DTC 15

Code 15 Diagnostic Chart:

Code 15 Circuit Diagram:

CIRCUIT DESCRIPTION:

The Coolant Temperature Sensor uses a thermistor to control the signal voltage at the ECM. The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore, the ECM will sense a high signal voltage. As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine operating temperature, the voltage will measure about 2.5 to 3.0 volts at the ECM terminal "17." Coolant temperature is one of the inputs used to control:

^ Open Loop timing.
^ Fuel delivery (M/C solenoid operation).
^ EFE (Early Fuel Evaporation).

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

1. Checks to see if code was set as result of hard failure or intermittent condition. Code 15 will set if:

^ The engine has been running for 5 minutes.
^ Signal voltage indicates a coolant temperature below 0°C (32°F).
^ Conditions are present for more than 5 minutes.

2. This test simulates conditions for a Code 14. If the ECM recognizes the grounded circuit (low voltage), and displays a high temperature, the ECM and wiring are OK.

3. This test will determine if there is a wiring problem or a faulty ECM. If CKT 452 is open, there may also be a Code 33 stored. Be sure to carefully check terminals at the engine harness connectors,

DIAGNOSTIC AIDS:

A "Scan" tool reads engine temperature in degrees Centigrade and Fahrenheit. After the engine is started, the temperature should rise steadily to about 194°F (90°C), then stabilize, when the thermostat opens. A Code 15 will result if CKT 410 and/or 452 is open. With the coolant sensor circuit open the ECM senses a cold engine and will increase the idle to above 2000 rpm by increasing the duty cycle of the mixture control solenoid. If Code 15 is intermittent, refer to DIAGNOSIS BY SYMPTOM under TESTING PROCEDURES.