Re: Thermostat sending unit troubleshooting.
With your voltmeter in the DC volts range, connect the black probe to the temp. gauge ground terminal..connect the red probe to the temp. gauge power terminal. This should read approx.12-13 volts with the key on. Next...disconnect the wire from the "s" terminal and temporarily ground the terminal using an unpowered test light to complete the circuit to ground. The meter should deflect slowly towards hot. (If you don't use a test-light to complete the circuit, you run the risk of damaging the gauge.) The sending unit can be checked with an ohmmeter. I don't have a spec for its resistance at room temp. but it should be something reasonable like 2000-30,000 ohms (pending on the make and model.) not open circuit.
