Powertrain Controls - ECM/PCM

Circuit Description
The coolant temperature sensor uses a thermistor to control 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 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 200°F) the voltage will measure about 1.5 to 2.0 volts.

Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Code 14 will set if:
^ Signal voltage indicates a coolant temperature above 135°C (275°F) for 3 minutes.
2. This test will determine if CKT 410 is shorted to ground, which will cause the conditions for Code 14.

Diagnostic Aids
Check CKT 410 routing for a potential short to CKT 452 or ground.

"Scan" tool displays engine temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90°C (195°F) then stabilize, when thermostat opens.

The "Temperature to Resistance Value" scale, at the right, may be used to test the 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.