A/C System Familiarization and Troubleshooting

This thread is to give the Average Person a general understanding of a vehicle Air Conditioning Troubleshooting procedures and Refrigeration Principals. The concept also applies to home HVAC and refrigeration systems.

Refrigeration is the process of moving “heat” from one place to another.

The heart of the system is the compressor; it builds pressure and circulates Freon throughout the system.

When Freon gas leaves the compressor towards the condenser, it is under high pressure and is full of “heat.” As it passes through the condenser, the gas “condenses” into a liquid as it is cooled by the airflow over the condenser. The air flowing over the condenser takes the “heat” out of the system and passes it to the ambient air outside the coil.
This cooler condensed gas is now a liquid when it gets to the “orifice tube.”

The orifice tube is responsible for restricting the flow of refrigerant and regulating the pressure in the system creating a high and low side.

Once the refrigerant passes through the tube, there is a huge pressure reduction to around 45psi (the low side). At this point the liquid refrigerant starts to boil off as it passes through the “evaporator.” The inside air flowing over the evaporator coils “heats” up the gases and that “heat” is absorbed into the system resulting in the cooler air coming out of your vents.

This gas that is “full of heat” then goes back out to the compressor and to the high side to remove the “heat” and start the process over again.

System Troubleshooting

All automotive systems after 1995 are required to use r134a. Pre 1995 is more than likely is running r12 and must be serviced by an EPA licensed technician; unless there has been a r134a retrofit kit installed. Make sure you are wearing eye protection during this process and gloves are recommended due to the high pressure gases within the system. If you are uncomfortable with high pressure gases please consult a qualified A/C technician.

The A/C system can only be properly diagnosed and checked when the compressor is engaged and running.

There is a low pressure safety switch on the low side of the compressor that will prevent the compressor from engaging if the refrigerant “charge” is too low. Usually around 25-30 psi. This switch can be bypassed for “testing purposes only” with a paperclip to check for compressor functionality.

When the system is “not” running, you should have at least 50psi in the system. More is better and it can be as high as a 200-300 or more depending on the outside and engine temperatures.

When the compressor is engaged, the low pressure should be around 35-40psi +/- 10psi depending on the ambient temperature.

With the compressor engaged the high side should be 200-350psi. Ambient air temperature and air flow over the condenser plays a big part in this pressure reading. Higher on a hot day and higher with poor or restricted airflow over the condenser.

For the most accurate diagnosis of the A/C system you need to check the high and low side pressure. With that information, you can determine the cause of your performance issue.

-Low side and High side have lower than normal psi= Low refrigerant charge

-Low side and High side have higher than normal psi= Possible overcharge of refrigerant or blocked/restricted airflow over the condenser. Check condenser for obstructions/debris/cleanliness/bugs/rocks/bent fins. Could also indicate an inoperative condenser fan.

-Low side is extremely low or negative and high side is too high= bad or blocked expansion valve/orifice tube. Orifice tube will need to be replaced and inspected for cause of blockage. Debris or early signs of compressor failure.

-Low side is too high and high side is too low= compressor failure or possible clutch slippage. Compressor is unable to provide needed flow and needs to be replaced.

If you find yourself needing to add refrigerant every couple of weeks, you need to have the system checked for leaks and serviced by a qualified person. Some leakage is expected out of these system on a yearly basis due to the smaller size of the r134a molecules.

Common problems for 3rd gen Rams

-Compressor lead wires short out on compressor housing or engine and cause vehicle performance issues.

-Compressor clutch fails. Power is going to the compressor but the clutch is not engaging. Try tapping the clutch with a wooden handle to see If it engages. A failing clutch can also draw too much amperage from the TIPM resulting in the system shutting down.

-No power to compressor. Check low pressure switch for functionality. Check system for too low of charge resulting in low pressure safety cutoff.

-TIPM failure can result in the compressor not being allowed to turn on even if the low pressure switch is bypassed.

-System compressor is cutting on and off steadily and the low side pressure drops to ~20psi then raises as the compressor disengages. Possible low charge resulting in low pressure safety cutout.

-System is properly charged but is not cool enough. Check for restrictions in the condenser preventing inadequate airflow. Thoroughly wash the Condenser with soapy water and a degreaser to remove road debris and rocks, also check for bent fins. Vehicles equipped with electric fans may not be functioning properly and need to be check for functionality when the A/C system is on. Could also be possible broken blend door within dash.

There is probably more scenarios but that is all that i can think of off the top of my head.