Battery Disconnect with relay up front

[dust]

Trunk mounted battery with disconnect switch (4 post). I'd like to run a relay up front to avoid having more hot wires running the length of my car.

Where are the best places to slice the relay into?

1972/74/77 wiring.

 

[67Dart273]

The strategy of a 4 post (DPST) switch is to kill the ignition so that the alternator cannot continue to power the engine

With the disconnect hooked in the battery main line to the big posts of the switch, run a pair of no 14 or 16 wires from the two small switch contacts to "up front

"Break" the output of the ignition switch coming out of the bulkhead, connect the switch output to one of your two no 14's going to the disconnect. Take the remaining no14 coming back up front to one of the coil connection of your relay, and ground the other relay terminal, or, run it off to a grounded anti-theft switch.

Feed one of the switch contacts with appropriate fuse from the starter relay or your main junction. Run the cold side contact off to your ignition feed..........the one you cut in the first place

If you use a "continuous duty solenoid" or other heavy duty relay you can use this to feed you pump(s), ignition, alternator field/ regulator, and electric fans.

Run your big battery cable "up front" and either hook it to the starter post, to an aftermarket junction post, or to the starter relay post. Run a no4 jumper from there to the starter

 

[BillGrissom]

One option for relay is the Battery Brain. I recall it is rated ~200 A. I have one on all my cars, to protect from running down the battery. Some models have a remote to disconnect the battery for storage or security. List price is fairly high, but I got most ~$30 on ebay.

 

[dust]

The strategy of a 4 post (DPST) switch is to kill the ignition so that the alternator cannot continue to power the engine

With the disconnect hooked in the battery main line to the big posts of the switch, run a pair of no 14 or 16 wires from the two small switch contacts to "up front

"Break" the output of the ignition switch coming out of the bulkhead, connect the switch output to one of your two no 14's going to the disconnect. Take the remaining no14 coming back up front to one of the coil connection of your relay, and ground the other relay terminal, or, run it off to a grounded anti-theft switch.

Feed one of the switch contacts with appropriate fuse from the starter relay or your main junction. Run the cold side contact off to your ignition feed..........the one you cut in the first place

If you use a "continuous duty solenoid" or other heavy duty relay you can use this to feed you pump(s), ignition, alternator field/ regulator, and electric fans.

Run your big battery cable "up front" and either hook it to the starter post, to an aftermarket junction post, or to the starter relay post. Run a no4 jumper from there to the starter

Do you have a picture of this set up? A visual would help my understanding in what you're trying to explain.

 

[67Dart273]

All you are doing is "cutting" where the ignition switch feeds out of the bulkhead. That switched "ignition run" normally comes from the switch, out of the bulkhead, and feeds the ignition and regulator on 69/ earlier cars

On 70/ later it branches off and feeds some other stuff, including the later alternator blue field wire, idle solenoid on 6 packs etc. All you are doing is cutting that feed and using the power coming from the key to activate a relay / solenoid. THAT activation line is wired in series with the two small wires coming up front from your disconnect so that when you pull the disconnect, the relay drops out and kills both the alternator and the ignition

You do understand? That simply cutting the battery will not kill the engine. The alternator, once activated will continue to charge and to operate the system. This in fact is "an old wives tale" which goes "If you disconnect the battery and the engine dies, the alternator is bad."

One of our members drew this up. The solenoid could also be a relay, a good heavy one, heavy enough to switch whatever underhood switched loads you have. If you don't run a pump, or use a dedicated pump relay, and same with fans, etc, a good quality "Bosch" relay is probably good enough

Step 6 is not really necessary.

Follow along...........

The "cut blue" coming out of the bulkhead is your old ignition feed coming FROM the ignition switch. You want this to turn on the relay / solenoid THROUGH the small terminals of the disconnect. So from the bulkhead, goes to the rear through your small conductors, through the small terminals of the switch, back up front on the second small conductor, and to one of the small terminals of the solenoid / relay. The second terminal goes to ground. YOU COULD run that second solenoid / relay terminal to ground through a second hidden switch for anti theft.