GMC 4000 Series Service Manual page 396

Assembly and Installation (Fig. 6)
1. Apply thin film of light engine oil to bush­
ing, diaphragm, and body before assembly.
2. Install expander spring around inside of
diaphragm, then install these two parts in body.
3. Install rubber retainer on bushing.
4. Place valve in recess of bushing, then in­
stall in body.
5. Install plunger spring and inlet valve plunger
6. Set unloader assembly in place on cylinder
head and fasten with two bolts.
7. Connect tube assembly from unloader to
governor.
8. Test assembly with air pressure. Unloader
should withstand a pressure of 100 psi without
leaking.
DESCRIPTION
The governor, operating in conjunction with
air com pressor unloading mechanism, automatic­
ally controls air pressure in the air brake or air
supply system between the desired, predetermined
maximum and minimum p ressures. The air com­
pressor runs continually while the engine runs,
but actual compression of air is controlled by the
governor which stops or starts compression when
the maximum or minimum pressures are reached.
M AINTENANCE
Every 500 operating hours or after every
15.000 m iles, clean or replace governor filters.
Every 3,000 operating hours or after every
100.000 m iles, disassem ble the governor and clean
and inspect all parts. Repair governor if necessary.
GOVERNOR TESTS
OPERATING TEST
Start the engine and build up air pressure in
system . Observe reading on air pressure gauge in
gauge panel when governor cuts-out, stopping com­
pression of air by the com pressor. Reading on
gauge when governor cuts-out should be within
range shown on chart at right.
With the engine still running, slowly reduce
air pressure in the system by applying and releas­
ing brakes. Observe pressure registered by gauge
when governor cuts-in and compression is resum­
ed. Gauge reading when governor cuts-in should be
within range shown on chart at right.
Before condemning or adjusting the governor,
be sure the dash air gauge is registering accurate­
ly. Use an accurate test gauge to check pressure
registered by the dash gauge. If the pressure se t­
G M C S E R V I C E M A N U A L
AIR COMPRESSORS A N D GOVERNORS
AIR COMPRESSOR GOVERNOR
tings of the governor are inaccurate or it is nec­
essary that they be changed, adjust governor as
described in the "Adjustment" procedure.
LEAKAGE TEST
its exhaust port in both cut-in and cut-out posi­
tions. In the cut-in position, check exhaust port
for inlet valve leakage by applying a soap solution
at the port. Leakage could also be past the bottom
piston grommet (B-W only). In the cut-out position
check the exhaust port to determine if leakage is
present at the exhaust valve seat or stem grommet.
In this position leakage could also be past the upper
piston grommet (B-W only).
haul governor.
Governor
All Midland-Ross
All Bendix-
Westinghouse
falls below 15 psi or if the pressure settings are
incorrect. Before making any adjustments, check
governor filter (10) and governor supply line (res­
ervoir) for any restriction, then proceed as follows:
MIDLAND-ROSS (Fig. 7)
Figure 6 — U n lo a d e r A s s e m b ly (M -R)
Leakage checks on the governor are made at
If there is excessive leakage, replace or over­
GOVERNOR ADJUSTMENT CHART
Cut-In
P ressure
85 psi
86-90 psi
GO VERNO R ADJUSTMENT
Readjustment may be necessary if the range
1. Loosen adjusting plug lock nut (2).
2. Build up pressure to 115 psi and maintain
Sec. 6T
Page 385
Cut-Out
Pressure
100-107 psi
103-111 psi