Seville STS V8-4.6L VIN 9 (1998)
Transmission Temperature Sensor/Switch: Description and Operation
AUTOMATIC TRANSMISSION FLUID TEMPERATURE (TFT) SENSOR
The automatic Transmission Fluid Temperature (TFT) sensor controls Torque Converter Clutch (TCC) and line pressure. The TFT sensor attaches to
the upper control valve assembly. The TFT signals the Powertrain Control Module (PCM) regarding transmission fluid temperature. The sensor is a
negative temperature coefficient thermistor which receives a five volt signal from the PCM.
When the temperature is cold, the sensor resistance is high; therefore, the PCM sees a high signal voltage. As the fluid warms, the sensor resistance
decreases, the voltage drop across the sensor becomes lower.
The TFT sensor may be tested with a voltmeter across pin terminals L and M of the 20-way transmission connector. Nominal voltage readings at
specified temperatures are given in voltage levels at the PCM.
The temperature sensor mode operates in the following two ways:
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Hot mode enable: Above 130°C (266°F) - TCC ON in 3rd gear at 30 mph with 0 - 10 degree throttle angle; and 4th gear at 39 mph with 0 - 10
degree throttle angle. This reduces transmission temperature by decreasing the amount of heat generated in the torque converter. It also provides
maximum cooling by routing transmission fluid directly to the transmission cooler in the engine radiator.
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Super hot mode disable on the 275 HP RPO LD8 ENGINE: Above 140°C (284°F) - TCC OFF with 4th gear inhibited. Super hot is a condition
where a hot engine adds heat to the transmission through the transmission cooler. By inhibiting TCC and 4th gear operation, the engine works
under less load. This reduces the heat added to the transmission fluid by the transmission cooler. As both the engine and the transmission cool off,
TCC and 4th gear are permitted again.
A PCM internal shunt circuit comes into play as the temperature increases beyond 50°C (122°F). As the temperature decreases, the internal shunt
comes out at 40°C (104°F). There is a 10°C (50°F) overlap.
