Description Adn Operaton (page 5 of 6)

 
The heater controls the temperature of the air supplied to the distribution ducts, as directed by the ATCM. The heater is 
installed on the vehicle center-line, between the instrument panel and the engine bulkhead. The heater consists of a 
casing, formed from a series of plastic molding, which contains an evaporator, heater core and control doors. Internal 
passages integrated into the casing guide the air through the casing and separate it into two flows, one for the LH outlets 
and one for the RH outlets. 
 
When the Air Conditioning (A/C) system is operating, the evaporator cools the air entering the heater. 
 
The heater core provides the heat source to warm the air being supplied to the distribution ducts. The heater core is an 
aluminum two pass, fin and tube heat exchanger, installed across the width of the heater housing. Two aluminum tubes 
attached to the heater core extend through the engine bulkhead to connect with the engine cooling system. When the 
engine is running, coolant is constantly circulated through the heater matrix by the coolant pump. On vehicles with a Fuel 
Fired Booster Heater (FFBH), when the FFBH is active the coolant flow is assisted by an electric circulation pump. For 
additional information, refer to   

Auxiliary Heater

 (412-02B Auxiliary Heating) 

 
 
Two temperature blend doors, one LH and one RH, regulate the flow of air through the heater core to control the 
temperature of the air leaving the heater. On the automatic system, the two temperature blend doors operate 
independently to allow different temperatures to be set for the LH and RH outlets. On the manual system, the temperature 
blend doors are coupled together and produce a common temperature for the LH and RH outlets. On the automatic 
system, separate stepper motors operate the LH and the RH side temperature blend doors. On the manual system, a 
single stepper motor operates the two doors. 
 
A drive arm is attached to the drive spindle of each temperature blend door stepper motor. The end of the stepper motor 
drive arm engages with a slot in the drive arm of the related temperature blend door.  
 
Operation of the temperature blend door stepper motors is controlled by the ATCM. For additional information, refer to   

Control Components

 (412-04 Control Components) 

41 
 

Air Flow Through Heater 

1

 -

Coolant outlet pipe

2

 -

Coolant inlet pipe

3

 -

Windshield distribution door motor

4

 -

RH temperature blend motor

5

 -

Heater core

6

 -

Heater casing

7

 -

LH temperature blend motor (automatic system only)

8

Evaporator

9

Face and feet distribution door motor