$74 (P1799) - Calculated Oil Temperature In Use

Name of Code: $74 (P1799) - Calculated Oil Temperature in Use

When Monitored: Whenever the Engine is running.

Set condition: The code is set if any of the following conditions exist for three consecutive key starts:
- The thermostat voltage is out of range (below 0.07 volts or greater than 4.94 volts)
- If continuous erratic thermostat voltage is sensed.
- The thermostat temperature stays below 80°F for an extended period of time.

Theory of Operation: The TCM uses a thermostat to monitor the transmission's sump temperature. This temperature is used to determine which shift schedule the TCM is to use. (See "Transmission Operation and Shift Scheduling at Various Sump Temperatures"). If the thermostat circuit fails to operate properly the TCM will use the calculated oil temperature routine found in prior model year TCM's. If this occurs for three consecutive key starts, the code will be set. The TCM will then test the thermostat circuit after every 35 OBDII warm-up starts. If the thermostat circuit is OK, the thermostat data is used in place of the Calculated Oil Temperature Routine.

Transmission Effects: If the thermostat indicates a temperature below 0°F or above 240°F at start up, the TCM compares the calculated oil temperature to the indicated thermostat oil temperature. If the calculated oil temperature differs significantly from the thermostat value, the calculated oil temperature will be used for that key start. This code does not cause the transmission to go into limp-in mode.

Possible Causes:
- Wiring or Connector problems in the transmission temperature sensor signal circuit.
- Defective TRS
- Defective TCM