Fan Temperature Sensor

For the fan to work, it needed a temperature sensor. I had bought a simple sensor and a fitting to place it in the lower radiator hose. I would need to drain the coolant, cut a piece out from the hose and add the aluminum fitting in. Then I could just screw in the sensor and attach the wires.

But I ran into a problem. The largest fitting I could find was 42 mm in diameter but the hose was 48 mm. No matter how much I searched and asked around, I couldn't find a larger one. Next I thought about building one myself. Should be simple to buy a short pipe, drill a hole to the side and weld in a nut. But I didn't have a welding machine...

I called US-Parts in Helsinki and they had the solution. They were selling a Hayden Automotive fan sensor model 3647 that I could use. It had some good and bad sides. The good news were that now I wouldn't need to drain the coolant, cut the hose and that I could adjust the temperature settings. The bad were that I would need to add new wiring for the sensor and that I didn't know how accurate the sensor was when it wasn't actually touching the coolant. Despite these, I bought it.

I attached the control box to the driver side fender and checked the wiring. Luckily I could use almost all of the wires in my own harness and only needed a few new ones. Here's how I connected them up:

• Red wire - permanent power: I still had room in my power junction so I connected the wire directly to it.
• Black wire - ground: There was a large ground wire meant for the actual fan near the sensor so I split it up to two wires and used the other for grounding the sensor.
• Yellow wire - ignition: I had an ignition wire already in my harness as I had meant to use it to power the other fan sensor. So I just connected it to the new sensor. Oh, and originally the ignition signal was spliced from the windshield washer power wire.
• Green wire - AC power: This signal would force the fan to turn on when AC was on. As the AC had been removed from my car, this wire was unused. This wire could be used if I'd like to override the sensor with a switch later.
• Blue wire - fan 1: Connected directly to the fan itself.
• Orange wire - fan 2: Not connected as I only had a single fan.

Final bit was to attach the sensor itself. The instructions said that I should place it as close to the upper hose as possible. I found this a bit strange as shouldn't the fan be turned on when water exiting the radiator gets too hot? If air flow by itself is not enough to cool the engine? Isn't this the reason that normally fan sensors are in the lower hose? Based on this I installed the sensor as low in the radiator as the wire permitted. Unfortunately this was not very low as the wire was so short. Oh well, I could tune the fan activation point from the control box so it was OK.

And now I had a complete electrical fan control system in my car. I just needed to test it and adjust the fan activation point.