Temp gauge playing up? Have I wired it up wrong?

[edmason]

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?

[Savannah Buzz]

Sounds like the flat top sensor is a switch, rather than a variable resistor to operate a guage. Have you been able to find a wiring diagram in the RAVE or other downloads for this age vehicle?

Here is a link to something I found, but no temp guage. http://www.aloharovers.com/tech/iiawire.htm

You can check the "sensor" by disconnecting the chassis wiring from it, and read with an ohm meter. It should be some resitance to ground when cold, and that will change (usually decrease) when it warms up. If it reads "open", then when heated "closed" it is a switch for a warning light, etc.