765 A Coolant Temperature Sensor

The coolant temperature sensor (CTS) operates on signal voltages that range from approximately 0.2-4.8 V. Generally, a fault code will only be registered if the voltage falls outside these values. This means that an engine may operate with a defective coolant temperature sensor. A likely outcome of this is that the CTS may indicate a cold engine reading to the ECU when in fact the engine is hot. A probable outcome could be that the engine may attempt to operate on a mixture that is too rich for the conditions. Tests of the type shown below will verify the sensor through its operating range and the CTS can then be eliminated from the possible causes of a problem.

1 Defective TPS pattern.

2 Spikes in a downward direction indicate a short to ground or an intermittent open in the resistive carbon strips.

3 Voltage decrease identifies enleanment (throttle plate closing).

4 Minimum voltage indicates closed throttle plate.

5 DC offset indicates voltage at key on, throttle closed.

6 Voltage increase identifies enrichment.

7 Peak voltage indicates wide open throttle (WOT).

Fig. 7.24 Analysis of voltage trace for a potentiometer-type throttle position sensor

Fig. 7.24 Analysis of voltage trace for a potentiometer-type throttle position sensor

1 Throttle at position other than closed. Not necessarily wide open throttle.

2 Reference voltage.

3 Transitions should be straight and vertical.

4 Throttle opening and voltage transitioning.

5 Throttle plate closed.

6 Ringing may indicate worn contacts or loose throttle return spring.

Fig. 7.25 The throttle switch type of voltage pattern

It is likely that the ECM will detect voltage as measured at the sensor terminals and this means that a voltmeter or oscilloscope can be used for sensor checks. Figure 7.26 shows the Bosch portable oscilloscope being used to examine coolant temperature sensor performance.

• Measurement conditions for the temperature sensor test

- Turn the key ON, Engine OFF. With the sensor wiring harness connected, measure the output voltage (engine COLO)

- Run the engine and monitor the voltage decrease (NTC) as the engine warms,

- This same test sequence can be performed while monitoring the resistance value of the sensor. The sensor must then be disconnected.

• MultiScope key sequence for the temperature sensor test

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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