TRUCK SERVICE MANUAL
TRANSMISSION
DESCRIPTION AND OPERATION
Para 2-26/2-27
r. Automatic Downshifts
s. Downshift and Reverse
(1) Automatic downshifts, like upshifts, are
(1) Inherent in the system, as a result of valve
controlled by rear governor and modulator pressures.
areas and pressure calibrations, is a means for
Downshifts occur in sequence as rear governor pressure
preventing downshifts at a too high vehicle speed or a
and/or modulator pressures decrease. Low modulator
reverse shift at an excessive forward speed.
pressure (open throttle) will hasten the downshift; high
modulator pressure (closed throttle) will delay downshifts
(2) Progressive downshifts occur because the
regulated hold pressure is calibrated, along with the
(2) In any automatic downshift, the shift signal
valve areas, to shift the signal valves downward against
valve acts first. This exhausts the shift signal holding the
rear governor pressure only when the governor pressure
relay valve downward. The relay valve then moves
decreases to a value corresponding to a safe downshift
upward, exhausting the applied clutch and applying the
speed. Thus, if speed is too great, rear governor
clutch for the next lower gear.
pressure is sufficient to hold the shift signal valve
upward against hold pressure.
As rear governor
pressure decreases, the shift signal valves move
downward in sequence.
691