843 / 1003
3-7
Go to
Chapter Start - Table of Co
ntents- Index
Chapter Review
• There are various methods for controlling air temperature
and movement in a heater or HVAC system. In a simple
system there is usually an on-off switch for the fan or blower
motor. Most have three speeds plus “off.” The water valve
may be cable controlled, as is the defroster door. More com-
plex systems may have air pressure switches and air con-
trols, vacuumcontrols, electronic controls, and various doors
to control air flow direction.
• In heater systems air outlet temperature is controlled by one
of three methods:
1. Thewater valve which varies the amount of coolant
passing through the heater core.
2. A blend air door where cool air is mixed with heated air
to obtain the desired temperature.
3. Electronic controls, which we will cover in Chapter 5 of
this manual. • Full fresh air system designs use 100% fresh air in all but “off ” and “maximum” air conditioning control settings. These systems offer advantages you should be aware of. Fresh air
increases air pressure in the cab, thus any openings leak
outward. This eliminates cold spots and provides a more
uniform cab temperature. Air moving out of the cab openings tends to act as a sound barrier and reduce cab noise levels. The fresh air leaking out of the cab also provides a continu-
ous change in the cab air and purges smoke, other air con-
taminants and any excess moisture.
• Some systems activate the air conditioner in the defrost mode
to remove moisture from the heated air. This ensures quick
defogging of the windows.