GMC 4000 Series Service Manual page 53

Sec. 1
Page 46
CONVENTIONAL CAB
ROTATE TO C H A N G E
CABLE LENGTH
Figure 3 0 — Heater Control Cable Adjustment Sleeve
part way, part tempered and part untempered air
p asses through the heater.
The control on dash marked "AIR" regulates
the flow of outside air which passes through heater
and into cab. This control must be positioned all
the way towards the right in order to obtain heat.
IMPORTANT: DO NOT operate the blower
motor when "AIR" control lever is in the full left
position. The heater fan speed is controlled by
moving blower switch lever from left to right posi­
tions "OFF-LOW-SPEED," "MEDIUM SPEED,"
and "HIGH SPEED."
The "DEFROST" lever controls flow of air
to the windshield defroster ducts. With knob all
the way to the right, entire heater output is divert­
ed to the defroster ducts for defrosting.
For warm weather ventilation, place "DE­
FROST" and "TEMP." control knobs in extreme
left positions, move "AIR" knob to extreme right,
then place fan switch lever to desired fan speed
position. The fan speed control resistor unit is
attached to the blower shroud with two screw s. If
resistor should become defective, the result will
be high speed blower motor operation with blower
switch lever in both the "LOW" and "MEDIUM"
speed positions.
HEATER UNIT REPLACEMENT
(HEAVY DUTY AIR-FLOW TYPE)
REMOVAL (Fig. 29)
1. Drain cooling system to just below level of
heater core.
2. Mark hoses for identification, then discon­
nect hoses at heater core fittings.
3. At front side of cowl, remove single bolt
which attaches the heater blower bracket to cowl.
G M C S E R V I C E M A N U A L
move protective shield and the small spring-type
retaining washer from "TEMP" control lever,
then remove the control wire clamp screw. Pull
wire from lever.
"AIR" control wire in same manner.
tributor hose.
separate the wiring connection at the blower speed
control resistor.
motor. Pull outward on connection.
remove four bolts which attach heater unit to cowl
extension.
support bracket to bottom of dash panel. This same
bolt attaches the blower motor ground wire.
the unit rearward and down from compartment.
assembled for inspection or replacement of the
blower, motor, core, etc.
INSTALLATION
over heater core water inlet and outlet pipes.
into position, then install all attaching bolts.
wire is sandwiched under head of heater support
bracket attaching bolt at dash panel.
sistor unit and also at the blower motor. Make
sure connections are secure.
to heater assembly. Lower view in figure 29 shows
connections.
heater hoses prior to disconnecting, connect the
hoses to respective heater core piping.
hose is connected to small core pipe and that the
3/4 inch I.D. hose is attached to the larger pipe. If
hoses are not installed as directed, poor heat out­
put and/or a ruptured heater core will result. The
core internally is designed to accept the higher
pressure at the inlet (5/8 inch I.D. pipe) side of
core only.
operate engine until normal operating temperature
is obtained.
operation of heater and controls.
4. At front side of heater core housing, re­
5. At top of blower motor, disconnect the
6. Disconnect the blower adapter-to-air dis­
7. At top *of blower housing, near the cowl,
8. Separate wiring connector at rear of blower
9. Underneath the dash, at right side of core,
10. Remove single bolt which attaches heater
11. Using care not to damage heater unit, work
12. If desired, the unit can be readily d is­
1. Make sure the cellular dust pad is located
2. Place the heater core and blower assembly
IMPORTANT: Make sure blower motor ground
3. Connect wiring at blower speed control re­
4. Connect the blower air outlet tube.
5. Connect the "AIR" and "TEMP." controls
6. Referring to identification marks made on
IMPORTANT: Make sure the 5/8-inch I.D.
7. Replenish cooant in system then start and
8. Check for possible leakage and check the