44RE (A-500) 4 Speed
GOVERNOR PRESSURE SOLENOID VALVEThe solenoid valve generates the governor pressure needed for upshifts and downshifts. It is an electro-hydraulic device and is located in the governor body on the valve body transfer plate.
The inlet side of the solenoid valve is exposed to normal transmission line pressure while in forward gears. The outlet side of the valve leads to the valve body governor circuit.
The solenoid valve regulates line pressure to produce governor pressure. The average current supplied to the solenoid valve controls governor pressure. One amp current produces zero psi governor pressure. Zero amps sets the maximum governor pressure. Current is regulated by modulation of the pulse width of a 512 Hz driver frequency (512 cycles per second).
The Powertrain Control Module (PCM) supplies electrical power to the solenoid valve. Operating voltage is 12 volts (DC) and is provided through the battery terminal on the module.
The solenoid is polarity sensitive. The PCM energizes the solenoid by grounding it through the power ground terminal on the Powertrain Control Module.
GOVERNOR PRESSURE SENSOR
The governor pressure sensor measures output pressure of the governor pressure solenoid valve.
The sensor output signal provides the necessary feedback to the Powertrain Control Module. This feedback is needed to accurately control pressure. The unit is an absolute pressure device and the output is calibrated to be 0.35 to 0.65 volts at 14.7 psi (normal barometric pressure). Since this is an absolute pressure device, 0 psi calibration is required often to compensate for changing atmospheric pressure or altitude. This voltage measured at 0 psi is referred to as zero pressure offset.
GOVERNOR SHIFT SCHEDULES
The electronic governor has several governor curves possible as opposed to a conventional governor which has a single governor curve with two stages. These transmissions are mechanically and hydraulically the same as the ones they replace.
As with all-hydraulic transmissions, the vehicle shift speeds are determined by balancing a hydraulic pressure signal proportional to transmission output speed (called governor pressure) against a pressure signal determined by throttle position (called throttle pressure). The four curves are used during the following operating conditions.
Low Transmission Fluid Temperature - When the transmission fluid is cold at or below 30 °F the conventional governor can delay shifts, resulting in higher than normal shift speeds and harsh shifts. The electronically controlled low temperature governor pressure curve is higher than normal to make the transmission shift at normal speeds and sooner. The PCM uses a temperature sensor in the transmission oil sump to determine when low temperature governor pressure is needed.
Wide-Open Throttle Operation - In wide-open throttle (WOT) mode, adaptive memory in the PCM assures that up-shifts occur at the preprogrammed optimum speed. WOT operation is determined from the throttle position sensor, which is also a part of the emission control system. The initial setting for the WOT upshift is below the optimum engine speed. As WOT shifts are repeated, the PCM learns the time required to complete the shifts by comparing the engine speed when the shifts occur to the optimum speed. After each shift, the PCM adjusts the shift point until the optimum speed is reached. The PCM also considers vehicle loading, grade and engine performance changes due to high altitude in determining when to make WOT shifts. It does this by measuring vehicle and engine acceleration and then factoring in the shift time.
Normal Operation - Normal operation is refined through the increased computing power of the PCM and through access to data on engine operating conditions provided by the PCM that were not available with the previous stand-alone electronic module. This facilitated the development of a load adaptive shift strategy - the ability to alter the shift schedule in response to vehicle load conditions. One manifestation of this capability is grade "hunting" prevention - the ability of the transmission logic to delay an upshift on a grade if the engine does not have sufficient power to maintain speed in the higher gear. The 3-2 downshift and the potential for hunting between gears occurs with a heavily loaded vehicle or on steep grades. When hunting occurs, it is very objectionable because shifts are frequent and accompanied by large changes in noise and acceleration.
GOVERNOR OPERATION
The electronic governor control system replaces the old centrifugal governor pressure control and is located on the valve body. The control system uses a governor pressure solenoid that can vary pressure, a pressure sensor, and the output shaft speed sensor.
The electronic governor control system regulates pressure to control shifts in the first three gears. Output shaft and throttle position are used to determine target pressure. Actual governor pressure is read from the sensor and the difference between the target pressure and actual pressure are used to determine duty cycle correction.
The duty cycle is the amount of time the governor pressure solenoid needs to be OFF to meet the target pressure.
Speed of the output shaft, throttle position, controller calculations, and shift lever position determine different governor pressure curves.
Governor pressures can be different at the same output shaft speed. The desired pressure is determined by many things including the acceleration of the vehicle. There is no need for concern if at the same output shaft speed there are different requested pressures. There is a need for concern if the target pressure and actual pressure are not within three PSI for five seconds or more. If this occurs the control system could result in erratic shifting.
The only time the governor control system stays at zero is when the gear selector is in park, neutral, reverse or drive with the vehicle at a stop. When the transmission is in park, neutral, or reverse no line pressure is supplied to the governor pressure solenoid making governor pressure zero.