Theory Of Operation

WARNING: This page is about a different car, the 2014 RAM 1500. However, it is still accessible from the selected car via links, so may be relevant.

The purpose of the monitoring is to detect a fault in more than one of three temperature sensors.

The transmission has three different temperature sensors. If any one of the three sensors fails, then the other two sensors are used as substitute with no impact on transmission control, operation or monitoring. Therefore a fault is only set on a multiple sensor failure.

The three sensors consist of a discrete analog sensor outside the Transmission Control Module (TCM) which is located in the oil sump of the transmission (Oil Temperature Sensor) and two substrate (or chip) temperature sensors located in different positions on the TCM. As the TCM is embedded in the molded wiring harness lead frame, all temperatures have a strong correlation.

Normally the oil temperature is used for the control of the transmission. Substrate temperatures are only used for over-temperature shutdown and rationality monitoring.

Chip Temperature Sensors: These sensors are located in ASICs on the substrate of the TCM in die form (no discrete sensors). The ASICs have built in temperature sensor circuitry, A/D converter, temperature register and SPI interface to communicate digital values of the actual temperature to the microcontroller. Therefore there is no possibility for electrical monitoring (circuit continuity), but out-of-range monitoring (transmitted temperature too low or too high) and a check of all ASIC monitoring capabilities (e.g. communication line rationality, timeout, etc.) is done.

Oil Temperature Sensor: The transmission oil temperature is measured with a thermistor which is connected at two pins of the TCM. A connection to sensor supply voltage and ground level is realized over an internal resistor network.