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Best way to add a fused circuit and/or relay

EFI Wiring Fundamentals

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Which video would I need to watch, or where can I learn the best way to add a circuit that only turns on with the key’s accessory position?

I see a lot of people use the audio-mod approach where an “add-a-circuit” would piggy-back on a fuse location such as the windshield wipers. My concern is that based on how they are wired, it may put the load on both fuses. I am not yet sure the size I will need so if I need a 20 amp, I don’t want to be limited by the 10 amp of a pre-existing accessory.

I’m assuming they tie into the existing relay panel so it is safe though.

My car is a 2007 Mustang GT, and I need to power:

- reverse solenoid for TR6060 transmission

- various gauges and boost controller

- brake line-lock solenoid

- exhaust cutout

- accessory lights and power (door panel, mirror, etc)

- Fore Innovations triple pump fuel system (using FC3 controller)

My hope was to come up with the best method of running a separate power and chassis ground that only turns on when the car is on or in the accessory position.

Thank you for any direction you can provide. Meanwhile, I am finishing up the Wiring fundamentals course and then will move on to the club level wiring course.

The current draw of the fuel pumps is probably going to be the deciding factor but if someone were to ask me to do it simply, cleanly and bullet proof I'd just run a contactor or high capacity relay such as this:


To power some sort of PDM. For simple straightforward and fairly small ancillary setups I go with this:


Strap the contactor and PDM to a panel, fuse and pull juice off the battery through the contactor to the PDM and then control the contactor via your 'audio-mod' approach. Current draw on the control circuit you choose will be negligable.

This is what I'd do/tend to do on race vehicles that don't want to sack up and run a solid state and based PDM. Hope this helps.

P.S. There are a myriad of different PDM configs you can sub in to suit and if you're feeling froggy Eaton has released what seems like a nice compromise in their CAN capable mVEC PDMs but I haven't used them.

Thank you so very much for that thorough reply. I’ve done a bunch of research on my setup, and it turns out I am fortunate that the FC3 controller I have coming from Fore Innovations has relays built in and handles 90 amps total (30x3). I will power it with a fused wire (80 amps), and trigger the internally relays with the OEM inertia wire and then just disconnect the OEM FPDM.

For the other accessories, they are seemingly low on amp requirements (3 amps and less), and there is apparently an open blade slot in the SJB under my glove box inside the kick panel, so I think i can use an add-a-circuit there @20amps, then branch it out to 14 gauge or so for each accessory. A friend recommended I switch the gauge backlights by tapping into the headlight switch.

Hopefully all this pans out, as it sounds like a safe way from what I’ve learned today. Anything raise red flags to you?


Now I am not so sure, because all of these add-a-circuit’s have 16awg wire, which I see in the charts is rated around 7amps. I’m not sure why they would sell something like that and rate it for 20amps. Any idea?

I like your idea of using a PDM and triggering it with the add-a-circuit, so are you aware of one that is straight forward as a single relay with several fuse links since i only need the one relay? I’ve been looking but I’m getting lost in the options.

For the physical location, it would seem that I would want the PDM mounted in the rear by the battery so that the fuse is close to the source, then I would extend the add-a-circuit from the front SBJ to the rear of the car at the PDM, and run the wires back up to the front. It is a lot of cable length, but is that the safest approach since it is fused within 1-2 feet of the source?

If you don't need to be able to switch the circuits just run a single relay to power a single bus fuse panel.

Thank you. I’ll see what I can find.

Does the last part of my updated post make sense? I wanted to confirm that I would keep the relay and fuse panel in the trunk with the battery so it is close to the power source, but that would mean i would need to have the add-a-circuit for key-on run all the way from the SJB in the front to the trunk, then run the smaller accessory power wires back up to the front. That’s fine of course and I would do it neat and orderly from what I’ve learned in these vids so far, if i go this route. Just wanted to make sure it’s the right approach.

It turns out that the OEM inertia wire for the fuel is a 16awg wire, and is fused at 20amps, so now I don’t know what to believe. It seems some charts show 7 amp max and others show 22 amp max for 16awg.