Auxiliary fuse box relay

I have lots of aftermarket electric gadgets in my car—fog lights, rear window defogger, cruise control, gauges, wiper delay, etc. It didn’t take long before I had to install an auxiliary fuse box. Since I didn’t want to run the box and an extra 20 amps or more through the ignition switch, all those accessories could be turned on and left on with the engine off.

Though an engine-off/appliance-on alarm helped with the rear defogger (the only device you couldn’t see working that also drew a lot of amps), it was still possible to drain the battery overnight with even a relatively small load. And, mechanics always seem to find a reason to randomly turn switches on and not turn them off again.

The solution, of course, is an auxiliary fuse box relay. It runs off an accessory fuse on the main box, so everything on the auxiliary box stays off unless the ignition key is in the “on” or “accessory” position. And the relay doesn’t add a significant load to the ignition switch.

Without further ado, here is the schematic. All parts are easily found at an electronics or auto parts store. Note: The bypass and disable switches (which can be combined into a single switch) are just a little extra bonus.

Relay schematic

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