Troubleshooting (or why do I need gauges if they won’t tell me the amount of charge?)Refrigerant is considered a condensable, not a compressible. That is to say, the more refrigerant you put in a closed system, the more liquid you get. The pressure does not increase unless the temperature increases. Each refrigerant has a very specific relationship between pressure and temperature. Knowing this, the pressures can tell us a lot about what is going on inside the system.
Under normal conditions with the system not running and everything at an equalized temperature, the static pressures at both the HP port and the LP port are equal to expected pressure at ambient temperature as indicated on a temperature/pressure chart for that specific refrigerant.
Once the system is running, as described above, the system divides itself into two halves: the High Pressure side and the Low Pressure side, each with a port to monitor the pressure. Once running, typical pressures for R-134a are LP =30-31psi, HP 204-210psi. R-12 has a narrower range, typically LP = 32-33psi, HP=185-190psi.
The above is rule of thumb only. Actual normal pressures may vary by ambient temp and humidity. Reference should be made to the
R-12 performance chart or
R-134a performance chart for greater precision when deciding if a pressure should be considered low, normal or high under the exact circumstances. The same goes for the vent temperature – a system in perfect working order might blow near-freezing air out the centre vent when ambient temperatures are 75°F and RH is 40 or 50%, but might only achieve 70°F when the mercury climbs over 90° and humidity approaches 100%.
LP low, HP normal to slightly low. Probable causes:
1. Incorrect adjustment of LP switch (note: some vehicles use a TXV valve control to perform the function of the orifice tube used in the B-bodies. The symptoms associated with the orifice tube would apply, with modification, to the TXV valve in those vehicles)
2. Restriction in the low side of the system. EG: plugged orifice or screen.
3. Moisture in the system (may freeze at orifice inlet, causing very cold inlet).
To verify between 2 and 3, turn off AC and allow to stabilize for 15 minutes, then turn back on. If gauge reading immediately goes to abnormal condition, the screen is probably clogged. If the gauge readings are normal for a few minutes, then goes abnormal, it is probably excess moisture.
LP very low to low, HP lowProbable causes:
1. Low refrigerant charge
2. Clogged inlet screen
3. Defective or plugged orifice tube
4. Moisture in the system (as above)
5. High-side restriction in the high side before the orifice tube (eg: crushed condenser tube).
Loss of refrigerant is usually due to a leak, although all systems leak a tiny bit as a normal condition. R-134a systems are usually more sensitive to the correct charge than R-12 systems. R-12 systems usually have a sight glass, and low charge will show up as bubbles in the sight glass. The compressor will usually cycle rapidly, as there is insufficient liquid refrigerant available to supply the compressor intake. The evaporator outlet line may also be warm, as there is insufficient refrigerant to keep the evaporator full of liquid.
LP low, HP high to extremely high. Probable cause:
1. Restriction in the high side of the system
2. excessive oil charge
In the case of a restriction, it could be anywhere from the compressor outlet to the fixed orifice tube. The closer to the compressor, the higher the high-side gauge pressure will be (because the refrigerant has less time and space to cool). Probably a kinked or bent line or tube. A marked temperature change will often mark the location (as it acts like an orifice tube) with the cooler side downstream of the restriction.
Excessive oil charge may result in vibrating or pulsating hoses. The oil acts as both an insulator and takes up space. The compressor must work harder against higher pressure and the lines may vibrate as the refrigerant is pushed through a pool of oil in a line. The outside temperature of the high-side line might be cooler than the temperature/pressure chart would indicate as the oil is preventing heat transfer from the refrigerant to the line.
LP high, HP low Probable causes (electrical):
1. compressor LP cycling switch
2. PCM pressure sensor
3. ambient air temperature switch
4. engine coolant temperature sensor
5. throttle position sensor
(mechanical)
6. defective/misadjusted clutch
7. defective compressor
The compressor may or may not be running – look closely at the centre hub. If it is not turning, or not turning continuously with the pulley then there is an electrical problem or a clutch problem. If it is turning, it is probably the compressor itself or a mechanical problem with the clutch.
Disconnect the LP cycling switch connector (on the accumulator). With a piece of jumper wire, briefly short the two wires and verify if the compressor starts. The switch is adjustable, using the small slot-headed screw inside the connector. The switch can also be replaced without evacuating the system as it has a Schrader valve beneath it.
Using a scan tool, verify the AC pressure as seen by the PCM and compare the reading to the actual measured pressure. If the indicated pressure is much higher than the measured pressure, the PCM may be cycling the compressor each time the sensor hits the high limit. Similarly, the PCM will shut down the compressor if it detects an overheated engine, full-throttle operation, or low ambient temperature.
If the clutch fails to engage, only engages when travelling uphill or only engages when the centre is tapped then the clutch gap may be excessive and should be reduced. The clutch coil may be burnt out or shorted. A shorted coil will draw excessive current and usually blow it’s fuse. An open coil can be determined with an ohm meter. A slipping clutch is rare, but can occur if a rivet or arm has broken and it will make a racket. You won’t need gauges to figure out what’s wrong if the clutch is slipping.
Once you have ruled everything else out, the only thing left is a worn out or damaged compressor. Unless someone else worked on the system and forgot to reinstall the orifice tube.
LP high, HP high to extremely highProbable causes:
1. Air (or other contaminants) in the system
2. Overcharge of refrigerant
3. Excess oil in system
4. Condenser fins clogged or obstructed or debris trapped between condenser and radiator
5. Defective cooling fan(s)
6. Overheating engine
7. Incorrect refrigerant
An overcharge of refrigerant will often result in a cool to warm evaporator outlet pipe and while cooling will be poor at idle, it may be OK on the highway. Air in the system can be very similar, but can sometimes be identified by turning the system off while watching the gauges – the pressure will drop 20 or 30 psi very quickly, then taper off very gradually as the two sides equalize.
Poor cooling of the condenser (fans, obstructions to airflow) can usually be temporarily corrected by misting the condenser with a garden hose and sprayer. The HP will usually plummet to near-normal almost immediately when the water hits the condenser.
An overheating engine causes additional heat load, keeping the condenser warm by radiation.
Air contamination can result from improper charging practices of course. However, one Florida study suggested that as many as 25% of commercial AC shops have contaminated recovery equipment, probably by servicing vehicles after the owner tried and failed. Similarly, the owner may have tried to recharge the system with some other refrigerant. If the shop does not test the system contents with an expensive refrigerant identifier, the contaminants get spread to successive customers.
Home 
Thu Aug 21, 2014 6:44 pm
