I can’t get the temperature gauge to work properly on my Land Rover. My Series IIA has the 2.5l petrol defender engine. On the block there are 3 sensor terminals: 1. On the filter casing there’s a terminal, connected to the oil pressure warning light, which works. 2. On the n/s/f of the block, at the top, just behind the thermostat housing, is a terminal in a flat topped sensor, held in by 3 bolts, which is connected to the temperature gauge. When the engine is cold the gauge shows cold, and a test light to the terminal doesn’t light. As soon as the thermostat opens the gauge goes straight to hot, and the test light lights. 3. On the o/s/f of the block, at the side, near No 1 plug, is a brass hex with a terminal, not connected to anything. Test light = nothing, engine hot or cold. So, have I got a faulty gauge, a faulty sensor, or is something wired up wrong? Is one of these sensors for an oil pressure gauge? I’m trying to sort out a thermostatic fan. I’ve got a rad (from the Defender I assume) with a sensor/switch just below the top hose entry, which has 3 wires, black, red & white. I assume that red should be a +ve supply, black earth, and white should switch the fan (I’ve used a relay), but I can’t get it to work, and without the temp gauge working it’s hard to know what’s going on. Can someone help please?

 

I found this diagram on the web http://www.lrfaq.org/Series/images/SIIA+VE.jpg

There is a electrical switch sensor called the mixture thermostat switch, but that is not designed to operate a guage. It looks like it can turn on a heating elemnt for the carb until engine warmed up, which matches your description.

Don't see a temp guage. But the guages of that era used a sensor that would be screwed into the metal to provide the ground side, and a single wire went off to the guage, thru the guage, and on to 12 volts switched on by the key.

So you can measure your sensor with wires disconnected, and between frame ground and the sensor you should see a value of resistance. When the engine warms up, the resistance will decrease. If it reads open, or closed, then it is a switch for something. Reading with a test light may be inconclusive. Might want to try reading that brass nut you mentioned.