Code 79, Transmission Fluid OVERTEMP: Notes

The transmission fluid temperature sensor, located within the fluid pressure switch assembly, is a thermistor which controls the signal voltage to the PCM. The PCM applies and monitors voltage on circuit No. 1227 to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM will see high signal voltage.

As the transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts. If fluid temperature becomes greater than 304°F (150°C) but less than 309°F (154°C) for 15 minutes, Code 79 will set. If fluid temperature becomes greater than 309°F (154°C), Code 58 will also set.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 309°F (154°C). 2) This test simulates a Code 59.