Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1989 Geo Spectrum L4-91.5 1.5LFACTORY OEM SERVICE PROCEDURE
Coolant Temperature Sensor/Switch (For Computer): Description and Operation
The coolant sensor is a thermistor (a resistor which varies resistance inversely with a change of temperature) mounted in the engine coolant stream. As coolant temperature increases, sensor resistance decreases. The temperature sensed is used by the ECM to determine when to go into closed loop mode. The information from this sensor is also used to change the open loop operation for fuel delivery (M/C solenoid control). The ECM provides a 5.0 volt source to the coolant sensor and measures the voltage in the sensor circuit. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the ECM knows the engine coolant temperature. Engine coolant temperature affects most systems the ECM controls.
Factory Service Tools
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.