DTC 15

Flowchart Code 15:

Coolant Temperature Sensor Circuit:

CIRCUIT DESCRIPTION
The coolant temperature sensor uses a thermistor to control the signal voltage to the Electronic Control Module (ECM). The ECM applies a voltage on Circuit (CKT) 410 to the sensor. When the engine is cold the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.

As the engine warms, the sensor resistance becomes less and the voltage drops. At normal engine operating temperature (85°C to 95°C), the voltage will measure about 1.5 to 2.0 volts at the ECM.


TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. Code 15 will set if Signal voltage indicates a coolant temperature less than -39°C (-37.5°F) for 3 seconds.

2. This test simulates a Code 14. If the ECM recognizes the low signal voltage, (high temperature) and the "Scan" reads 130°C, the ECM and wiring are OK.

3. This test will determine if CKT 410 is open. There should be 0 volt present at sensor connector, if measured with a DVOM, with CKT 410 grounded.




DIAGNOSTIC AIDS
A "Scan" tool reads engine temperature in degrees centigrade. After engine is started the temperature should rise steadily to about 90°C then stabilize when thermostat opens.

A faulty connection, or an open in CKT 410 or 455 will result in a Code 15.

If Code 23 or 63 is also set, check CKT 455 for faulty wiring or connections. Check terminals at sensor for good contact. Refer to Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions