Tech Tips
Below are a few quick Tech Tips from Tech Choice Parts, which will allow you easily understand some of the most common issues both Professional AC Mechanics and the typical Shadetree Mechanic tend to face on a regular basis:
What Vent Temps. should you expect from your vehicle?
Table of Refrigerant Pressures vs. Ambient (outside) Temperature
| Ambient Temperature | Low Side Pressure | High Side Pressure | Center Vent Temperature |
| 60°F | 28-38 psi | 130-190 psi | 44-46°F |
| 70°F | 30-40 psi | 190-220 psi | 44-48°F |
| 80°F | 30-40 psi | 190-220 psi | 43-48°F |
| 90°F | 35-40 psi | 190-225 psi | 44-50°F |
| 100°F | 40-50 psi | 200-250 psi | 52-60°F |
| 110°F | 50-60 psi | 250-300 psi | 68-74°F |
| 120°F | 55-65 psi | 320-350 psi | 70-75°F |
Comments
Tech Choice Parts - A/C Compressors 
Why do I need Air Conditioning in the Winter?
The quick answer is this: defrost. To explain further, the A/C system doesn’t just cool the air, it ‘conditions’ it. Whether you want cold or warm conditions is your choice. The A/C system is also a dehumidifier, removing moisture from the air. Most defrosters run with the A/C system these days. Also, because most engines have a single serpetine belt driving all the accessories, a locked up compressor can cause further problems, including a broken drive belt, resulting in a stranded motorist. If you are not going to replace your locked up compressor, do not turn on your A/C or defrost.
‘Tight’ General Motors Compressors
General Motors compressors, including R4, HR6, HD6, HT6, and V5, are normally very difficult to turn by hand right out of the box. Each piston end has a Teflon piston ring that is pressed onto the piston. These rings ‘flex’ and created a positive seal in the cylinder. Similarly would be that new jar of grape jelly. Just because you can’t get the lid off doesn’t mean it’s locked up and the jelly is bad; you just need the right tool. In the case of GM compressors, that tool is called a spanner wrench. Most auto parts stores and technicians have these.
General Motors Compressor, Shaft Nut Missing
Most new GM compressors do NOT have a shaft nut. TSB number 93-T-03, GM Corp reference 261204R and 111402R, dated October 1992: Subject: “Compressor Shaft Nut Eliminated”, all R4 and H6-series compressors. The details are: “Effective after July 13, 1992 for the R4 compressor or April 28, 1991 for the HR compressor, the shaft nuts were/are no longer produced for these units. This affects both production and service compressors. The elimination of this nut has been made possible by improvements in the clutch driver to compressor shaft press fit. If you receive an air conditioned vehicle with an R4 or HR compressor and there is no compressor shaft nut present, no attempt should be made to install a nut.”
My A/C does not cool at idle
Poor Cooling At Idle
If you experience good cooling when driving at higher speeds, but poor cooling when stopped or at low speed, have your cooling fans or fan clutch checked. Good airflow across the condenser and radiator are important for a properly working system. Also, make sure the condenser is clean; free of bugs, debris, large plastic bags that drift up there. Blocked airflow will cause poor cooling.
High High-Side Pressure
A bad compressor DOES NOT pump 200, 300, or 450psi. We occasionally hear from someone with an extremely high high-side reading, stating that their compressor is not working. The compressor is a pump; all it does is pump refrigerant through the system. If your high side reading is high, the compressor is working. Check temperatures at the condenser inlet and outlet (see Temperature Testing below) and gauge readings; likely restricted condenser or bad expansion device.
Temperature Testing
Temperature testing is the most effective method of verifying and diagnosing an A/C system. Gauges measure pressure. Pressure is temperature. Gauges only read at 2 points in the system; where the high and low couplers are. With a good temperature probe one can take temperature readings all over the system, and pinpoint problems. Here’s how to temperature test:
AC should be set on MAX, blower on High. Doors and hood should be open. (max heat load)
There should be a minimum 30 degree drop from 1 foot in front of the condenser (front of vehicle) to the center duct in the vehicle.
There should be a 20-40 degree drop from the condenser INLET to the OUTLET. Less than 20 degrees or more than 40 degrees indicates poor airflow, improper refrigerant charge, or INTERNAL RESTRICTION. A very large temperature drop almost always indicates a restriction. The refrigerant is building up at the restriction, causing high pressure (and high heat), while little to no refrigerant is coming from the outlet, causing the big drop in temperature.
