Powertrain Controls - ECM/PCM

DTC 15 - Engine Coolant Temperature (ECT) Sensor Circuit (Low Temperature Indicated):

Engine Coolant Temperature Sensor Wiring Circuit:

Circuit Description:
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage at the Powertrain Control Module (PCM). The PCM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore, PCM terminal "PC10" voltage will be high.

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 PCM terminal "PC10".

Engine coolant temperature is one of the inputs used to control:

^ Fuel delivery.
^ Ignition Control (IC).
^ Idle Air Control (IAC).
^ Torque Convertor Clutch (TCC).
^ Evaporative (EVAP) canister purge solenoid valve.
^ Electric cooling fan.

DTC 15 Will Set When:
^ ECT signal voltage indicates the engine coolant temperature is less than -39°C (-38°F).
^ Engine run time is greater than 60 seconds.

Action Taken (PCM will default to): The Malfunction Indicator Lamp (MIL) will illuminate, the PCM will default to 85°C for engine coolant temperature (Tech 1 scan tool will not display defaulted value), and the cooling fan will enable.

DTC Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.
1. This checks to see if DTC 15 was set as a result of a hard or intermittent condition.
2. This test simulates a DTC 14. If the PCM senses the low signal voltage (high temperature) and the scan reads 130°C, 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: A Tech 1 scan tool displays engine coolant temperature in degrees Celsius. After the engine is started the temperature should rise steadily to about 90°C, and then stabilize when the thermostat opens. It is normal for engine coolant temperature to fluctuate slightly around 95°C.

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

DTCs 15 and 21 stored at the same time could be the result of an open CKT 808.

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

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