Part 1 of 2

Code Chart 14:

ECT Circuit:

CIRCUIT DESCRIPTION:
The Engine Coolant Temperature (ECT) sensor utilizes a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM applies a reference voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high. The ECM will then sense a high signal voltage.

As the engine warms up, the sensor resistance decreases and the voltage drops. At normal engine operating temperature, the voltage will measure about 1.5 to 2.0 volts at ECM terminal "B4."

Coolant temperature is one of the inputs used to control the following:

^ Cooling fan.
^ Fuel delivery.
^ Ignition Control (IC).
^ Idle Air Control (IAC).
^ Torque Converter Clutch (TCC).

A separate thermistor within the ECT sensor provides a signal to the coolant temperature gauge located in the instrument panel.

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

1. DTC 14 will set if:

^ The engine has been running for 2 minutes or longer.
^ Signal voltage indicates an engine coolant temperature below -34°C (-30°F).
OR
^ Signal voltage indicates an engine coolant temperature above 143°C (289°F) for 3 seconds or longer.

2. If the ECM recognizes the grounded circuit (low voltage) and displays a high temperature, the ECM and wiring are OK.

3. This test will determine if there is a wiring problem or a faulty ECM. If CKT 452 is open, there may also be other DTC(S) stored.

DIAGNOSTIC AIDS:
A scan tool should display engine coolant temperature in degrees Celsius.

After the engine is started, the temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens.

If the engine has been allowed to cool to an ambient temperature (overnight), Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) may be checked with a scan tool and should read close to each other.

When a DTC 14 is set, the ECM will turn "ON" the engine cooling fan.

^ If DTC 14 is intermittent or a "Hard Start" symptom is present, check engine coolant temperature with a scan tool on a cool engine. Temperature displayed should be within 5 degrees of the ambient. If not, check the ECT sensor using the "Diagnostic Aids" on the next part. If sensor is OK, check connections.