Hoses

• Three hoses, each with an identifying color, and two gauges make up a basic manifold gauge set. The red hose is the high-pressure service port, and the blue hose is the lower pressure service port; both are connected to the vehicle during service. The yellow hose is attached to the refrigerant cylinder or vacuum pump if the vehicle is being charged or evacuated of air, respectively.
  
  

Gauges

• The gauge colors correspond to the blue and red hoses. The blue low-pressure gauge -- called the compound gauge -- ranges from minus 30 to 120 and measures pressure in psi gauge (psig) or vacuum in inches of mercury. The red high-pressure gauge ranges from 0 to 500 and measures only pressure. When the vehicle is turned off, both gauges should register approximately the same number, called a static pressure. Although a static pressure reading will not determine the extent to which the system is full, it can determine if liquid refrigerant is present.
  
  

Short Cycling

• A process known as short cycling is used to detect low refrigerant and other problems. When the static pressure reaches at least 45 psi, turn the car on and begin to test at stable engine speed -- idle or just above. The pressure on the low side will drop from 45 to 20 psi and then gradually reach equilibrium with the high side again at about 45 psi. A rapid short-cycling process indicates low refrigerant. This can be remedied, using the manifold gauge, through a process called charging.
  
  

Other Usage

• A knowledgeable technician can pinpoint a number of problems in a car's air conditioning system using a manifold gauge. If there is a lower-than-normal pressure on the red gauge and the engine is sluggish to reach equilibrium when short cycling, an orifice tube is clogged. Similar measurements on the high and low gauges when the car is running indicate insufficient suction and a bad compressor. The low-pressure gauge drawing into a vacuum -- negative pressure -- indicates a bad expansion valve that is stuck open.