Temp gauge not working

Hi all its maybe been asked before but I'm gonna ask.
I have a 1993 RRC lse vogue, that several engine's (from the various grind marks on chassis) have been put into and taken out of. I have just put a 300 tdi and R380 gearbox into it.
My problem, as the title states DUH, is that no matter what I have tried I cannot get the temp gauge to work. I have the electrical troubleshooting guide for my model and have tried all the troubleshooting given in the manual but still am at a loss.
I'm now at the point where I didn't have 12 volts at the female end of the cable, I do now, and when I short it off the body the temp gauge goes all the way to hot and back down when released, but when I clip it onto the coolant sensor I get nothing. I have removed the sensor from a friends 300tdi which works in his Disco but not in my one.
Would I be right in saying that I may have to find the sensor that suits the orignal engine (which I have no clue what it could be) or am I just clutching at straws.

Any help is greatly appreciated

Paul

You may find the resistance values of the 300TDi and the Orignal 4.2 V8 are totally different.....

You could try an original Temp Sender if you can get it to fit, but not sure what the running temperature of a 300TDi is compared to the V8....you may find it indcates running either cool or hot...but still be at the correct working temp....if that makes sense....

I think the running temp on both is the same, well from what I can make out from the RAVE cds they both run at 88 degrees. But you could of hit the nail on the head regarding the resistance values I know that it should be x ohms when cold but the rate it increases or decreases which ever suits when the engine heats could be problem
I'll give trying to get an orignal sensor to work I have a spare housing so if it needs to be tapped out I won't f**k up my one.
Thanks Saint.V8 .

P.S would a 3.9 sensor work as I can't find a 4.2 on any of the U.K based suppliers

bet they are the same.ie any rrc temp sender..cos they use the same gauge.

if you still av the one out of the engine you removed,and that worked,and it will fit then use that..

or..it could be you aint got any coolent??so no reading..

I don't have the one from the engine that came out as I bought it minus engine and box. There is plenty of coolant in the engine, plus I took out the thermostat just to be sure that it wasn't seized shut, to no avail.
Off to buy a sensor for a 3.9/4.2 and see what happens.