Transmission Control Systems: Description and Operation

TRANSMISSION CONTROL

Note: Transmission Control is accomplished by the PCM.

The PCM operation includes control of 3 and 4 speed automatic transmissions utilizing electronic governor pressure control, eliminating the need for a separate transmission controller. Transmission control is achieved through regulation of governor pressure using a variable force solenoid valve. (Valve position is controlled by pulse width modulation. Torque convertor lock-up and overdrive solenoids are also controlled by the PCM, as are the transmission protection relay and dashboard overdrive lamp. PCM inputs affecting transmission operation include the throttle position sensor, output shaft speed sensor, vehicle speed sensor, engine speed sensor, brake switch, cruise control switches, ignition, overdrive on/off switch and sump fluid temperature thermistor.

Electronic Governor Pressure Control
Regulation of governor pressure enables control of WOT shift points to within +/- 50 RPM resulting in improved shift quality (as opposed to control to within +/- 300 RPM in transmission utilizing mechanical governors). The elimination of the mechanical governor resulted in a physically smaller and lighter package. Torque calculation can be used to determine when upshifting is advantageous.

Anti-Hunt Shift Strategy
This strategy was added in order to maintain fuel economy over a variety of load conditions. This technique allows for relatively early upshift in mid-range throttle settings under light load conditions, resulting in improved fuel economy. Under heavier loading, shifts are delayed to prevent upshifting into a condition where insufficient torque is available to maintain vehicle speed, resulting in downshift/upshift cycling.

Sump Pump Temperature Sensing
A thermistor is attached to the lock-up solenoid and submerged in the sump fluid. The sump fluid temperature is a factor in the calculation of WOT shift strategy, low temperature shift compensation, sump temperature limiting strategy, overdrive strategy, torque converter clutch strategy, and governor pressure transducer calibration.