For some reason the temperature gauge on the Marlin has started to show a slightly higher reading over the last couple of months. Whereas it used to be rock steady on 80C it seems to have crept up to 90C. Just enough to make me a bit anxious.

So I decided to see if there was anything obviously wrong. On the modified Zetec engine there is a water rail which contains the thermostat and next to it is a tapping for the water temperature gauge, and another one for the ECU temperature sensor. The ECU is from Omex and is supposed to put the fan on, via a relay, at 90c and turn it off at 85.

I fitted a LED to the relay output and tested the relay worked using a wire to activate it. The LED and fan came on, so I positioned the LED to it just stick out under the bonnet and could be seen when driving.

I planned to drive to Montacute house near Yeovil to marshal on todays Parkrun. As I joined the A303 the gauge showed a temperature of 50C but the LED came on. Bugger I thought that is not good and pulled into a layby. As I did the LED went out. I checked the fan wasn’t running, and the engine certainly was not hot. Back on the A303 and the LED came on again, until I slowed until less than 50, then it went out. Meanwhile the gauge was slowly going up, but hadn’t got to 70C yet.

As I approached Montacute the gauge was indicating 90c, the LED was on until I slowed down and then the LED went off.

Puzzled I did my marshalling stint, then on the way home, with the same kind of results I realised what was going on.

A motor and a generator are mechanically the same, except with a motor you put electricity in and get rotation out, but with a generator you put rotation in and get electricity out. So the fan was windmilling when I was going fast and producing enough electricity to light the LED.

I’m now going to change the temperature sensor.