Coolant Temperature Sensor

First and foremost make sure your coolant level is correct and the system doesn't have any air in it. Also be sure to check the Engine Cooling page as well.

Our temperature sensors work by variable resistance, the higher the temperature the lower the resistance. You can view the live data from the ECM/PCM(DiagnosticCodes) and it's easy to test the sensor with an ohm meter. Don't forget to check the sensor and wiring for corrosion.

Starting with your engine at ambient temperature, your readings should be within 5-10f of ambient, you can then start your engine and watch the temperature rise up to the thermostat opening point. If the temperature never quite gets there, it could be a faulty temperature sensor, but most of the time is a failing/failed/wrong temperature thermostat.

When a temperature sensor has failed, you will usually see a really high temperature(shorted circuit), a really low temperature(open circuit), or the resistance/temperature just doesn't match the measured temperature.

Nine times out of ten it's the sensor that went bad, but it only takes a few minutes to test. If your temperature sensor tested fine, check for loose connections and wiring for corrosion/defects.

Astro/Safari vans can have multiple temperature sensors. Common locations are in the drivers side head and next to the thermostat housing. Earlier years have separate sensors for the ECM/PCM and the dash. You can check the connection between the sensor and the gauge/computer by inserting a 240ohm resistor in place of the sensor, this would show around normal on the gauge and about 195 on the live data from the ECM/PCM.

Single wire sensors use the body of the sensor as a ground. There are some single wire sensors that are only used for idiot lights(dashboards without a temperature gauge), they only make contact after a certain temperature is reached. On the two wires sensors, the ECM/PCM sends 5vdv down one wire, the resistor resists depending on the temperature sending a lower voltage back to the ECM/PCM.

Temperature Sensor Resistance Chart: