Powertrain Controls - ECM/PCM

DTC 15 Chart:

Engine Coolant Temperature Sensor Circuit:

Circuit Description:
The Engine Coolant Temperature (ECT) sensor is a thermistor which varies the signal voltage at the PCM. The PCM applies 5 volts to the sensor on CKT 410. The sensor's resistance is inversely proportional to the temperature it is sensing.

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 PCM terminal "A31."

Coolant temperature is one of the inputs used to control:

- Fuel delivery.
- Ignition Control (IC).
- Idle Air Control (IAC).
- Torque Converter Clutch (TCC)
- EVAP canister purge.
- Exhaust Gas Recirculation (EGR).
- Cooling fan operation.

DTC 15 Will set When:
A DTC 15 will set when the ECT sensor indicates an engine coolant temperature less than -38°C (-37°F).

Action Taken (PCM will default to):
The PCM will use a default value of 90°C (194°F) for fuel delivery calculations. The cooling fans will be enabled and the MIL will become illuminated.

DTC 15 Will Clear When:
A current DTC 15 will clear when the low temperature failure is no longer detected [temperature rises above -38°C (-37°F) threshold]. A history DTC 15 will clear after 50 consecutive ignition key cycles without a current DTC 15 being stored.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. This is to verify that the temperature fault is not intermittent.

2. This test simulates a DTC 14. If the PCM recognizes the low signal voltage (high temperature), and the Tech 1 scan tool reads 130°C (266°F), the PCM 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:
Check harness routing for a potential open in CKT 410. This circuit is routed from the PCM through the 10-way INJ/PCM connector and then to the engine coolant temperature sensor.

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

A faulty connection, or an open in CKT 410 or 470 will result in a DTC 15.

DTCs 15 and 21 stored at the same time could be the result of an open CKT 470 which would also turn the temperature warning indicator "ON."

The "Temperature to Resistance Value" scale at the right 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.

Refer to "Intermittents" in "Diagnosis By Symptom." - Intermittent Malfunctions