With 4L80E Transmission

Transmission Fluid Overtemp:

Transmission Fluid Overtemp Circuit:

CIRCUIT DESCRIPTION
The Transmission Fluid Temperature (TFT) sensor is a thermistor that controls the signal voltage to the Powertrain Control Module (PCM). The PCM supplies a 5 volt reference signal to the sensor on CKT 1227. 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 approximately 1.5 to 2.0 volts.

DIAGNOSTIC TROUBLE CODE (DTC) 79 WILL SET WHEN
- No DTC 58.
- Trans fluid temp greater than 146°C (295°F).
- All conditions met for 30 minutes.

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 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.
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 TCC complaints. Check transmission fluid. Refer to Fluid Checking Procedure. Procedures