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HVAC Service Documents

Gauge Diagnostics

Summary Table for Gauge Diagnostics

Low Pressure Side is TOO HIGH
The low pressure side normally becomes too high when the high pressure side is too high.

Cause	Inspection	Remedy
Defective Thermoswitch	The magnetic clutch switch turns off before the outlet air temperature is sufficiently low.	Replace the thermoswitch
Poor Expansion Valve temperature sensor contact	The high and low pressure sides gauge pressures equalize when the magnetic clutch is turned off (within a short time duration)	Replace Compressor
Expansion Valve opens too far	Frost has adhered to the suction hose / pipe	Install the temperature sensor against the low pressure pipe and cover with insulating tape.
Clogged compressor suction filter	Compressor fitting is cool but the low pressure hose is not.	Remove and clean the filter.

Low Pressure Side is TOO LOW

Cause	Inspection	Remedy
Insufficient Refrigerant	There is little temperature difference between the low and high pressure sides.	Repair any leaks and recharge the refrigerant to the correct levels.
Clogged liquid tank (receiver dryer, accumulator)	Considerable temperature difference between the inlet and outlet sides of the receiver dryer during operation. Also, tank may be frosted	Replace liquid tank (receiver dryer)
Clogged expansion valve	Expansion valve inlet side is frosted. Generally the low side pressure is near vacuum.	Replace expansion valve.
Expansion valve temperature sensor gas leak (damaged capillary tube, etc..)	Expansion valve outlet side is chilled and low pressure side is LOW.	Clean or replace expansion valve.
Clogged or blocked piping	Low pressure readings decrease or a negative reading is shown. Indicates piping is clogged or blocked between the evaporator coil and the compressor. A frost spot may indicate the point of blockage.	Clean or replace piping.
Defective thermoswitch (cold control)	Evaporator is frozen showing ice along the face of the coil, not just frost on the manifolds of the coil.	Adjust temperature sensing tube to a cooler part of the coil or replace the thermoswitch.

High Pressure Side is TOO HIGH

Cause	Inspection	Remedy
Poor Condenser cooling	Dirty or clogged condenser fins, Cooling fans do not operate correctly.	Clean and / or repair the condenser core / fans.
Excessive refrigerant	Verify by gauge reading	Utilize your refrigerant recovery equipment to capture excess refrigerant. Charge to the correct refrigerant levels.
Air in the system	Pressure is high on both high and low sides	Evacuate, vacuum and recharge with refrigerant.

High Pressure Side is TOO LOW

Cause	Inspection	Remedy
Insufficient refrigerant	Little temperature difference between the low and high pressures.	Repair any leaks and recharge the refrigerant to the correct levels