As noted in last month’s article, troubleshooting an air conditioning system often concerns refrigerant, airflow and mechanical problems, either individually or in combination. This installment in this series of articles will deal with an air conditioner’s liquid and suction line restrictions, overcharges and undercharges, and liquid subcooling in the condenser. A restricted liquid line will starve the evaporator of refrigerant, thus causing low pressures in the evaporator. If the evaporator is starved of refrigerant, the compressor and condenser will also be starved of refrigerant, so the evaporator will not absorb much heat for the condenser to reject. However, most of the refrigerant will be in the condenser and not necessarily cause high head pressures because of the reduced heat load on the evaporator. Because most of the refrigerant charge is in the condenser, liquid subcooling in the condenser will increase. This is a big difference from an undercharge of refrigerant, which will cause low condenser subcooling. If the system has a receiver, much of the refrigerant will be in the receiver, causing lower-than-normal head pressures. A restricted suction line will cause low su...