There is a well known problem in the defender’s original thermometer. The thermometer is electrical and in order for it to work well it needs a good ground connection to the chassis. Heavy consumers of an electric current like the high headlights and the air conditioner blowers make the thermometer show a high value, that settles when the lights turned off. Those sides affects together with the fact that the original thermometer is not sensitive enough causes a situation when you can’t trust the value on the original thermometer.

That was the reason that pushed me into this little project. I wanted another thermometer, more accurate and trustable. My first thoughts were to add a mechanical thermometer of a tractor. Then a temperature controller of an old retired machine rolled in to my hands. The controller has a display that I used as a thermometer, entry voltage of  12 Volts DC, entrance to thermocouple (the measuring unit) and an exit of two internal relays that can operate any electrical device- in our case I used it to operate a simple buzzer.

The temperature tracker is a thermocouple in a shape of a ring. External diameter is 15mm and the cord is 2 meters long. When you order a thermocouple, specify that it should be the kind that fits the controller, my type was “J isolated”.

Most of the thermocouples in the industry don’t isolate the thermocouple. These types of thermocouples will not fit to a car where the chassis grounded.
That causes problems and actually the controller did not work and showed a fault message until I solved that isolation problem.

The thermocouple is connected to the block of the engine, using a screw in the center of the ring.

For the installation of the controller I made on a special bracket for all the switches that I added, as you can see in the picture above.

From electricity point of view the connection is pretty simple:

Foot #9 – gets the minus (-)
Foot # 8 – gets the plus (+) after the switch  and sends the (+) also to the buzzer’s positive leg.
Foot # 11 and 12  – gets the lines from the thermocouple (and if the reading doesn’t make any sense – switch them)
Foots  #3 connects to the buzzer’s negative (relay controlled) leg.
In addition there is a bridge from the controllers minus (in our case #9) to foot # 2 – the relay’s “active” foot.

The temperature measured in a usual drive is between 82 to 86 c degrees.
And this is how I adjusted the temperature in witch the buzzer is set to 95 degrees.
That already proved to alert before a real damage happened.

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