Saturn Coupes/Sedans/Wagons 1991-1998 Repair Guide

Coolant Temperature Sensor (CTS)

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OPERATION



This sensor is a thermistor type, meaning as the temperature increases, resistance decreases. The PCM sends a 5 volt reference signal to the sensor (pin B) and a sensor reference ground to the sensor (pin A). The PCM calculates coolant temperature depending on the return signal.

TESTING



See Figures 1, 2 and 3

If Code 14 (1991-95 models), P0117/P1114 (1996-97 models) or P0118 (1998 models) is set, the PCM will turn the coolant fan ON at all times. The thermostat opens at 190°F (88°C) and coolant temperature should stabilize there at idle.

If Code 17 (1991-95 models) or P1628 (1996-98 models) has been set, the fault is in the PCM and cannot be repaired. Code 14, P0117/P1114 or P0118 indicates that the CTS signal is above sensor range. Code 15 (1991-95 models), P0118/P1115 (1996-97 models) or P0117 (1998 models) indicates that the signal is below sensor range. Test the sensor first.

The sensor is threaded into the coolant passage in the back of the cylinder head.

  1. Unplug the connector from the sensor and use the chart to check the sensor resistance with an ohmmeter. This can also work if the sensor is removed and immersed in a container of water.
  2.  
  3. With the ignition switch ON , there should be some voltage across the connector terminals. If not, check the wiring between the sensor and the PCM.
  4.  
  5. If using a scan tool, the reading should be the same as the actual coolant temperature with the ignition switch ON .
    1. If the reading is less than -32°F (-35°C), unplug the sensor connector and jumper the PCM circuit terminals together. If the reading goes to above 266°F (130°C), the problem is in the PCM connector, ground or in the sensor. If the reading does not change or go high enough, the problem is in the wiring or the PCM.
    2.  
    3. If the reading is more than 284°F (140°C), unplug the sensor connector. If the reading goes below -32°F (-35°C), the problem is in the sensor. If it does not change or does not go low enough, the problem is in the wiring or the PCM.
    4.  

  6.  
  7. If a scan tool is available, allow the engine too cool to the ambient temperature overnight, then compare the CTS and the ATS readings. Temperatures should be within 4 degrees of each other with the ignition ON and the engine not running.
  8.  



Click image to see an enlarged view

Fig. Fig. 1: Schematic for the coolant temperature sensor circuit