TRUCK SERVICE MANUAL
HT 700D SERIES TRANSMISSIONS
calibrated springs. Each of the shift valves and springs
(3) When any clutch (except forward) is
are calibrated to insure that the valves will shift at the
applied, apply pressure is sent also to the upper end of
proper time and in proper sequence. At a given
the trimmer valve. Initially, the valve and plug are forced
governor pressure, an increase in modulator pressure
downward against the trimmer spring until oil escapes to
will upshift a signal valve. A decrease in modulator
exhaust. This escape of oil, as long as it continues,
pressure will cause a downshift if governor pressure
reduces clutch apply pressure. However, oil flows
alone will not hold the valve upward.
through an orifice in the trimmer valve to the cavity
between the trimmer valve and plug. Pressure in this
(4) A t the trimmer regulator valve, modulator
cavity forces the plug farther downward, to the stop. The
pressure assists in regulating the trimmer regulator
plug stops, but flow through the orifice in the trimmer
pressure against the spring at the top of the valve.
valve continues. The pressure below the trimmer valve,
because it is acting upon a greater diameter than at the
(5) A t the lockup valve, modulator pressure
upper end, pushes the trimmer valve to the top of the
causes lockup to occur at a lower vehicle speed.
valve bore. This throttles, then stops, the escape of oil to
exhaust. When escape of oil is stopped, clutch apply
i. Trimmer Regulator Valve
pressure is at maximum value. The plug remains
downward, against the stop, until the clutch is released.
(1) The trimmer regulator valve reduces main
pressure to a regulated pressure.
(4) When the clutch is released, the trimmer
pressure i s raised or lowered by change s in modulator
spring pushes the trimmer components to the top of the
pressure (red and green).
bore. In this position, the trimmer is reset and ready to
repeat the trimming action when its clutch is again
(2) Trimmer regulator pressure is directed to
the lower side(s) of the trimmer regulator plug(s) to vary
the clutch apply pressure pattern of the trimmer valves.
(5) A trimmer boost accumulator valve is
A higher modulator pressure (closed throttle) will reduce
connected to the trimmer regulator pressure circuit. The
trimmer regulator pressure. This results in lower initial
accumulator will absorb surges in the trimmer regulator
clutch pressure. Conversely, a lower modulator pressure
pressure and provide a more uniform regulator pressure.
(open throttle) results in higher regulator pressure and a
k. Lockup Circuit (Actuated
higher initial clutch pressure.
by front governor)
j. Trimmer Valves
(1) The front governor circuit originates at the
(1) The purpose of the trimmer valves is to
front pitot. Rotation of the vaned pitot collector ring
avoid shift shock. The valves reduce pressure in the
directs oil against, and into, the pitot orifice. This
clutch apply circuit during initial clutch application, then
produces a pressure in the front governor circuit which
gradually returns the pressure to the operating
varies with the rotational speed of the collector ring.
maximum. This applies the clutch gently, and harsh
Pressure increases as speed increases. This pressure
shifts are prevented.
is directed to the top of the lockup valve.
(2) Although each trimmer valve is calibrated
(2) When front governor pressure (or front
for the clutch it serves, all trimmers function in the same
governor pressure assisted by modulator pressure) is
manner. Each trimmer includes (from top to bottom) an
sufficient to compress the spring at the bottom of the
orificed trimmer valve, trimmer valve plug, trimmer spring
valve, the lockup valve moves downward. In this
and stop pin.