Over Temperature Alarm

Over Temperature alarm for less than $15.00 from Rob Bradford

Keeping an eye on the temperature gauge as a TR owner has become instinctive however there are many instances where this habit is still not enough to detect a cooling system failure such coolant loss, water pump demise or fan belt failure.

We are fortunate that our cars normally use a capillary type temperature gauge which does eventually respond to these situation and will provide an indication of pending disaster if we happen to be looking at the instrument rather than dealing with complex situations perhaps in competition or just spirited driving. There are also many vehicles which use electronic temperature gauges and therefore get no warning of a sudden increase in engine temperature, as these units only work if there is coolant surrounding the probe.

I feel more comfortable if my inattention to the instruments is protected so I have installed a warning light for inappropriate temperature at the back of the cylinder head, which gives me plenty of time to do something before there is any risk to any part of the engine. The back of the cylinder head usually runs about 6 degrees hotter than the front and about 10 - 12 degrees hotter than the coolant at the thermostat, so it seems logical to measure the temperature at the back of the head, by installing a disc thermostat onto the rearmost head bolt/nut and have this trigger an LED or a buzzer.

I just bent the tabs and used a spring to hold it in place but there are many options which will work.

I chose a SELCO Normally Open auto reset disc thermostat from RS Components on line for $8.98

  • There are 2 suitable switches which close on Rise in Temp at 200 F or at 220F.
  • I am using the 220 F one (at the moment).
  • Update: Rob suggests that you contact RS by phone (1300 656 636) and ask for RS stock number: 111-7177 (Mfr part # CA-225)  OR   111-7176 (Mfr part # CA-200)

I think it is important to wire the system so that the power goes to the warning device, either LED or Buzzer and the earth is run through the switch, so that there is no live wires hanging around.

Just solder wires to the tabs, one to earth and the other to the warning device in the dash. I also have an oil pressure switch in the same circuit so that either low oil pressure or high temp will trigger the alarm.


The next thing is to select either an LED or a buzzer which are both available from Jaycar. NOTE: Jaycar also stock a cheap range of Disc thermostats but not the N/Open ones with the right temperature. Buzzers are $3.95 and LED’s are $5.50 so either one will bring you in under $15.00

This the chrome bezel (below) fitted to the dashboard.

I intend to use a similar system on the V8 stag motor I am doing so am sure this simple device has the potential to save a lot of unnecessary damage and worry caused by cooling issues.

Cheers and Happy TRing

Rob Bradford