If the third sensor is there to operate the aircon fans on an engine overheat, it will be a normally open thermal switch with a set point of 105/110C, so it would normally be open circuit. A temperature gauge sensor on the other hand is a variable resistor of one sort or another. So with the black sensor there would be no gauge movement.
Using a potentiometer to test does not always work, it depends on what sort of sensing element is being used. The sensor is screwed into the head and has 2 wires so it could effectively have 3 connections, and there are other 2 wire sensor elements that cannot be mimicked with a pot.
I reckon it's odds on that fitting the correct sensor will give you a working gauge.
 
If the third sensor is there to operate the aircon fans on an engine overheat, it will be a normally open thermal switch with a set point of 105/110C, so it would normally be open circuit. A temperature gauge sensor on the other hand is a variable resistor of one sort or another. So with the black sensor there would be no gauge movement.
Using a potentiometer to test does not always work, it depends on what sort of sensing element is being used. The sensor is screwed into the head and has 2 wires so it could effectively have 3 connections, and there are other 2 wire sensor elements that cannot be mimicked with a pot.
I reckon it's odds on that fitting the correct sensor will give you a working gauge.

Heres hoping Keith.
Will let you guys know when fitted and checked.
 
Just an update guys to let you know new sensor arrived to day and fitted this afternoon.
:clap2::clap2::clap2::clap2:
Jobs a good un,turns out it was the wrong sensor in place after all.

Big big thanks to Keith and Tony for all your help guys.

Nice to have a working temp gauge at last.
I owe you a beer guys
Ed
 

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