With 4L80 Transmission

Coolant Temp Sensor Circuit:

Circuit Description:
The Coolant Temperature Sensor (CTS) sensor is a thermistor that controls the signal voltage to the PCM. The PCM applies a voltage on CKT 410 to the sensor. When the engine coolant is cold, the sensor (thermistor) resistance is high, therefore, the PCM will sense a 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 (194°F), the voltage will measure about 1.5 to 2.0 volts.

A Code 15 may inhibit TCC operation.

Coolant Temp Sensor Circuit Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Code 15 will set if:
^ Signal voltage indicates a coolant temperature less than -33°C (-27°F) for one second.

2. This test simulates a Code 14. If the PCM recognizes the low signal voltage, (high temperature) and the "Scan" reads 130°C or above, 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 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 then stabilize when thermostat opens.

A faulty connection, or an open in CKT 410 or CKT 452 will result in a Code 15.


The "Temperature To Resistance Value" scale 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.