Scott, it will be a mismatch between the new guage and the old sender unit. See what the make of new guage is and try to find a new sender unit that matches that. My bet is that the thermostat will be fine, and that you actually have no problem with the cooling system. It is just that the guage is giving you a false reading. Wiring the sender up (complete with an earth wire) and putting it into a pot of boiling water as mentioned by Angelo will certainly prove it.

Temp is measured in Celcius
Old gauge in previous dash worked
Old gauge in current dash worked
Cut wire sent to that unit.
Wired in after market gauge
Temp soars past 100degrees.

Hmm. Pulling the temp sender off tomoz and putting in a jug with hot water. Good idea!! And the thermostat is coming out for a new one.

Cheers everyone.

Scott
Davo

I think most of the aftermarket companies sell temp senders to match their temp gauges. Otherwise, they should at least say what the resistance should be at certain temps.
I usually rig up a bunch of wires and hook up the gauge and sensor out of the car, then drop the sensor in some boilling water. I usually compare the gauge with a different thermometer just to see if its close, or at least reads 100C/212F when the water is at full boil (i'm near sealevel). A camping stove works great for this. It keeps you and your junk off of the kitchen counter.

If it is an aftermarket temperature gauge and you are using the standard Datsun temp sender unit there will be a mismatch between the gauge and the sender. You will need to get the sender unit that goes with that gauge to make it work properly. They always come as a matched pair. Hope this helps

OK, are you talking temp. in Ferinhite or Celcius? it makes a difference.
for those who use "F" water boils at 212 degrees, and for those who use "C" water boils at 100 degrees.
there are exceptions to this however, the main one being that if you pressurize the system, it will raise the boiling temp.
I can't give you all the metric equivilants to this , but you should be able to figure it out .
for every 1 pound of pressure increase, you raise the boiling temp 2 degrees. EXAMPLE; you have a 10 pound radiator cap on you car and it will increase the boiling temp. from 212 deg. to 232 deg.
don't worry if the temp. at normal operating temp reads over boiling unless it really is overheating.
not all OEM or aftermarket gauges are accurate, they will only give you a close idea.
this is why car manufacturers stopped putting temp. on the factory gauges. the best thing you could do would be to get a known accurate thermal measuring device and use it to check your coolant temp at normal operating temp. them you will know for sure what temp the engine is at when fully warmed up.

hi davo, did the last guage work properly? (the standard one) if it did then i would bet that the guage is faulty. if you try and get a hold of another guage to check it with. i hope this helps davo.
DAVE

OK. A qeustion for the A series owners.. Hears the story - Put a new dash in the 1200 and hooked up all the wires. Started it up and stocko temp gauge worked and didnt go over the threshold point. Kool. All good. Now, this dash has a after market temp gauge were the cigi lighter went, hooked up all the wires - postive, negative and one to the temp sender. All good, hooked up and everythings reading after starting again. Now, temp rises slowly, gets to about 90 degrees and thinking it should start to level off. No, it rises up passed 100 degrees and kept creeping at a steady rate. Why? After that, i switched it off and the radiator was fairly cold and stuck my finger in the water and was only mildly warm.....

I have nailed it down to either a faultly thermostat or the actual thermostat is in backwards? But i doubt it. So what other reasons could cause this. Any input is helpfull, but a thermostat change is in order i think...

Scott
Davo