Engine Control Module ECM

Diagnostic Trouble Code 15:

ECT Sensor Circuit:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) sensor is a thermistor which controls 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 a high signal voltage.

As the engine warms, the sensor resistance becomes less and the voltage drops. At normal engine operating temperature, 85°C to 95°C (185°F to 203°F) the voltage will measure about 1.5 to 2.0 volts.

DTC 15 WILL SET WHEN
Signal voltage indicates a coolant temperature less than -35°C (-31°F) while engine not running or coolant temperature less than -35°C (-31°F) while engine running less than 90 seconds or coolant temperature less than -35°C (-31°F) when engine starts in "clear flood" mode.

ACTION TAKEN (ECM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate.

DTC 15 WILL CLEAR WHEN
Conditions for fault are no longer present and Tech 1 "Clear DTCs" function used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and battery feed voltage is removed for 10 seconds.

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

1. Check to see if DTC was set as result of a hard failure or intermittent condition.

2. This test simulates a DTC 14. If the ECM recognizes the low signal voltage (high temperature), and the Tech 1 scan reads 130°C (266°F) or above, 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 J 39200 voltmeter. This will determine if there is a wiring problem or a faulty ECM.

DIAGNOSTIC AIDS
A scan tool reads engine coolant temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens.

A faulty connection, or an open in CKTs 410 or 452 will result in a DTC 15.

Refer to Diagnosis By Symptom. Testing and Inspection

The "Temperature To Resistance Value" scale in the chart may be used to test the engine coolant sensor at various temperature levels to evaluate the possibility of a "skewed" (mis-scaled) sensor. A "skewed" sensor could result in poor driveability complaints.