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 15 will set if:
^ Engine running longer than 5 minutes.
^ Coolant temp. less than -30°C (-22°F), for 5 minutes.
2. This test simulates a Code 14. If the ECM recognizes the low signal voltage, (high temperature), and the "Scan" reads 130°C (275°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 a DVOM.

Diagnostic Aids
A "Scan" tool reads 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.

If Code 12 or 21 is also set, check CKT 452 for faulty wiring or connections. Check terminals at sensor for good contact.

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.