General System Operation

The 42RE Transmission Control Module (TCM) controls operation of the converter clutch, overdrive clutch, and governor pressure solenoid. The TCM determines transmission shift points based on input signals from the transmission thermistor, transmission speed sensor, engine speed sensor, vehicle speed sensor, and throttle position sensor. Operating voltage is 12 volts and is provided through the battery terminal on the TCM. The ignition voltage signal is provided through TCM terminal E9.

The TCM continuously checks for internal transmission problems, electrical problems, and some hydraulic problems. When a problem is sensed, the control module stores a diagnostic trouble code. Any of these codes cause the transmission to go into "default" mode. When the TCM detects a problem, the transmission will default to third gear. When this happens, the only transmission functions are:

^ PARK and NEUTRAL
^ REVERSE
^ THIRD GEAR

No upshifts or downshifts are allowed. The position of the manual valve alone allows the three ranges that are available. Although engine performance is seriously degraded while in this mode, it allows the owner to drive the vehicle in for service. The transmission can be shifted manually by quickly downshifting into 1st to achieve 1st gear, then shifting to 2nd, then to third. However, default mode will not allow 4th gear or any Electronically Modulated Converter Clutch (EMCC) operation.

Once the DRB is in the 42RE portion of the diagnostic program, it constantly monitors the transmission control module, updating the DRB screen with switch, sensor, and input/output states, as well as displaying diagnostic trouble codes and default status.