16-51 Figure 16-84.  Cross-section of an R12 refrigerant service valve.  Figure 16-82.  Examples of electric motor driven vapor cycle air  conditioning compressors. Figure 16-83.  A vapor cycle air conditioning condenser assembly  with an integral fan used to pull outside air through the unit during  ground operation.  Figure 16-81.  A typical belt drive engine driven compressor. The  electromagnetic clutch pulley assembly in the front starts and stops  the compressor depending on cooling demand. R134a systems use valves that are very similar to the Schrader  valve in function, operation, and location. As a safety device  to prevent inadvertent mixing of refrigerants, R134a valve  fittings are different from Schrader valve fittings and do not  attach to Schrader valve threads. The R134a valve fittings  are a quick-disconnect type. Another type of valve called a compressor isolation valve  is used on some aircraft. It serves two purposes. Like  functioning normally fully condenses all the refrigerant  flowing through the condenser. All vapor cycle air conditioning systems are closed  systems; however, access is required for servicing. This  is accomplished through the use of two service valves.  One valve is located in the high side of the system and the  other in the low side. A common type of valve used on  vapor cycle systems that operate with R12 refrigerant is the  Schrader valve. It is similar to the valve used to inflate tires.  [Figure 16-84]  A central valve core seats and unseats by  depressing a stem attached to it. A pin in the servicing hose  fitting is designed to do this when screwed onto the valve’s  exterior threads. All aircraft service valves should be capped  when not in use.  Service Valves