2–10 Copyright © 1996 General Motors Corp.
AT 500, 1500 SERIES AUTOMATIC TRANSMISSIONS
and the selector valve is shifted to
1
(Drive 1), the
transmission will not immediately shift to first range.
Instead, it will shift 4–3–2–1 as speed decreases (it
will remain in fourth range if speed is not decreased
sufficiently to allow an automatic downshift).
(2) The progressive downshift occurs be-
cause the regulated hold pressure is calibrated to move
the signal valves downward against governor pressure
only when the governor pressure decreases to a value
corresponding to a safe downshift speed. Thus, if
speed is too great, governor pressure is sufficient to
hold the shift signal valve upward. As governor pres-
sure decreases, the shift signal valves move downward
in sequence.
n. Trimmer Valves
(1) There are four trimmer valves in the hy-
draulic system. These are: first clutch trimmer, second
clutch trimmer, third clutch trimmer, and fourth clutch
trimmer. The purpose of a trimmer is to avoid harsh
shifts.
(2) All four trimmers function in the same
manner. Each trimmer includes an orificed trimmer
valve, a plug, a trimmer spring or springs, and a stop.
The trimmer spring or springs hold the plug and trim-
mer valve to the top of the valve bore.
(3) When any clutch (except forward) is ap-
plied, apply pressure is sent also to the upper end of
the trimmer valve. Initially, the valve and plug are
moved downward against the plug spring until the
circuit exhausts. This escape of fluid, as long as it
continues, reduces clutch apply pressure. The trans-
mission fluid flows through an orifice in the trimmer
valve to the cavity between the trimmer valve and
plug. Pressure in this cavity moves the plug downward
to the stop. Main pressures above and below the trim-
mer valve equalize. The pressure below the trimmer
valve, because it is acting upon a greater diameter,
pushes the trimmer valve to the top of the valve bore.
This throttles, then stops, the exhaust of the fluid.
When escape of fluid stops, clutch-apply pressure is at
maximum value. The plug remains downward, against
the stop, until the clutch is released.
(4) When the transmission is shifted into re-
verse, clutch-apply pressure is directed to the top and
bottom of the fourth clutch trimmer valve. This equal-
izes the pressure at both ends of the valve, thereby
holding the trimmer valve and valve plug at the top of
the bore. No trimming action occurs in reverse.
(5) Whenever a clutch is released, the
spring(s) push the trimmer components to the top of
the bore. In this position, the trimmer is reset, ready to
repeat its trimming action when the clutch is again en-
gaged.
o. Priority Valve
(1) The priority valve ensures that the control
system upstream from the valve (governor, modulator,
forward clutch) will retain sufficient pressure during
shifts to perform its automatic functions.
(2) On models before S/N 34501, main
pressure is routed directly to the 1–2 relay valve, thus
eliminating the priority valve function until the
selector valve is moved. On models after S/N 34500,
main pressure is routed directly to the priority valve,
thereby including the 1–2 signal valve in those
receiving pressure priority.
p. Trimmer Regulator Valve
(1) The trimmer regulator valve reduces
main pressure to a regulated pressure circuit (red and
yellow). The regulated pressure is raised or lowered by
changes in modulator pressure.
(2) Trimmer regulator pressure is directed to
the spring end of the first and second clutch trimmer
valves to increase the clutch-apply pressure of the
trimmer valves. A higher modulator pressure (closed
throttle) reduces trimmer regulator pressure, resulting
in lower clutch-apply pressure. Conversely, a lower
modulator pressure (open throttle) results in higher
regulator pressure and higher clutch-apply pressure.
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