III-75
These readings at the vents will be higher and temperature swing slower
and not as obvious. Also blower speed will cause the temperature, levels to
read higher (high air speed) or lower (low air speed) at the same thermostat
setting. When you measure air temperature, an electronic thermometer/py-
rometer is a great tool to have. You can easily measure cab air temperature
at several locations quickly.
Swing temperatures vary depending on where you measure temperature,
and on outside temperature, humidity and altitude. The chart in Figure 7-6,
shows some examples of typical temperature variables. Don·t forget that
cab and sleeper area temperatures can vary within the same vehicle. Also,
electronic controls used in newer HVAC systems often keep the tempera-
ture spread within a narrower range.
2. Check Clutch Cycling Under Load
The following operating inspections, visual and by feel, are done outside the cab
while you wait for the system to stabilize.
A. Lift hood – With the hood up (or cab tilted) observe the clutch cycling
under load.
Note: If the condenser is hood mounted you may not have adequate air
flow through it.
B. Touch suction and discharge lines – Soon after system start up you can
safely feel the suction and discharge lines and note their change in tempera-
ture. The discharge line will get hot (after a while it may be to hot to touch)
and the suction line will get cooler.
3. Check Sight Glass
The sight glass is the only point where you can actually see inside the air condi-
tioner during operation. Check the sight glass through the window on the top of
the receiver-drier (or the separate in-line sight glass). If the system is functioning
properly and cooling the cab adequately, the sight glass should be clear (you will
not see anything in it). If it is not clear when the system is first turned on, wait a
few minutes for the system to stabilize, then look again. Figure 7-7 illustrates and
explains what you may observe in the sight glass. Roof mounted condenser fans
may run continuously or cycle on and off. If you can·t tell by sound you may have
to climb a ladder and observe the fan blades.
Inspection & Maintenance
Figure 7-6
The chart of AC system and
cab temperature range shows
you typical variables.
AIR TEMP.
DEGREES F.
AIR QUALITY
CENTER OUTLET
AIR TEMP.
DEGREES F.
OUTLET AIR TEMP.
RANGE DEGREES F.
HUMID
70°
80° 90° 100°
DRY
43°
to
47°
LEFT & RIGHT AIR OUTLET TEMPERATURE WILL VARY
40°
to
41°
LEFT RIGHT
LEFT RIGHT LEFT RIGHT LEFT RIGHT
HUMID DRY HUMID DRY HUMID DRY
40°
to
44°
44°
to
48°
40°
to
44°
47°
to
51°
40°
to
44°
52°
to
56°
41°
to
45°
41°
to
44°
41°
to
45°
43°
to
47°
46°
to
52°
47°
to
54°
48°
to
55°
50°
to
56°
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