240 Auto Heating & Air Conditioning
these switches can be diagnosed by looking for a frozen
condensation at the evaporator inlet. The compressor on
an orifi ce tube system will operate continuously. Typical
system pressures are shown in Figure 15-18.
After all checks have been made, return the engine
to idle, shut off the HVAC system and engine and go to
Step 4.
Leak Detection
One of the most common refrigeration system
dia gnostic jobs you will do is locating leaks. It has been
estimated that over 50% of all refrigeration problems are
caused by system leaks. Leaks either cause performance
problems or lead to failure of a system part, usually the
Figure 15-17. Gauge readings for a restricted condenser.
Condenser Restriction
Low Side (Suction)
Symptoms: Poor cooling at low speeds, engine overheats, sight glass clear. Ice or frost
forms on the condenser. Verify proper fan and cooling system operation.
Orifice Tube
Exp. Valve
VDOT
(Varies with ambient temperature) (Varies with ambient temperature)
Normal to High
Normal to High
Normal to High
High
High
High
High Side (Discharge)
0
10
20
30
40
50
60
70
80
90
10
0
110
120
0
50
100
15
0
200
250
300
350
400
450
500
Figure 15-18. Gauge readings for a misadjusted or defective pressure switch.
Defective or Misadjusted Pressure or Thermostatic Switch
Low Side (Suction)
Symptoms: Cooling OK initially, then air warms, continuous compressor operation (all systems),
evaporator outlet iced, sight glass clear. Blown thermal limiter, if used.
Orifice Tube
Exp. Valve
VDOT
(Varies with ambient temperature) (Varies with ambient temperature)
Low
Normal to Low
Low
Normal
Normal
Normal
High Side (Discharge)
0
10
20
30
4
0
50
60
70
80
90
100
110
120
0
50
100
15
0
200
250
300
350
400
450
500