1956 AMC Hudson Technical Service Manual Supplement

48
FOURTH SPEED D-4 RANGE
When the car reaches sufficient speed, G-1 pressure on the large land of the 3-4 shift
valve and G-2 pressure on the 3-4 governor valve will overcome the 3-4 shift valve
spring and modulated T.V. pressure on the 3-4 shift valve and regulator plug. The 3-4
shift valve will open and direct pump pressure through the transition valve to the
coupling valve. Pump pressure on the coupling valve moves it to the right, opening the
valve and causes the following action:
Pump pressure is directed through the coupling valve through two separate passages.
Pump pressure will go through the signal oil passage to the front unit coupling closing
the two exhaust valves. Pump pressure through the limit valve and coupling valve will
also be directed through the feed oil passage to fill the front unit coupling. When the
front unit coupling is filled, the front sprag clutch is automatically released and the front
unit is in direct drive.
When G-2 pressure moved the 3-4 governor valve to the right, it directed pressure on
the center of the valve to a land on the pressure regulator valve to lower pump pressure
in fourth speed (line drop). The transmission will operate on less pressure in fourth gear
which will lower horsepower requirements. To prevent pressure from dropping until
after the front unit has changed, the pressure to the pressure regulator valve is also
directed to a ball check in the front unit coupling feed oil passage. As the front unit starts
to fill, pressure on this side of the ball check will drop and the higher pump pressure on
the other side will unseat the ball and assist in filling the front unit coupling. When the
front unit coupling has filled, pressure will be equal on both sides of the ball and the
spring will again seat the ball. Pressure will then act on the pressure regulator valve to
lower pump pressure.
With the front and rear units in direct drive, the transmission is in fourth gear with no
reduction.