Review of Frequent Problem Areas
3. Refrigerant Lines, Hoses, and Fiftings
4. Refrigerant Metering Valves
5. Other Problems
1. Belts and Compressor Clutch
Let's review problem areas listed at the beginning of Chapter 7. The most
frequent repairs are replacing belts and servicing or replacing the com-
pressor or clutch. Heavy duty vehicle operation puts a lot of stress on
these parts. There are several main reasons.
There is often continuous operation for long periods of time. There may
be frequent sudden RPM variations when shifting gears up or down. For
this reason the AC clutches used in heavy duty systems usually have
double row ball bearings. Vibration and road shock contribute to loose or
broken mounting brackets, electrical connections and fittings. Belts, bear-
ings and compressor reed valves wear out.
Various compressor clutch cutout switches are used because the AC
designers know about compressor operating conditions. System leaks,
high operating pressures, malfunctioning engine cooling system compo-
nents--all cause compressor problems and failures. When refrigerant and
refrigeration oil leaks out of a system or there is contamination blocking
oil flow, the compressor will be starved for oil and seize.
Condensers get dirty and the dirt reduces heat movement by insulating
the condenser. The fittings come loose or break from stress if the con-
denser or connecting hoses are not secured properly to keep the effects of
vibration at a minimum.
Heat transfer efficiency and pressure in the condenser are affected by
the amount of outside air flowing through condenser fins. A lack of air
flow can mean the refrigerant doesn't give up enough heat energy to the
outside air (it doesn't change state). The refrigerant arrives at the evapo-
rator as a gas and can't pick up any heat energy from cab air. In the cab,
air from the vents is only slightly cool or warm.
One possible cause of condenser malfunction could be the engine cool-
ing system. This is why fan clutches and radiator shutters are often
controlled or overridden by AC switch function. In fact, we can add fan
clutch, radiator shutters and also fan motors to condenser problems. If
they don't function to allow sufficient air through the condenser, pressure
inside the system may become dangerously high. A lack of air through the
condenser fins can raise high side pressure and blow out the weakest
point in a system, or damage the compressor.
3. Refrigerant Lines, Hoses and Fittings
Problems with these parts may be caused by normal deterioration, vibra-
tion damage, lack of maintenance or human error (improper installation
or replacement). All rubber parts are attacked by ozone (oxygen) in the
air. Rubber parts break down slowly and become more vulnerable to the
effects of vibration with the passage of time.