Thanks but think I don't have a bleed tee as standard.
Am I correct in thinking that this smaller sensor is just for the gauge? The larger sensor in front of it on the water rail is for the ECU and more important functions and that the fan either uses this or has it's own sensor near the radiator. Put another way, is the car probably ok to use?
This is the common one used...
Oh this is frustrating. From notes above . . . . .
1/ Tested gauge by earthing to battery negative terminal. Needle spins round to maximum so not a gauge problem. Just for good measure, removed it from dash, checked the connectors and put it back. Tested by earthing again to make sure and still ok.
2/ Before fitting new sensor (part above), I removed the female spade connector again from the current sensor and connected it to the new one. Immersed to c. 1cm in both warm and hot water (between 40 and 90C). Ignition on and gauge doesn't register anything.
So what else can it be? It's such a simple little circuit so all I'm thinking is
a) does the new sensor need to be in-situ?
b) have a got a dud new sensor?
Private Message sent.
I see. Have just redone the test correctly on the the new sensor. Hot water about 70C gives a resistance on my 20kohm meter of 1.04 then as it cools to ambient temperature I watched the resistance steadily rise to 1.81. This suggests the new sensor is okay.
I then removed the old sensor and same test showed same 'OL' (open circuit) resistance readings regardless of temperature. Looks like the old sensor is the issue. Fitted the new sensor in and connected morning with negligible loss of coolant in the process. Run engine up to temperature, gauge moved smoothly round from 40 to low 90s, then thermostat opened on cue, fan started and ran for about 2 mins to take it back down to high 80s. Heater working too.
Decent run out later on in the afternoon and perfect. Thank you everyone for input.