GMC 4000 Series Service Manual page 54

AIR DISTRIBUTOR REPLACEMENT
(HEAVY DUTY AIR-FLOW HEATER)
The air distributor can be readily replaced
after disconnecting the attaching air hoses and the
"DEFROST" control wire, then removing the three
screw s which attach distributor to dash and cowl
(fig. 29).
HEATER CONTROL CABLE
ADJUSTMENT
Each control wire conduit is equipped with a
threaded-sleeve (fig. 30) which can be turned to
obtain full opening and closing of air doors and to
even up the heater knobs on control panel. Make
adjustment to control wire conduit if desired.
SERVICE DIAGNO SIS
The following applies to both the Standard and
Heavy Duty Air-Flow Heater systems:
POOR OR NO HEATING
1. Heater hoses are kinked, deteriorated, or
switched around at the core piping.
2. Heater core plugged.
3. Poor blower motor ground connection.
4. Blower malfunctioning.
5. Low coolant temperature.
BLOWER INOPERATIVE
1. Check dash switch and blower motor con­
nections.
2. If blower operates at high speed with switch
in either "MED." or "SLOW" position, the speed
control resistor at top of blower shroud is defec­
tive and should be replaced.
NOTE: Resistor is attached to the blower motor
scroll by two screw s. Only one-half of resistor
G M C S E R V I C E M A N U A L
winding may be defective; if so, replace the r e s is ­
tor assembly.
(fig. 31) permits outside air to enter cab under
dash panel. The vent is controlled by "VENT" con­
trol handle located at top center of dash panel.
Pulling "VENT" handle rearward opens the vent.
Vent closes when handle is pushed forward.
be replaced if deteriorated or damaged. Before
installing new seal, remove all the old seal and
adhesive from seal channel.
anism shown in closed position. If vent cover fails
to close firmly against seal, the link rod under
dash can be bent slightly as required to increase
the closing pressure.
control handle will fail to remain in closed detent
position.
CONVENTIONAL CAB
3. Check the blower motor.
VENTILATION
A single vent intake at front center of cab cowl
Sponge rubber seal under vent cover should
Figure 31 illustrates the vent control mech­
NOTE: Do not bend rod excessively as the
Sec. 1
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