How to wire an electric fan

Many cars are fitted with a engine driven radiator fan, these run all the time, wasting power. And worse still, in stationary traffic, when you need the most air flow, they run at their slowest speeds. To improve this you can fit an electric fan(s) to the radiator.

It is important that the fan fits as close to the radiator as possible, and ideally has a surround to allow the air to be pull thru the radiator, not just in along the edges.

fan no surround
The previous setup

As can be seem above the original set-up on the coupé wasn't ideal as far as surrounds go, it was a fan off a Rover mounted about 10mm back from the radiator.

twin fans
Twin fans

The twin fan (more of that later) set up is mounted right up against the radiator. It is better, if you can, to attach the fan to the edges of the radiator, not with the cable tie style pins, as these can damage the radiator core. You will notice, I also fitted a rubber seal to the tops and side of the radiator to make sure all the air flowed thru the core.

I decided to fit a proper thermal switch, rather than the type you again push into the core, or radiator pipe. Before you take this route, check to see if you radiator has brass tanks, otherwise you will not be able to solder the boss on.

The first thing needed was a way of fitting the switch and the switch itself, so at the local scrap yard, you need to find a VW of some sort with a broken radiator end tank, there will be loads, as they are just plastic and tend to be broken in any frontal impacts. Unscrew the thermal switch, and cut off the plug as far up the wiring loom as you can. Now you need to break the plastic away around the switch hole, so you can access the brass threaded boss.

switch and boss
switches and boss

I removed a second switch from a Jetta, as these provide a dual stage switch. On the side of the switches should be the temperature they work at, try to find one that opens at approximately 15o to 20o above your thermostats temperature. The VW uses a common M22 thread, so look on other cars for a suitable switch.
My Thermostat opens at 78°C so I chose the Jetta switch that turns on at 95°C, off at 81°C, stage two turns on at 102°C and off at 91°C.

Once you have your boss and switch, you need to decide on a position on the radiator to fit it, it need to be a close to the return connector as possible. Then drill a suitable hole (22mm) and solder the boss above it. This was done using plumbing flux, solder and a propane torch.

boss
boss fitted
boss and switch
switch fitted

Now it is just a case of wiring the fan(s) up. This needs a relay as the switch is unable to handle the currents required by the fans, and would quickly fail. The wiring is show below, for both a single and double fan setup. The diode is needed to stop any back EMF from damaging the switches.

single fan wiring
single fan wiring

twin fan wiring
twin fan wiring