DTC 79

DTC 79 Transmission Fluid Overtemp:

Fluid Pressure Switch Assembly Circuit:

Circuit Description:
The transmission fluid temperature sensor is a thermistor within the fluid pressure switch assembly, that controls the signal voltage to the PCM. The PCM applies voltage on CKT 1227 to the sensor. When the transmission fluid is cold, the sensor resistance is high and the PCM will sense high signal voltage.
As the transmission fluid temperature warms to normal transmission operating temperature 100°C (212°F), the sensor resistance becomes less and the voltage decreases to 1.5 to 2.0 volts If the fluid temperature becomes greater than 150°C (304°F) but less than 154°C (311°F) for 15 minutes, a DTC 79 will set. If the fluid temperature becomes greater than 154°C (311°F), a DTC 58 will also set.

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

1. DTC 58 will set if:
^ Signal voltage indicates a transmission temperature above 150°C (3O4°F) and less than 154°C (311°F).
2. This test simulates a DTC 59.

Diagnostic Aids:
Check harness routing for a potential short to ground in CKT 1227.
Scan tool displays transmission is operating, the temperature should rise steadily to about 1OO°C then stabilize.
Refer to "PCM Intermittent Diagnostic Trouble Codes or Performance."
The temperature to resistance value scale at the right, may be used to test the transmission sensor at the various temperature levels to evaluate the possibility of a "skewed" sensor. A "skewed" sensor could result in delayed garage shifts or TCC enabled complaints.
Check transmission fluid.