RAM 2500 Van V8-5.2L Prop VIN 2 (2000)
Fluid Pressure Sensor/Switch: Description and Operation
42RE (A-500) 4 Speed
GOVERNOR PRESSURE SOLENOID VALVE
The 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.
