12v supply to the temp gauge, it earths through the temp sender.

When temps cold, high resistance through sender unit, no needle movement.

When temps hot, low resistance through sender unit, big needle movement.

Go straight to the back of the gauge check supply with multimeter and ring out resistance between gauge and sender unit, my bet is that it's open circuit. Or vibration has shaken a wire free on the back on the gauge.

Either way would recommend you get familiar with a multimeter........ you could of just saved yourself the price of a temp sender! :-)

dan1978 wrote:It is actually the temperature sender unit that is connected to the thermostat housing, that I replaced. I have tried earthing the wire to the engine. There aren't any fuses to check for it. I guess it is either the wiring or the gauge itself. Hopefully it is just the wiring as that should be a lot cheaper to fix.

Ah ok! It got a bit confusing reading all the posts with the mention of Temp Sender Switch & Rad Fan Switch...
So I'm guessing the needle didn't move when you earthed the wire?
Do all the other checks and if it turns out to be the guage itself, let me know as I've got the internal part of one off an NC29 that you can have; it should be the same as the NC35, it certainly looks the same!

The needle didn't move at all when I earthed the wire. How do I check the circuit with a multimeter?

It seems to be ok now. When I turn on the ignition the needle moves and it moves more when the engine is warming up now. I don't know what was actually wrong with it in the end, it definately needed a new temperature sender unit anyway. I am not sure if it just kind of sorted itself out, or if it was a loose connection on the back of the gauge as 80sman said.

That's good to hear; do keep an eye on the temperature though.
Maybe the thermostat was stuck???