Page created: 8-Feb-04

Re-published: 1-Nov-09

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Ancillary Wiring

An engine pre-heater is a device that's designed to independently heat the water in the cooling system of an engine so that it's much nearer to operating temperature when it's first started. This aids cold starting, ensures the engine reaches operating temperature quickly and can help defrost a vehicle prior to a journey. They consist of a pump, a heat source and a heat exchanger, plus usually some method of control, and generally fall into two categories: a self-contained fuel burning unit or a mains operated unit that requires an external power supply.

This is one of those extra's that I've always wanted but struggled to justify, primarily for reasons of cost! I did have a Kenlowe Hot Start unit fitted to my Stage 1 V8, which is a mains powered engine heater, but they are less than a quarter of the price of a self-contained fuel burning unit. I eventually convinced myself though that the latter would be a good idea for my Discovery. 

After a bit of research, I chose a unit offered by Webasto. Again, the cost was a big factor - they are cheaper than the equivalent Eberspacher unit, which is the common alternative - although the fact that they are fitted as standard to some Land Rover models also helped. The model I've chosen is called the Thermo Top and I've opted for remote control operation (the other alternatives being a timer or via mobile phone).

The unit is mounted in the engine bay on the drivers side inner wing between the brake master cylinder and the header tank. On a 300 series Discovery this is a very tight fit and requires a small modification to the header tank bracket to move the tank forward slightly. This problem doesn't exist on a 200 series model as the fuse box arrangement is different and the header tank is in a different position.

Once positioned, the unit is plumbed into the cooling system pipework and also the fuel pipework near to the fuel pump. An electrical feed is taken from the battery and the control wiring is run to the remote control pick-up fixed to the windscreen and the switch for the interior heater fan. There is also an exhaust for the diesel heater, which comprises a small twin-wall flexible stainless steel tube that is routed down around the suspension turret and out behind the front wheel.

In operation the unit runs off fuel stored in its' own small tank, which is replenished by the vehicle fuel pump whilst driving. The tank holds enough fuel for about an hours operation although in practice it only needs to run for 30 - 40 minutes to achieve a suitable temperature. Once that has been reached the unit will switch on the interior heater fan to warm up the inside of the vehicle and defrost the windscreen.

The remote control, which has a range of up to 600m, enables a number of the units' settings to be altered. This includes the run time and activation of a 'summer' mode that only switches the interior fan on without any heating to provide ventilation.