DTC 15

Fig. 027 Code 15 Coolant Sensor Circuit:

Fig. 028 Code 15 Coolant Sensor Circuit:

CIRCUIT DESCRIPTION:
The Coolant Temperature Sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, so the ECM sees 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 1.5 to 2.0 volts at the ECM terminal "B8."
Coolant temperature is one of the inputs used to control:
^ Fuel delivery
^ Electronic Spark Timing (EST)
^ Cooling Fan
^ Convertor Clutch (TCC)
^ Idle (IAC)

TEST DESCRIPTION: Numbers below refer to circled numbers on Diagnostic Charts.

1. Checks to see if code was set as result of hard failure or intermittent condition.
CODE 15 WILL SET IF:
^ Signal voltage indicates a coolant temperature below -30 degrees C (-22 degrees F) for 60 seconds.

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 21 stored.

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

^ If the engine has been allowed to cool to an ambient temperature (overnight), Coolant and MAT temperature may be checked with a "Scan" tool and should read close to each other.
^ When a Code 15 is set, the ECM will turn on the Engine Cooling Fan.
^ A Code 15 will result if CKTs. 410 or 452 are open.
^ If Code 15 is intermittent refer to DIAGNOSIS BY SYMPTOM.