GMC 4000 Series Service Manual page 146

3. Attach motor w ires to switch term inals.
The red or longer wire attaches to bottom term ­
inal. Tighten inside terminal nuts (19) firm ly.
4. With a screwdriver inserted into slot of
drive screw (20), install bearing (12) on end of
screw with shielded side of bearing toward inside.
Retain bearing with bearing lock nut (13).
5. Run nut to center of screw, then insert
slotted end of screw into housing, meshing slotted
end with armature shaft.
NOTE: Fiber bumper contact on drive nut
must be toward switch.
6. Install gasket (14) and cover (15) with three
screws and lock washers (16).
firm ly.
NOTE: R em ainder of a sse m b ly of sh ift unit
used on Rockwell axles must be completed when
unit is installed on housing as described previous­
in the section under "Two-Speed Shift Unit
ly
Replacement."
VACU U M CONTROL VALVE
Control valve (fig. 22) requires very little
maintenance, unless cleaning of air cleaner has
been neglected and valve has become contamin­
ated as a result of neglect. However, should tests
indicate that valve is faulty it should be repaired
or replaced.
REMOVAL
1. Remove manifold to valve vacuum line, air
cleaner line, and speedometer shift line from re­
spective valve ports.
2. Remove two axle shift chamber lines from
valve ports.
3. Loosen cable clamp nut at top of valve.
4. Remove two mounting screw s, separate
valve from mounting surface, and loosen cable
trunnion screw.
5. Remove valve from control cable.
DISASSEMBLY (Fig. 22)
1. Remove four screw s, and separate cover,
cover gasket, valve body assembly, body gasket,
and lower body.
2. Remove two tension springs from ends of
poppet lever and trunnion pin.
3. Remove two retaining washers, poppet
springs and poppet valve assem blies from valve
body. Remove discs from valve stem s.
INSPECTION
1. Check poppet valve seats in valve body and
lower valve body for evidence of nicks or scratches
which may cause vacuum leak.
2. Inspect poppet levers, springs, operating
levers, trunnion pin, and pivot pin for wear or
other damage.
G M C S E R V I C E M A N U A L
new condition, since these items are included in
a repair kit.
ASSEMBLY (Fig. 22)
of Lubriplate, or equivalent, to all friction sur­
faces.
ends of pin in cable trunnion.
end of operating levers, and align to valve body.
Tighten screws
nion pin off-center toward operating levers. Install
pivot pins, driving pin in until ends are flush with
valve body.
stem s. Install poppet springs.
body and through elongated holes in poppet lever.
Install new retaining washers.
attaches to trunnion pin and opposite end to notch
in poppet lever arm.
trunnion up and down several times by hand, while
noting if "over center" action of the tension springs
cause poppets to operate freely and without binding.
align body with valve body.
tioning new gasket and valve cover to top of valve
body. Install and tighten four screws.
BENCH TEST
has been correctly assembled and is operating
properly without serious vacuum leakage.
install a shut-off valve in line.
ports leading to axle shift chamber. Each line
should have a vacuum gauge.
valve with valve trunnion in "up" position. One
gauge should read 20 inches of mercury and op­
posite gauge should read zero.
ometer adapter outlet port. Reading on vacuum
gauges will now be opposite to previous reading.
nion, which should occur when trunnion is not less
than 1/8" from valve body in each direction.
graphs indicate that leakage is greater than 1 inch
in 15 seconds, the valve must be inspected for
faulty seats.
REAR AXLE AND CONTROLS
3. Inspect poppet discs and replace if not in
During assembly operations apply a light film
1. Place rounded end of operating levers over
2. Align poppet lever between holes at pointed
NOTE: Be sure parts are assembled with trun­
3. Install new poppet discs to ends of poppet
4. Install poppet valve stem s through holes in
5. Install tension springs. One end of spring
NOTE: Test operation of valve by moving
6. Install new gasket to lower valve body and
7. Hold these two parts in place while posi­
The following test will determine if the valve
1. Attach vacuum source to vacuum port and
2. Attach a vacuum line to each of the valve
3. Supply 20 inches of mercury vacuum to
4. Push valve trunnion down and close speed­
5. Check "snap over" position of cable trun­
6. Whenever tests outlined in previous para­
Sec. 4A
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