Powertrain Controls - ECM/PCM

Trouble Code 15 Chart:

Engine Coolant Temperature Sensor Schematic:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) sensor is a thermistor used to modulate a 5 volt reference signal applied to circuit 410. When the engine is cold, the sensor resistance is high which causes the voltage at pin "A31" to remain high. The Powertrain Control Module (PCM) calculates a high signal voltage as a low engine coolant temperature. As the engine warms, the sensor resistance drops and voltage at the PCM terminal drops. At normal engine operating temperature, the voltage will measure approximately 1.75 volts on circuit 410.
Engine coolant temperature is one of the inputs used to control:
^ Fuel delivery.
^ Idle air control.
^ Purge control.
^ Cooling fan operation.
^ Ignition control.
^ Torque Converter Clutch (TCC) operation.
^ Exhaust Gas Recirculation (EGR) operation.

DIAGNOSTIC TROUBLE CODE (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) for more than 2 seconds.

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 Malfunction Indicator Lamp (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
Numbers below refer to circled numbers 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 135° C (275° F), the PCM and wiring are OK.
3. This test will determine if circuit 410 is open. There should be 5 volts present at sensor connector if measured with a DVOM.

DIAGNOSTIC AIDS
A Tech 1 scan tool reads engine temperature in degrees centigrade. After the engine is started, the temperature should rise steadily to about 90° C (194° F) then stabilize when the thermostat opens. Circuit 410 is routed from the PCM through an injector harness connector, and then to the (ECT) sensor. A faulty connection, or an open in circuit 410 or 452 will result in a DTC 15.
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.
Refer to Computers and Controls / System Diagnosis / Procedures / Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions