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A/C testing and service
Before testing and service, technician must be certified by
ASE, MACS, or IMACA
A/C temperature testing True test of systems ability to cool Is normally measured at center outlet Procedure:
– After running system on medium fan and engine at 1500-2000rpm, turning blower on low will slow down air flow over evaporator
– This will be coldest reading used for testing– Basic rule of thumb: cooling should be 20
degrees reduction over incoming air
A/C temperature testing
More accurately tested with electronic thermometers– Often used in conjunction with DMMs
called thermocouples
A/C pressure testing MUST WEAR EYE PROTECTION!!! Manifold pressure gauge set normally
consists of two gauges and three hoses
Low side gauge– A compound gauge reads both pressure
(usually to 150 psi) and vacuum (usually to 30”hg)
– Hooked only to low side through a blue hose
A/C pressure testing
High side gauge– A high pressure gauge that reads
usually to 500 psi– Hooked only to high side with a red
hose– May need high side adapter
A/C pressure testing
Center hose is usually yellow– Yellow hose is hooked to service
equipment such as a vacuum tank or refrigerant bottle
Two hand valves for controlling flow through gauge set
All hoses must be anti-blowback design by law
Test ports and valves
R12 systems will use ¼” flare fittings for the low side ports
R12 systems can use ¼”, 3/16”, 1/8” flare fittings for high side test ports to avoid incorrect hookup
Test ports and valves
R134a systems will use a 13mm O.D. quick disconnect connector for low side test ports
R134a systems will use a 16mm O.D. quick disconnect connector for high side test ports
Test ports
Test ports will be covered by a protective cap, mandatory to avoid slow leakage
Test ports will contain a special schrader valve designed for operation in refrigerant
Service valves at compressor
Used to isolate compressor from rest of system for repairs
Will have three positions - backseated, mid position, frontseated
Pressures - low side Low side pressures should reflect
evaporator temperature R12 systems will run 25-35 psi typically
– Variable displacement compressors can run into the 50psi range
– Cycling clutch systems will pull down to below 30 psi and then usually cycle off to about 45-55psi before turning on again
– R134a systems will run 18-40 psi typically
Pressures - high side
High side or head pressures should reflect condenser temperature– All systems vary widely based on ambient
temps– R12 will typically run 150-250 psi
System with electric cooling fans will cycle high with fan off then low when the fan turns on
– R134a systems will run somewhat higher - up to 300 psi
Diagnosis thoughts for complaints / pressure problems
Low refrigerant
All pressures will be low Compressors will cycle often in a
CCOT system LOOK FOR LEAKS
Excessive refrigerant
High side high but low side is metered down to normal/near normal pressures
In CCOT, all pressures high because volume is too high
Damaged compressor / stuck open TXV
Will not pump refrigerant and cause very little pressure change from high to low side
Restrictions
Cause high pressures before and low after
Temperature drops are greatest after restrictions (icy hoses or components)
Commonly plugged orifice tubes
Overheating condenser
Will cause high head pressures and high low pressures
Usually inadequate air flow
Iced evaporator
Usually reflected in lower than normal low side pressures
Often reduced air flow out ducts
A/C leak testing Electronic leak detectors
– Must be used under suspected leak– Most common today
Dyes– Added to A/C like refrigerant– Can be visible to naked eye or fluorescent type
Flame type tool (antique)– Obvious dangers with phosgene gas– Will not work on R134a systems
Retrofitting Changing R12 system over to R134a Remove most of mineral oil Flush condensor Replace dessicant Manufactures recommend multiple
component replacements Change over all fittings and add labels Add only portion of R134a with new style oil
Servicing AC systems
ON CAR
END