GMC 1976 ZEO 6083 Maintenance Manual page 187

Overtravel Lever
Inlet
(Air Inlet Position '
Valve
Lever
Note : Air Pressure Enters
Through Check Valve and
Passes Out Open Inlet
Valve To Bellows
LOADING
Overtravel Lever
(Neutral-Center Position)
Note : No Movement
of Air Pressure
(All Valves Closed)
NEUTRAL
Overtravel Lever
(Air Exhaust Position)
Note : Air Pressure From
Bellows Flows Out Exhaust
Valve To Atmosphere
UNLOADING
The height control valve contains an intake
valve, air bellows outlet, exhaust valve, delay piston,
and overtravel control body . The overtravel control
body contains a spring-loaded nylon piston which
protects valve parts if overtravel lever is moved
beyond normal operating range.
HEIGHT CONTROL VALVE OPERATION
Loading
When vehicle is being loaded, frame tends to set-
tle. Since valve is linked to control arm, and valve is
bolted to wheel well, valve moves downward with
frame as vehicle is loaded . As valve arm and control
shaft turns, a force is applied to the delay piston
which moves slowly and allows the intake valve lever
to move against the intake valve core . As pin is
pushed in, air pressure flows through height control
valve into bellows . Increased air pressure expands
the bellows and raises frame.
Inlet valve is "PROTECTED" by check valve in
inlet adapter. Light spring in core freely admits
reservoir air, but return flow of air is blocked.
Neutral Position
As increased air pressure expands bellows and
lifts frame, the height control valve moves upward
with frame. As frame is returning to normal ride
height, valve arm and shaft return to a neutral posi-
tion . Inlet valve lever also moves away from inlet
valve core and inlet valve closes . This stops the flow
Figure 4-Operation of Height Control Valve
of the air into bellows. The exhaust valve remains
closed . Since the exhaust valve is closed, and the
check valve in the inlet adapter prevents compressed
air from returning to air reservoir, air is trapped in
bellows and in valve. No further valve action or air
pressure change takes place until load is increased or
decreased, moving valve arm out of neutral position
for four seconds or more to actuate intake valve or
exhaust valve .
Unloading
When part of load is removed, air pressure in
bellows lifts frame. Valve arm, linked to axle, is
pulled downward from neutral position . This applies
a force on the delay piston, which moves it slowly .
The exhaust valve lever moves with the delay piston .
The outer end of exhaust valve lever fits around stem
of exhaust valve core . As soon as lever moves beyond
free-travel range, lever pulls on stem and opens ex-
haust valve. Inlet valve remains closed . Compressed
air from bellows then flows through the open exhaust
valve and out exhaust fitting to atmosphere . As the
compressed air is exhausted from bellows, the frame
lowers until overtravel lever and shaft are again in
normal (neutral) position .
Valve Arm Free Travel
REAR SUSPENSION 4- 3
With vehicle in motion and frame at normal ride
height, control valve arm and shaft are in neutral
position . Small irregularities in road causes slight up
and down movement of valve arm. Clearances are
provided between operating levers and cores of inlet