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.
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HOMESERVICE MANUALSCADILLAC1991FLEETWOOD FWD V8-300 4.9LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSCOOLANT TEMPERATURE SENSOR/SWITCH (FOR COMPUTER)DESCRIPTION AND OPERATION
1991 Cadillac Fleetwood FWD V8-300 4.9L
Coolant Temperature Sensor/Switch (For Computer): Description and Operation
1991 Cadillac Fleetwood FWD V8-300 4.9LSECTION Description and Operation
Coolant Temperature Sensor:
The coolant sensor is a thermistor (a resistor which varies its resistance value based on temperature) mounted in the engine coolant stream. The coolant sensor is a two wire component which receives a reference voltage signal from the PCM/ECM. Low coolant temperature produces a high resistance, while a high temperature produces low resistance. The PCM/ECM supplies a voltage signal to the coolant sensor through a resistor in the PCM/ECM and measures the signal voltage output. The voltage will be high when the engine is cold, and the voltage will be low when the engine is hot.
The sensor provides coolant temperature information to the PCM/ECM for fuel enrichment, ignition timing, EGR operation, canister purge control, air management, idle speed control, and closed loop fuel control. A failure in the coolant sensor circuit should set either a code E14 or code E15.
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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.