connected to charging system via an isolator, it is in essence a high amp diode, power can only flow in one direction thru the isolator, so anything hooked up to the second battery is not connected to vehicle battery or any of it's electrics.


Some have used a starter relay, those older fender mounted relays, they can handle the amps.
You run a similar gauge wire that is on the alternators B+ post now from the B+ post to the new relay, then from the other large post on the relay a similar gauge wire to the second battery.
The smaller "S" post on the relay activates it(assuming relay case is Grounded), you can use a hot in run wire, or a switch on the dash.

You should put a mega fuse on B+ wire and second battery charging wire.

Hot in run wires can be found at the EVAP purge valve, Fuse 41 15amp in engine fuse box is hot in run

Last edited by RonD; 05-16-2015 at 11:18 PM.
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Old 05-17-2015
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I already have a 500 amp isolator and all the wiring. I will check the evap fuse and see about tying into it. Thank you!
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Old 05-17-2015
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You won't need a hot in run wire with most isolators, but they do come in 3 or 4 post designs, 4 post would use hot in run hookup.

Normal hook up would be to run the B+ wire to isolator, then one wire from isolator's labelled posts to their respective batteries.

Good look at wiring here: Dual Battery Setups

One of the EVAP purge valve wires will be hot when key is on, computer grounds other wire to activate valve, so remove connector to test which wire is hot with key on, and make sure it is off with key off, it is fused via the EEC relay power fuse, 30amp I believe.

Fuse 41 15amp, is just another key on source for 12v in engine compartment, not part of EVAP

Last edited by RonD; 05-17-2015 at 11:26 AM.
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Old 05-17-2015
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The one I have is a 4 post. I think Id rather tie into fuse 41 and use it as the trigger.
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Old 05-19-2015
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Fuse 41 works great. Thank you Ron!
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Old 05-19-2015
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Good work
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