3.2.3
TRANSMISSION CONTROL
The PCM operation includes control of 4 speed
automatic transmissions utilizing electronic gover-
nor pressure control, eliminating the need for a
separate transmission controller. Transmission con-
trol is achieved through regulation of governor
pressure using a Governor Pressure Solenoid valve.
Valve position is controlled by pulse width modula-
tion. Torque converter clutch and overdrive sole-
noids are also controlled by the PCM, as are the
transmission relay and dashboard overdrive off
lamp. PCM inputs affecting transmission operation
include the throttle position sensor, output shaft
speed sensor, vehicle speed, engine speed sensor
(CKP), brake switch, ignition, overdrive on/off
switch, torque converter clutch solenoid, transmis-
sion temperature sensor, and governor pressure
sensor.
The PCM continuously checks for internal trans-
mission problems, electrical problems, and some
hydraulic problems. When a problem is sensed, the
PCM stores a diagnostic trouble code. Any of these
codes cause the transmission to go into ‘‘default’’
mode. When the PCM detects a problem, the trans-
mission will default to third gear. When this hap-
pens, the only transmission functions are:
•
PARK and NEUTRAL
•
REVERSE
•
THIRD GEAR
•
MANUAL SHIFTING of FIRST, SECOND and
THIRD GEAR
No upshifts or downshifts are allowed. The posi-
tion of the manual valve alone allows the 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 trans-
mission can be shifted manually by quickly down-
shifting into 1st to achieve 1st gear, then shifting to
2nd, then to third. However, default mode will not
allow 4th gear or any EMCC operation.
Once the DRBIII
t is in the RE transmission
portion of the diagnostic program, it constantly
monitors the powertrain control module, updating
the screens with switch, sensor, and input/output
states, as well as displaying diagnostic trouble
codes and default status.
TRANSMISSION IDENTIFICATION
The transmission part/identification numbers
and codes are stamped on the left side of the case
just above the oil pan gasket surface. The first
letter/number group is the assembly part number.
The next number group is the transmission serial
number. Refer to this information when ordering
replacement parts.
GOVERNOR PRESSURE SOLENOID VALVE
The solenoid valve generates the governor pres-
sure needed for upshifts and downshifts. It is an
electro-hydraulic device and is located in the gover-
nor 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 gov-
ernor pressure. Zero amps sets the maximum gov-
ernor 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 ener-
gizes 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 .35 to .65 volts at 14.7 psi
(normal barometric pressure). Since this is an ab-
solute pressure device, 0 psi calibration is required
often to compensate for changing atmospheric pres-
sure 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 gover-
nor 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 hydrau-
lic 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, result-
ing in higher than normal shift speeds and harsh
shifts. The electronically controlled low tempera-
4
GENERAL INFORMATION