Powertrain Controls - ECM/PCM

DTC 15 Chart:

ECT Sensor Circuit:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM supplies a 5 volt reference and ground to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore, PCM terminal "PC1O" 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 between 1.5 and 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 valve.
- Electric cooling fan.

DTC 15 WILL SET WHEN
- ECT sensor signal voltage indicates 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, and the PCM will default to 85°C (185°F) 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 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 (266°F), the PCM and wiring are OK.
3. This test will determine if CKT 410 is open. There should always be 5 volts present at sensor connector.

DIAGNOSTIC AIDS
- After the engine is started, the temperature should rise steadily to about 90°C (194°F), and then stabilize when the thermostat opens. It is normal for engine coolant temperature to fluctuate slightly around 95°C (203°F).
- DTCs 15 and 21 stored at the same time could be the result of an open CKT 808.
- The "Temperature to Resistance Value" scale 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.