Cooling fan · relay · thermoswitch
Radiator fan relay wiring diagram
An electric cooling fan draws 15–25 A — far too much for a thermoswitch or an ECU output. A relay does the heavy switching; the switch or ECU only drives its coil. Here is how it is wired and how to size the fuse and the wire.
Open the editor · ▶ Run the live demo
The circuit
- Battery + → fuse (25–30 A for a typical fan) → relay terminal 30.
- Relay 87 → fan + ; fan − → ground, with a wire as thick as the feed.
- Coil: 86 to switched ignition +, 85 to the thermoswitch, which closes to ground when the coolant is hot.
- With an ECU: the ECU’s fan output grounds terminal 85 instead of the thermoswitch.
Fuse and wire size
A 250 W fan on 12 V draws about 21 A, so it takes a 30 A fuse, and the wire must carry 30 A: 4 mm² (12 AWG). Check your own fan and wire length in the fuse and wire calculator — the fan starts with a surge, so do not go below the fuse it needs.
Typical mistakes
- Fan wired straight to the thermoswitch — the switch contacts burn.
- Fan ground on a thin wire to the body — the fan runs slowly and the wire gets hot.
- Coil + taken from the fan side of the relay — the relay chatters.
Build it in WireForge
- Open the editor, drag in Battery, Fuse, Relay, a Cooling fan device, a Switch (the thermoswitch) and Ground.
- Wire 30, 87, 86 and 85 as above; set the fuse to 30 A and the fan wire to 4 mm².
- Press Simulate and close the switch: the relay pulls in and the fan turns.
- Click the fan wire to read its current; try a 1 mm² wire and watch the warning.
Questions
What size relay does a radiator fan need?
A 40 A automotive relay is the usual choice for one fan; a twin fan setup is better on two relays, one per fan.
Should the fan run with the ignition off?
Some cars do it to cool the engine after shutdown. Then 86 is fed from permanent +, not ignition — and the fan can drain the battery if the thermoswitch sticks.
Where does the thermoswitch go?
In the radiator or the thermostat housing; it closes at a set temperature and grounds the relay coil.
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