The short version
A relay lets a small signal switch a big load. Your ECU output or a dash switch can only handle a small current. A fuel pump or thermofan might want 10, 20 or even more than 30 amps. The relay sits between them: the ECU switches the relay, and the relay switches the pump.
Inside a 4-pin relay
Relays come in dozens of shapes and pin counts, but the basic 4-pin relay is the one to understand first. It holds two separate circuits that never touch each other electrically.
- The control side is a small coil of wire around an iron core. Put current through it and it becomes an electromagnet. It draws very little, typically around half an amp.
- The load side is a heavy contact called the armature. When the coil is magnetized, it pulls the armature closed and connects the load circuit. When the coil switches off, a spring pulls the armature open again and the load turns off.
What each pin does
| Pin | Side | Connect to |
|---|---|---|
| 86 | Control (coil) | A 12V supply that switches off with the ignition |
| 85 | Control (coil) | The ECU output or switch that pulls it to ground to activate |
| 30 | Load | Battery positive, through a fuse rated for the load |
| 87 | Load | The device being powered, such as the pump or fan |
The device's own ground then goes to the chassis, with wire just as heavy as its supply.
Ground switching vs power switching
In the layout above, the ECU switches the ground side of the coil. That is the most common arrangement in cars, and it suits ECU outputs, which usually pull to ground. It works the other way too: give pin 85 a permanent ground and let a switch send 12V to pin 86. Use whichever fits wiring that is already in the car.
Wiring it well
- Size the wire to the load. Pins 30 and 87, and the device ground, all carry full load current. Check what the pump or fan draws and choose the wire gauge to match.
- Fuse the supply. The fuse on pin 30 protects the wire, so pick one that suits the wire and the load, not the biggest one in the box.
- Keep the coil feed switched. Pin 86 should lose power with the ignition so nothing can run with the key off.
- Crimp properly. High-current terminals that are poorly crimped get hot. See our crimping guide.
- Ground properly. A relay can't fix a bad ground; our grounding article explains what good looks like.
Most ECU outputs are designed to switch a relay coil, not a motor. Asking one to carry pump current will damage it. Newer Haltech hardware with high-current outputs is the exception, but the rule for any output is to check its rating first.
Parts we use
- Haltech 30A power relay: a 5-pin changeover type. Its extra pin (87a) is connected when the relay is off, and you can simply leave it unused for a basic on/off job.
- 6-circuit Haltech fuse box or the waterproof mini fuse and relay box to keep relays and fuses together.
- 100A solid state relay for very high loads or fast switching.
- A Nexus PD16 PDM, which replaces relays and fuses with electronically protected outputs.
FAQ
How do I know if a relay has failed?
With the coil powered you should hear a click and see battery voltage at pin 87. A click with no output points to burnt contacts; no click points to the coil or its feed.
Can one relay run two fans?
Only if the relay, wire and fuse are rated for both fans combined. Often two relays is the cleaner answer.
Want to see relays planned into a whole harness? Watch how it's done in Dave's wiring vlog.
