Troubleshooting Gauges

[Duncan]

My car's been sleeping in a paddock for about 18 months, and when it came back the oil temp and water temp gauges both are not working.

Now the checking for 12v power, earth and 12v lights stuff I can do, but I want to confirm how the sender wiring works.

The water temp gauge just taps into the ECU water temp sender. Am I correct assuming that the sender works by showing a resistance compared to earth? Am I also right saying if I measured voltage compared to a good 12v source it would show something like a couple of volts when cold and closer to 12v when hot (depending on the exact calibration of the sender)? If not, how can I get a basic indication that the sender is connected properly and sending a reading to the gauge?

Same question for the oil temp gauge but it has 2 wires to the sender. Will one be earth (from the gauge) and the other the return signal? If I measure the resistance between the 2 it would decrease as the oil gets hotter?

[Duncan]

Actually I think the gtr workshop manual answers that question, in the water temp troubleshooting section it says to measure voltage between the sender output (ecu 28) and ground, and I should get:

20o = 3v

80o = 1v

[sleptema]

Both will be NTC thermisters, or negative temperature co-efficient. Which basically means when it is cold, it will read a high resistance, and as the temperature rises, the resistance will drop. Easiest way to check these would be consulting the workshop manual for values dependent on temperature, and using a multimetre between earth and signal and reading the resistance. If the resistance drops and is within range, the sender isn't the issue. To check the gauge, either use a gauge tester, or earth the signal wire breifly and the gauge should move to hot. (as earthing it will give very little resistance to ground, simulating a hot reading)

[sleptema]

Even though it is a sensor, not a sender (difference being 3 wires, not 2) it will still be changing resistance, only the ECU will be interpreting these values as a voltage, not a resistance, but the test should still be the same.