Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 15:

Engine Coolant Temperature Sensor Circuit:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM applies a voltage on CKT 410 to the sensor. When the engine coolant is cold, the sensor (thermistor) resistance is high, therefore, the ECM will sense high signal voltage.

As the engine warms, the sensor resistance becomes less and the voltage drops. At normal engine operating temperature (85°C - 95°C/185°F - 203°F) the ECT signal 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. DTC 15 will set if:

^ Engine running longer than 30 seconds.
^ Engine Coolant Temperature less than -33°C (-27.4°F).

2. This test simulates a Diagnostic Trouble Code (DTC) 14. If the ECM recognizes the low signal voltage, (high temperature) and scan tool displays 130°C (266°F) or above, the ECM and wiring are OK.

3. This test will determine if CKT 410 is open. There should be 5 volts present at sensor connector if measured with a Digital Volt Meter (DVM).

DIAGNOSTIC AIDS
A scan tool displays engine coolant temperature in degrees Celsius and Fahrenheit. After engine is started, the temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens.

If DTC 21 is also set, check CKT 452 for faulty wiring or connections. Check terminals at sensor for good contact.

Refer to Intermittents in Diagnosis by Symptom. Testing and Inspection