Maximize Vehicle Performance with Coolant Temperature Sensors

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Purpose

The coolant temperature sensor monitors engine coolant temperature. The vehicle’s computer uses this information, along with input from the oxygen sensor(s), to determine when the engine is warm enough to go into closed loop feedback fuel control. The computer also uses the signal from the coolant temperature sensor to perform a number of other functions based on current engine temperature, including:

Cold Engine

Spark advance and retard - To minimize vehicle emissions, spark advance is limited or inhibited until the engine reaches normal operating temperature.

EGR Valve Control - EGR flow is blocked to improve "cold engine" drivability.

Canister Purge - Canister purge is inhibited until the engine until the engine reaches normal operating temperature to improve "cold engine" drivability.

Idle Speed Control - Operation of the throttle kicker or idle speed control to improve "cold engine" performance.

Torque Converter Clutch Lock-up

Start-up Fuel Enrichment (fuel injected engines) - Fuel injector "dwell" time is increased to "enrich" the fuel mixture and improve "cold engine" performance.

Warm Engine

Fan Control - Operation of the electric cooling fan when a preset temperature is reached.

Location

The Coolant Temperature Sensor is typically threaded into the cylinder head, intake manifold engine block, where the sensor tip can contact the engine's water jacket.

Operation

Two basic types of coolant temperatures sensors are in general use: the variable resistance-type sensor, and the switch-type sensor.

Variable Resistance Sensor

The variable resistance-type coolant temperature sensor is most commonly used. The sensor is essentially a temperature-sensitive variable resistor called a “thermistor.” The electrical resistance of the sensor changes in direct proportion to changes in engine (coolant) temperature. During normal vehicle operation, the vehicle’s computer applies a control voltage to the sensor, and monitors the return signal voltage. The amount of resistance applied by the sensor determines the value of the return signal voltage. Most coolant temperature sensors are “negative coefficient” type sensors; the sensor’s resistance decreases as engine temperature increases. When the engine is cold the sensor’s resistance is high, and the return signal to the vehicle’s computer is high. As engine temperature increases, sensor resistance becomes lower, and the voltage output decreases.

Switch-Type Sensor

The switch-type sensor is generally found on older vehicles. The sensor opens or closes at a preset temperature, similar to a conventional temperature sending unit or thermostat.

Drivability Symptoms

Air temperature sensors can be damaged by backfiring in the intake manifold. Additionally, carbon deposits and oil contamination, as well as normal wear and “aging” can affect the sensitivity, accuracy and responsiveness of the sensor.

A malfunctioning or defective coolant temperature sensor may result in a variety of problems. The following symptoms may indicate a damaged or defective air temperature sensor, or trouble in a related system:

• The engine fails to enter closed loop feedback fuel control when the engine reaches normal operating temperature.
  
• Stalling, stumbling or rough idle
  
• Hesitation
  
• Decreased fuel economy
  
• Increased vehicle emissions
  

In some cases, symptoms generally associated with a faulty coolant temperature sensor may be caused by a failure or malfunction in a related system or component. Be sure to confirm a faulty coolant temperature sensor as the cause before replacing the sensor. Remember that that coolant sensor problems are more often due to damaged or defective wiring and loose or corroded connectors than failure of the sensor.

Testing

Operation of resistance-type coolant temperature sensors is easily checked with an ohmmeter using the following procedure. BE SURE the sensor is cold before testing.

• If necessary, drain a sufficient amount of coolant from the engine to prevent spillage when the sensor is removed.
• Unplug the wiring harness from the coolant temperature sensor, and remove the sensor from the engine.
• Connect an ohmmeter to the sensor’s signal pins. Note the sensor’s resistance.
  
• Heat the sensor tip with a blow dryer or other low temperature heat source. DO NOT use a propane torch or other high temperature heat source. DO NOT overheat the sensor.
• Observe the ohmmeter and verify that the sensor’s resistance decreases as heat is applied.
  
• Remove the heat source and disconnect the ohmmeter from the sensor. Allow the sensor to cool.
• Apply thread sealer to the sensor threads to prevent coolant leaks, then reinstall the sensor in the engine. DO NOT over-tighten the sensor when installing.
• Reconnect the wiring harness to the sensor’s connector.

Troubleshooting

If sensor resistance does not decrease when heat is applied, the sensor is defective.

Repair/Replacement

The coolant temperature sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

When replacing a coolant temperature sensor, keep the following general procedures in mind:

• In most cases, it is necessary to drain a sufficient amount of coolant from the engine to prevent spillage when the sensor is removed.
• Apply a suitable thread sealer to the sensor threads BEFORE installing the sensor to help protect against coolant leaks.
• DO NOT OVER-TIGHTEN THE SENSOR.
• Start and run the engine and turn the vehicle's heater on BEFORE refilling the cooling system. Open any bleed screws to vent air from the system while refilling.
• Refill to cooling system to the proper level using MANUFACTURER RECOMMENDED coolant. DO NOT USE PLAIN WATER.
• Drive the vehicle a short distance, then turn the engine off and allow to cool. Recheck the coolant level, and add additional coolant as necessary.

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