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Thermo Fan Install

Step-by-step fitment of a thermo fan.

By Michael Knowling

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Does your car run a bit warm? Well, here's how to fit a thermo fan...

Buying and Mounting

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Second-hand thermo fans can be bought cheaply at your local wreckers. This example was pulled from an early '80s Mercedes and cost just AUD$22. It measures 12 inches in diameter and is very slim, making it ideal for our particular application.

Buying Tips

It's a good idea to look for a thermo fan from a vehicle that's the same make as yours - there might be mounting similarities to make the installation easier.

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Also - where possible - grab the wiring all the way up to the loom connector. Snip the wires on the loom side of the connector so you've got a nice, factory-style connector to be used as part of your installation. Shown here are the male and female connectors for the Mercedes thermo fan.

The thermo fan can be mounted using brackets fabricated from aluminium strips. Note that the brackets should be quite sturdy and not thinner than about 2.5mm.

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As seen here, 2.5mm thick aluminium strip can easily be bent to the appropriate shape using a vice and a pair of multi-grips. You can use a piece of cloth between the jaws of the grips if you want to avoid marking the soft aluminium surface.

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Our installation required only two brackets to secure the fan - and here are the finished products. About an hour of effort was all they took.

Firing Up the Fan

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Due to the high current draw of a thermo fan you'll need to run its 12-volt power supply through a relay. You'll also need a method of switching the fan on and off at certain coolant temperatures and, of course, only when the ignition is on. There are a number of ways to achieve this, but one of the easiest is to buy a Davies Craig Thermal Switch kit. These kits retail for around AUD$80 and include the necessary relay, an adjustable temperature switch, mounting bracket and a complete wiring loom including terminals and a fuse holder.

Note that this kit dates back a few years so you might be lucky enough to find a second-hand one at the wreckers when you're looking for the thermo fan.

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The first step is to mount the kit's temperature sensor inside the top radiator hose. The copper sensor can be slipped beneath the radiator hose using a grooved rubber seal to prevent any leaks. The radiator hose clamp is simply retightened over the top as per usual.

The sensor's copper tail is connected to the adjustable temperature switching mechanism, which should be mounted on a (supplied) bracket near the radiator.

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Next, find a position to mount the thermo fan relay. Somewhere between the battery and radiator is ideal. There are four wiring connections to the relay - a 12-volt supply, earth, a switched input and the positive power output to the fan.

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The relay's 12-volt supply can come straight off the battery but make sure the supplied fuse holder is mounted close to the positive post to ensure maximum safety. The relay's earth can be either the battery's negative post or a reliable body earthing point. Note that the earth lead from the thermo fan can share the same earthing point as the relay - this makes installation easy.

Wiring the thermo fan's switched input requires a bit more effort. Find an existing wire under the dash that is switched on and off by the ignition (such as the switched power supply to the radio) and use the supplied scotchblock terminal to T in another wire.

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This new wire should then be connected to one of the terminals of the adjustable temperature switch mechanism. The second terminal of the switch mechanism should then be wired to the relay. (Note that the pins of the relay are clearly identified in the Davies Craig installation instructions) This arrangement means the fan will activate only when the ignition is on and the coolant temperature switch has been tripped.

The last step of the installation is to insert the appropriate spade fuse into the fuse holder. In the case of a 12-inch thermo fan (such as ours) you'll generally need a 20-amp fuse, while less powerful fans use smaller fuses.

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Start the car, go for a drive and watch that temp needle stay below the danger zone. Congratulations - job done!

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