Volvo 164 electric fan thermostat

While this circuit could be fitted to any vehicle, it was designed for use on my Volvo 164. The original mechanically-driven viscous coupling fan had a loose centre bearing and made alarming clanking noises at certain revs, so rather than try and arse about finding a replacement I figured I'd fit an electric fan and possibly save an mpg or two at the same time.

Of course, being a pigeon, I couldn't use a conventional temperature sensor in the radiator. For one thing, I'd have had to drill a hole in the rad to fit it. I don't really want to do that. Also, they're shit. I've lost count of the number of cars I've had to bodge by shorting the defunct temperature sensor so the driver can get home (most of them carry on driving like that indefinitely...) not to mention the number of shagged head gaskets from the same cause.

A favourite temperature sensing method of mine is to use the variation in voltage drop across a diode being driven from a constant current source. So this is what I did. Four 1N4007s in series on a scrap of veroboard are held to the radiator with a brass clip - the Volvo 164 has a proper radiator, made of copper and brass, so it is easy to solder a brass clip to it. In addition to the clip, the diodes are glued to the radiator with Araldite. This is more for reasons of thermal contact than mechanical fixing, because the Araldite goes all gooey and soft at operating temperature, so the clip does the work of holding the diodes in place.

Volvo 164 electric fan thermostat schematic

The above circuit diagram shows the rest of the gubbins. An op-amp circuit detects the fall in voltage across the string of diodes with rising temperature and operates a relay to switch the fan on. A multi-turn pot sets the switch-on temperature; I set mine to 92 deg C. A small amount of hysteresis is provided to ensure clean switching. The relay needs to be a beefy item to handle the switch-on surge of the fan, which is considerable. I used a glow plug timer relay off a diesel Peugeot. This also provided a handy housing to mount the circuit board in, fitting it in place of the original timer circuit board.

The two diodes shown dotted are required if you want to duplicate the function of modern electric fans in continuing to run when the ignition is switched off. I don't regard this as a very useful function unless you also have an electric water pump still running, so my unit does not include them.

The "command on" LED is for bench testing the unit without a fan connected. It may not be strictly necessary but all the best designs feature an LED. The "override" switch and "fan on" indicator light are dashboard fittings - the light to indicate that the fan is receiving power (I couldn't be arsed to go all the way and fit a rotation sensor) and the switch to bring it on manually if you don't like the look of the temperature gauge. The light provides the auxiliary function of "motorway speed limit indicator" - when I get up to about 70mph the fan starts windmilling and generates enough juice to make the bulb glow.

I am pretty sure the circuit diagram is correct - I have checked it over several times - but I cannot be totally certain as it is drawn from memory, and I can't be arsed to take the unit out of the car and dismantle it and trace it out. The reason for this vagueness is that the original circuit diagram tunnelled out of the folder of Volvo-related documents in which it was being kept. If by any chance someone reading this article has discovered the mysterious appearance of a very similar circuit diagram drawn in pencil on a bit of greasy lined A4, please contact me.

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