Transmission Temperature Sensor/Switch: Description and Operation

DESCRIPTION
The transmission temperature sensor is a variable, thermistor type. It reacts to temperature changes. At cold transmission oil temperatures, its resistance is high. As temperatures increase, its resistance will decrease.

The transmission temperature sensor is used on models equipped with an automatic transmission and a factory installed transmission cooler. It is also used on models equipped with certain heavy-duty options and an automatic transmission (8.0L V-10 / 5.9L turbo diesel with 47RH transmission).

Transmission Temperature Sensor:

LOCATION
The transmission temperature sensor is located in the transmission cooler outlet line, next to the transmission.

Transmission Temperature Warning Lamp:

An instrument panel-mounted transmission temperature warning lamp is also used.

PURPOSE
Its purpose is to help control transmission fluid overheating.

NOTE: Fourth gear and converter clutch operation in the 47RH are normally controlled by the PCM, but are also controlled by the temperature sensor under certain conditions.

OPERATION
The temperature sensor constantly monitors transmission fluid temperature. The sensor is in circuit with overdrive and converter clutch solenoid through the Powertrain Control Module (PCM).

The temperature sensor prevents overdrive and converter clutch engagement when fluid temperature is below 60 °F for the overdrive clutch; and below 70 °F for the converter clutch.

If the fluid temperature indicator lamp comes on, fluid temperature is either at, or above 285°F. This indicates an approaching overheat condition. The sensor will downshift the transmission to third gear and possibly disengage the converter clutch. At this point, it will be necessary to determine the cause of the overheat condition (e.g. cooler restricted, blocked, vehicle overloaded, not equipped for towing, etc.).

If transmission overheating has been determined by this sensor (fluid temperature exceeds 27O - 275 °F), an input is sent to the PCM. The PCM will then force a 4-3 downshift, or will prevent a 3-4 upshift. Once transmission temperature has cooled below specifications, a 3-4 upshift will be allowed.