TRUCK SERVICE MANUAL
TRANSMISSION
valve directs pressure to the lockup clutch piston, which
(3) The low clutch circuit connects the clutch to the
causes the lockup clutch to engage. Main pressure
1-2 relay valve and to the low trimmer valve. In neutral,
applies the lockup clutch in models not equipped with
the 1-2 shift valve connects to the top of the 1-2 relay
lockup cutoff valves. When lockup cutoff valves (para2-
valve. As long as main pressure is at the top of the 1-2
23) are included, clutch apply pressure from the drive
relay valve, the valve cannot move. Until the 1-2 shift
clutch engaged is used to apply the lockup clutch.
valve is charged by governor pressure, main pressure
will continue to hold the 1-2 relay valve down. Therefore
hydraulic fluid is directed to only one clutch. Since two
clutches are required to move the vehicle and since
(1) The priority valve insures that the control
governor pressure is negligible until the vehicle is
system upstream from the valve will retain sufficient
moving, neutral is maintained (refer to fig. 2-8).
pressure during shifts to perform its automatic functions.
(4) The low clutch in addition to being applied during
(2) Without the priority valve, the filling of a
neutral operation, is also applied in first and reverse
clutch might require a greater volume of oil (momentarily)
gears. Shifting the selector valve from neutral to drive 5
than the pump could supply and still maintain the
(D5) position or any other forward drive range (D4, D3,
necessary control pressures.
D2, D1) charges the forward clutch circuit and applies
the forward clutch. The low clutch remains charged.
O. Clutch Circuit, Drive Ranges
Hydraulic fluid is directed from the selector valve to the
forward clutch. It is also directed through the priority
(1) There are six clutches in the transmission.
valve to the 2-3 relay valve. The unit is now in first gear.
These are: low clutch; first clutch; second clutch; third
Automatic shifting from first gear, progressively, to higher
clutch; fourth clutch; and forward clutch. The clutches
gears (second, third, fourth, fifth) will occur when vehicle
are applied for various conditions as follows:
speed increases.
The highest gear attainable, in
Condition
Clutch(es) applied
automatic shifts, is determined by the range selected
Neutral ...........................Low
(refer to (7), below).
First gear .......................Low and forward
Second gear ..................First and forward
(5) Shifting the selector valve from neutral to
Third gear ......................Second and forward
reverse (R) charges the fourth clutch, while the low
Fourth gear ....................Third and forward
clutch remains applied. In reverse, fourth clutch (reverse
Fifth gear .......................Fourth and forward
signal) pressure is also directed to the bottom of the 2-3
Reverse gear .................Low and fourth
relay valve. The pressure at this point prevents the relay
valve from moving downward during reverse operation.
(2) Each of the six clutches has its own
circuit.
Each clutch except the forward clutch is
(6) When the circuits are charged, as
connected to a relay valve and a trimmer valve. The
described for the shift to drive 5 (D5), above, any forward
forward clutch is connected directly to the selector valve
shift from first to second, second to third, third to fourth,
and does not connect to a trimmer valve. It does not
and fourth to fifth can occur automatically. These shifts
require connection to a trimmer valve because its
occur as a result of rear governor pressure. At less than
application (except in neutral-to first gear on the HT
full throttle, modulation will assist governor pressure.
750CRD and neutral-to second gear on the HT 750DRD)
precedes the application of an additional clutch which is
trimmed.
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