True Mods Relay Wiring Diagram

Wiring relays correctly is where most people mess up their builds

I spent three weeks tracing a parasitic drain on a project car last year because someone had wired a 30-amp relay backwards through a standard automotive relay terminal block. The car wouldn't stay off. That's the kind of problem a proper wiring diagram prevents. The True Mods Relay Wiring Diagram is a straightforward reference for how automotive and automotive-adjacent relay circuits are structured. It maps out the five common pins you'll see on a standard 5-pin automotive relay and shows how they connect to your power source, ground, switch, and load. Without one, you're guessing. With one, you have a schematic to follow.

True Mods Relay Wiring Diagram

The diagram breaks down each pin clearly. Pin 30 is your constant battery feed. Pin 87 is the switched output that goes to your load. Pin 85 and Pin 86 are the coil terminals, and either one can be ground or switched positive depending on how you wire it. Pin 87a is the normally-closed contact, which most people don't use unless they're doing something specific like keeping a fuel pump running on backup. Here is how I actually wired it in practice. I ran a 10-gauge wire from the battery directly to Pin 30 with an inline ANL fuse within six inches of the positive terminal. From Pin 87, I sent a separate 10-gauge wire to whatever component I was powering. The coil side came from a low-current trigger circuit. I used 16-gauge wire for the trigger because the coil only draws about 80 to 120 milliamps depending on the relay. A simple toggle switch grounded through Pin 85, and Pin 86 went straight to a fused ignition source. The first time I used this setup, I hit a snag with a high-draw cooling fan that kept burning out the switch contacts. The switch was rated for 10 amps but the relay coil was pulling more current than I expected because I had grabbed a cheap relay instead of checking the datasheet. Some of those no-name relays from bulk packs have coils that draw 200 milliamps or more. The fix was swapping to a known-good Omron G5V series relay and rerouting the trigger through a transistor driver so the switch only handled microamps. The problem was completely gone after that.

One thing beginners consistently miss is that the coil polarity does not matter on these standard automotive relays. You can flip Pins 85 and 86 around and the relay will still pull in the same direction. I used to get confused by this until I tested it on a bench. The diagram shows them separately for clarity but electrically they are interchangeable. What matters more is getting your fuse placement right. A common mistake is putting the fuse on the trigger circuit instead of the main power feed to Pin 30. The fuse protects against a short between the battery and the relay. If it is on the trigger wire, you are protecting almost nothing of consequence. Another counter-intuitive point is that your load wire from Pin 87 does not need a separate fuse if the wire gauge is properly matched to the relay's contact rating. Most 40-amp automotive relays can handle 40 amps continuously. Running a 10-gauge wire from Pin 87 to a 30-amp load is fine without an additional fuse at the relay because the wire itself cannot carry enough current to start a fire before the relay contacts weld shut. That said, putting a fuse closer to the load is still a good practice because it protects against a short after the relay rather than just before it. The real weakness of any relay-based wiring setup is that it introduces more failure points. Every connector, every spade terminal, every ground strap is a place where corrosion or vibration can fail. I once had a setup that worked perfectly for two years and then started intermittently failing because water had gotten into a single Deutsch connector on the coil side. The diagram does not account for environmental sealing, which is something you have to add on your own.

If you are wiring anything above 30 amps, consider a contactor instead of a relay. Relays are rated for their contacts for a reason, and pushing them beyond that shortens their life dramatically. I learned this when a 40-amp relay started pitting its contacts after six months of running a 38-amp air compressor. The contactor lasted years under the same load because it was designed for continuous high-current switching rather than the intermittent use relays are built for. A downloadable version of the True Mods Relay Wiring Diagram is available at truemods.com/wiring-diagram. It is a single-page PDF with the standard 5-pin relay layout and a few common configurations mapped out. It is not exhaustive but it covers the vast majority of automotive and off-road applications you will run into. If your build involves something unusual like a 12-volt diesel glow plug system or a dual-battery isolation circuit, you will need to adapt the diagram yourself or look for a more specialized reference. The diagram shows a clean schematic, but real-world installations always require some adaptation. Wire length, fuse ratings, and connector selection all depend on your specific application. The core principle stays the same though. Constant power into Pin 30 with an inline fuse, switched ground or power through the coil, and the load connected to Pin 87. Get those three things right and everything else is just routing and protection.