The coolant temperature sensor on a 1993 Toyota Camry sends a signal to the car’s computer to adjust the fuel mixture, ignition timing and control engine temperature by cycling the cooling fan. A bad coolant temperature sensor will cause the spark plugs to foul and destroy the converter over time by allowing it to overheat. It is a thermistor, which exhibits a voltage drop through reduced resistance to ground as the temperature rises and, conversely, lower temperature increases voltage.
Locate the coolant temperature sensor by following the upper radiator hose to the thermostat housing on the engine. The two-wire sensor is next to the thermostat housing. Start the engine and allow it to warm to operating temperature. When it has reached normal operating temperature, shut the engine off. Wait 10 minutes to allow it to cool slightly.
Connect the voltmeter black lead to the negative terminal on the battery. Use the sharp point on the red lead probe to pierce the sensor signal wire on the electrical connector. Look at the sensor and locate the locking tab. Using the locking tab as the top of the sensor connector, pierce the wire to the left of the tab. Turn the ignition key to the “On” position without starting the car. If the sensor is good it will display 0.1 to 0.7 volts at 178 degrees. Withdraw the probe and the black lead from the battery negative terminal. Turn the ignition key off.
Unplug the sensor by pressing the tab and pulling the connector off. Switch the voltmeter to ohms. Probe the two male terminals in the sensor. A good sensor will show 200 to 400 ohms resistance. If it shows any discernible difference in either case, replace the sensor.
