Chapter 7 - Inspection & Maintenance
These readings at the vents will be higher and temperature swing
slower and not as obvious. Also blower speed will cause the tempera-
ture, 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/pyrometer 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 tem-
perature, 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 temperature spread within a narrower range.
The chart of AC system and
cab temperature range
shows you typical variables.
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.
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 temperature. 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
conditioner 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.