The INLET and OUTLET of the evaporator should be within 5 degrees of each other. This test is performed easiest on orifice tube systems, but if you can get to the evaporator side of the expansion valve, the test is still accurate. If the evaporator outlet is WARMER than the inlet, you have a low refrigerant charge, and need to add refrigerant. If the outlet temperature is COOLER than the inlet, there is too much refrigerant in the system. If evaporator inlet and outlet temperatures are the same, or within 5 degrees of each other, this indicates a perfectly charged A/C system.
Do I REALLY need to replace my Accumulator or Receiver Drier?
The Air Conditioning Receiver Drier, sometimes referred to as the Filter Drier, functions just as its name would indicate: as the filter for the refrigerant system. These “Filters” are cylindrical in shape and always have an Inlet and an Outlet. Receiver Driers are always used on AC systems which also have an Expansion Valve. When locating your Receiver Drier, you’ll find that they look similar to a metal tube or can, and will always be found on the high-side of the refrigerant system, usually between the Expansion Valve Inlet and the Condenser Outlet. Many later-model vehicles have the receiver drier built into or on the condenser assembly. Most of the time, when directly attached, the desiccant bag is replaceable by itself.
Many vehicles have what is also referred to as an AC Accumulator. Although the function of an Air Conditioning Accumulator is very similar to that of a Receiver Drier, it is typically much larger and is on the low-pressure side of the refrigerant system.
Accumulators and driers always have a desiccant bag inside. The desiccant bag, normally activated by moisture (oil and refrigerant), absorbs H2O (water) and traps debris. Once activated, it will trap any water in the system. When an A/C system is opened, the natural humidity in the air enters the A/C system, and the desiccant bag can immediately become completely saturated and be rendered ineffective. If the desiccant bag cannot absorb any additional moisture that remains in the system, a chemical reaction between water and refrigerant can create an acid that will eventually eat though seals and o-rings throughout the air conditioning system. For this reason, it is important that the accumulator or receiver drier be replaced any time the A/C system is opened to the atmosphere.
One of our favorite analogies we use to help consumers understand why it is important to change out the Receiver Drier and/or Accumulator is the Engine Oil example. When visiting your local repair shop to get an oil change, you always replace the oil filter to compliment the new oil added to your engine. The same holds true for the automotive refrigerant system. When performing any job where the Freon is evacuated and the system is opened up, it is imperative to replace the Drier/Accumulator along with any faulty parts. Doing so also ensures you preserve your warranty.
What about the Expansion Valve or Orifice Tube, Do I REALLY need to replace it?
Replacing the orifice tube is fairly quick and easy, and should be done any time the A/C system is serviced. The orifice tube is generally located in the condenser outlet, evaporator inlet, or sometimes in the liquid line hose assembly, particularly in Ford systems. Because orifice tubes have a fine mesh screen on them, they can easily becom clogged. Expansion valves, located on or near the evaporator, are usually more time consuming to replace, and are not required as part of our warranty, but they should be verified for proper operation. A good technician will know, with gauge readings and temperature readings, if your expansion valve is operating correctly.
All vehicles are equipped with either and expansion valve or an orifice tube. Vehicles with rear air conditioning may also have a combination of two valves, to orifice tubes, or an orifice tube and an expansion valve.
Why Flush the System?
If your original compressor has locked up, you must flush the A/C system. Failing to do this will leave metal debris in the system, and cause the replacement compressor to fail as well. Keep in mind, new multi-pass, 6mm tube condensers are nearly impossible to properly flush. You may need to replace the condenser. Talk to your A/C Technician for more information.
Blend Refrigerants. Or, What NOT to Use.
Use only R12 or R134a refrigerant. “Blend” or “Alternative” refrigerants are not approved by ANY of the major automobile manufacturers. If those refrigerants did what they say they do, the OEMs would be all over it. Some blend refrigerants, while they may improve duct temperatures, can cause problems down the road.
System Sealant. Or, What NOT to Use Again.
Avoid system sealers and leak stop. These products are band-aids, not repairs. Leak sealers react with air. If there is air in the system, the leak stop will congeal into a thick green jelly. Not only will it clog system components, your A/C Technician is not going to want to evacuate your refrigerant with his very expensive recovery machine. Further, he will be very upset if you don’t tell him you have it in your system, and he does run it through his very expensive recovery machine.
These are some basic technical facts. TechChoice Parts will be adding and updating information as it comes available, but if you have a specific question, feel free to e-mail or click “Sales Chat” for an instant online text session.