GMC 4000 Series Service Manual page 51

Sec. 1
Page 44
CONVENTIONAL CAB
SPEED CONTROL
RESISTOR UNIT
DEFROST
CONTROL
DISTRIBUTOR DUCT
AIR FLOW
M OTOR W IRIN G
[CONTROL
Figure 2 8 — Standard Duty Air-FIo w Heater
covering both types of heaters, is explained later
at end of the "Heaters" section in this group.
STANDARD AIR-FLOW HEATER
The standard air-flow heater provides heated
outside air for heating and defrosting requirements.
Outside air is drawn through the air inlet grille at
right side of cab cowl and through the heater by
the blower motor. The degree of heated air to be
expelled by heater is determined by the position of
"OFF-AIR-ON" lever at side of the air distributor.
IMPORTANT: Avoid operating blower motor
with this control lever in "OFF" position.
The control marked "ON-DEFROST-OFF"
regulates the flow of heated air to either the wind­
shield defroster outlets or onto floor, or to both.
The blower motor speed is regulated by the
dash switch and a resistor unit. Resistor unit is
attached to top of blower shroud with two screw s.
If resistor should become defective the result will
be high speed blower motor operation in both the
"MEDIUM" and "LOW" speed switch positions.
HEATER CORE, BLOWER AND
DISTRIBUTOR DUCT REPLACEMENT
(STANDARD AIR-FLOW HEATER)
Both the distributor duct and the blower a s­
sembly can be readily replaced separately without
having to remove the heater core unit from cab.
The replacement procedure following describes
the method for replacing the heater air distributor,
the blower and motor assembly, and the heater
core unit which are all shown in figure 28.
G M C S E R V I C E M A N U A L
AIR FLOW!
CORE UNIT
BLOWER SCROLL
CO N NECTO R
REMOVAL (Fig. 28)
1. Drain the cooling system to just below level
of heater core fittings.
2. Mark the heater hoses and piping at front
of cowl for identification purposes later at install­
ation, then disconnect hoses.
3. Separate the defroster air duct tubes from
air distributor.
4. Disconnect electrical wiring from heater
speed control resistor and from blower motor.
R esistor is located at top of blower scroll.
5. Remove screws which attach the air dis­
tributor to cowl and to blower scroll. Remove air
distributor.
6. To remove the heater core and blower unit
as an assembly, remove the screw s which attach
the heater core housing to the outside air inlet
duct. Carefully lower heater unit and remove from
compartment.
7. To remove the outside air inlet duct, re­
move screws which attach duct to fire wall and in­
strument panel, then remove duct and seal gasket.
8. Remove screws which attach heater core
in housing. Remove core.
9. Remove screws which attach blower motor
to scroll. Remove motor and blower fan assembly.
NOTE: The heater core can be repaired by a
competent radiator core repairman.
INSTALLATION (Fig. 28)
Assem ble and install the heater components
in reverse of "Removal" procedures using new
gaskets and seals where necessary.
IMPORTANT: When connecting heater hoses
to core piping make sure the 5/8 inch I.D. hose is
connected to sm all core pipe and that 3/4 inch I.D.
hose is connected to larger pipe.
If hoses are not installed as directed, poor
heat output and/or a ruptured heater core may re­
sult. The core, internally, is designed to accept
the higher pressure at the inlet (5/8 inchl.D. pipe)
side of core only.
HEAVY DUTY AIR-FLOW HEATER
The heavy duty air-flow heater operates on
full outside air and has a three lever control panel
and a blower motor control switch on dash (fig. 29).
Temperature control is by means of the air-
mix method; mixing heated outside air which has
passed through the heater core and outside air
which has by-passed the core in proportions nec­
essary to provide the desired temperature. The
control on dash for this purpose is marked "TEMP."
With lever to the left, all the incoming air by­
p asses the heater core. With lever to the far right,
all outside air must pass through the core, thus
providing maximum heat. When lever is positioned