With 4L60E Transmission

DTC 79 Transmission Fluid Overtemp:

Transmission Range (TR) Pressure Switch Assembly Circuit:

CIRCUIT DESCRIPTION
The Transmission Fluid Temperature (TFT) sensor is a thermistor within the Transmission Range (TR) pressure switch assembly, that controls the signal voltage to the Powertrain Control Module (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.

DIAGNOSTIC TROUBLE CODE (DTC) 79 WILL SET WHEN
Signal voltage indicates a transmission fluid temperature greater than 146°C (295°F) and has not cooled to less than 137°C (295°F) for 30 minutes, and no DTC 58 or 59 set.

ACTION TAKEN (PCM will default to)
DTC 79 will be stored in the PCM memory but will not turn "ON" the Malfunction Indicator Lamp (MIL).

DTC 79 WILL CLEAR WHEN
The fault condition(s) no longer exist.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test checks for skewed sensor or shorted circuit.
2. This test simulates a DTC 59.

DIAGNOSTIC AIDS
Check harness routing for a potential short to ground in CKT 1227.
Scan tool TFT displays should rise steadily to about 100°C then stabilize.
Refer to PCM Intermittent Diagnostic Trouble Codes or Performance. Testing and Inspection
The temperature to resistance value scale 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 Torque Converter Clutch (TCC) complaints.
Check transmission fluid. Refer to Fluid Checking Procedure. Procedures