Federal Emissions

Code 15 Diagnostic Chart:

Coolant Temperature Sensor (CTS) Circuit Diagram:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) 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, 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 the voltage will measure about 1.5 to 2.0 volts at the ECM terminal "C16".

Coolant temperature is one of the inputs used to control:
^ Fuel delivery.
^ EVAP canister purge.
^ Ignition Control (IC).
^ Exhaust Gas Recirculation (EGR).
^ Idle Air Control (IAC).
^ Cooling fan operation.
^ Torque Converter Clutch (TCC).

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 15 will set if:
^ Signal voltage indicates engine coolant temperature is less than 38.5°C (-37.30°F).

2. This test simulates a DTC 14. If the ECM recognizes the low signal voltage (high temperature), and the Tech 1 (or equivalent) scan tool reads 130°C (266°F), 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 DVM.


DIAGNOSTIC AIDS:

^ A Tech 1 (or equivalent) scan tool reads engine temperature in degrees centigrade. After engine is started the temperature should rise steadily to about 95°C (203°F) then stabilize when thermostat opens. CKT 410 is routed from the ECM to a mini harness, and then to the Engine Coolant Temperature (ECT) sensor.

^ A faulty connection, or an open in CKT 410 or 808 will result in a DTC 15.

^ DTCs 15 and 21 stored at the same time could be the result of an open CKT 808 which would also turn the temperature warning indicator "ON." The "Temperature to Resistance Value" scale in the chart may be used to test the engine coolant temperature sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor. A "shifted" sensor could result in poor driveability complaints.