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Many R107s have an auxiliary electric fan in front of the A/C condenser. It helps cooling in traffic and when the A/C is on. When it fails, the car can run hot at idle and the A/C gets weaker.
When the aux fan should run
On a typical US R107 with A/C, the auxiliary fan runs:
- When the coolant reaches a high temperature set by a switch in the cooling system
- When A/C pressure rises high enough, on cars with a pressure switch
- In some cases whenever the A/C is on
The exact triggers depend on year and model. Check the wiring diagram for your car.
Temperature switch and A/C trigger
The fan is controlled by a temperature switch, often a two-stage switch screwed into the cooling system, and on some cars by a pressure switch in the A/C system. The switch closes a circuit that powers the fan relay.
Relay and fuse
The fan has its own fuse and one or more relays. Relays get hot and fail with age. Look in the fuse box and relay area for corroded or melted connections. See fuse box and chart.
Test steps
- With the engine cold and ignition on, check the fuse.
- Unplug the temperature switch and bridge the connector terminals with a jumper wire. The fan should run. If it does, the switch is faulty or the coolant is not reaching its temperature.
- If it does not run, apply 12 V and ground directly to the fan (unplugged from the harness). If it runs, the problem is the relay or wiring.
- Spin the fan by hand: a seized or stiff motor will draw high current and blow fuses.
Replacement
Fans and switches are available new. When replacing the fan, check the shroud and mounts. Use the correct temperature switch for your engine; a lower-temperature switch makes the fan run constantly.
Quick answers
- Should the aux fan run with the A/C on? On many cars yes, at least at higher A/C pressure. If it never runs with the A/C on and the car running hot, test it.
- Can I wire the fan to run constantly? It would shorten its life and can make the engine run cooler than designed. Fix the switch instead.
- Is the aux fan needed if I have a good fan clutch? The mechanical fan does most of the work; the auxiliary fan matters in traffic and with A/C.
