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-
ture 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.
Transfer Case Low-Range Operation — On
four-wheel drive vehicles operating in low range,
the engine can accelerate to its peak more rapidly
than in Normal range, resulting in delayed shifts
and undesirable engine ‘‘flare.’’ The low range gov-
ernor pressure curve is also higher than normal to
initiate upshifts sooner. The PCM compares the
electronic vehicle speed signal to the transmission
output shaft speed signal to determine when the
transfer case is in low range.
Wide-Open Throttle Operation — In wide-
open throttle (WOT) mode, adaptive memory in the
PCM assures that up-shifts occur at the prepro-
grammed optimum speed. WOT operation is deter-
mined 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 re-
fined through the increased computing power of the
PCM and through access to data on engine operat-
ing 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 ‘‘hunt-
ing’’ 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 pres-
sure 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, con-
troller calculations, and shift lever position deter-
mine different governor pressure curves.
Governor pressures can be different at the same
output shaft speed. The desired pressure is deter-
mined 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 re-
quested 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 sole-
noid making governor pressure zero.
TRANSMISSION TEMPERATURE SENSOR
Transmission fluid temperature readings are
supplied to the powertrain control module by the
transmission temperature sensor. It is a thermistor
located in the governor pressure sensor connector.
The temperature readings are used to control en-
gagement of the overdrive clutch, the converter
clutch, and governor pressure. Normal resistance
value for the thermistor at room temperature is
approximately 1000 ohms.
The powertrain control module (PCM) prevents
engagement of the converter clutch and overdrive
clutch, when fluid temperature is below approxi-
mately -1°C (30°F).
If fluid temperature exceeds 126°C (260°F), the
powertrain control module will cause a 4-3 down-
shift and engage the converter clutch. Engagement
is according to the third gear converter clutch
engagement schedule.
The overdrive OFF lamp in the instrument panel,
also illuminates when the shift back to third occurs.
5
GENERAL INFORMATION