AC 102

[bromodragonfly]

A continuation of AC 101...

Some random facts and notes about the Beretta AC system.. and auto AC systems in general.

The pulley attached to your compressor is belt driven and always spinning. When you turn your AC on, a relay will send 12 volts to the compressor clutch - it is a metal piece (electromagnet) that will magnetize and 'bind' the pulley to the crankshaft in the compressor. It is only when the compressor clutch is engaged that the compressor will actually function. If you're looking at the compressor when the AC is switched on, you may see the clutch 'pull in' and hear it snap into place when it magnetizes.

There is one main safety and two operators in the Beretta AC system. First is the high pressure cutout. It is designed to cut power to the compressor clutch in the event that pressures on the high side of the system rise to higher than operational levels. (Note: High Side of the system refers to everything from the compressor discharge up to the metering device. The low side will be everything after the metering device and back to the inlet of the compressor.)

On my 87, next to the high pressure cutout is the fan cycling switch. I assume it works off pressure (although there are some that work off temperature), and engages when ambient temperatures are high, and/or pressure/temperature of the refrigerant is high, or you are idling and there is not enough airflow over the condenser.

There is also a low pressure operator (normally open). On my 87, it is mounted on the actual compressor body. It will only close and allow power to the compressor clutch when there is adequate pressure on the low side of the system. If the AC is running and the low side pressure drops too much, it will cut power to the clutch.

Static pressure in the system (pressure when the AC system is off) will always be equal on both sides of the system. If the AC was turned on, high side pressures would equalize into the low side by either flowing back through the metering device, or through the valves inside the compressor (the hissing sound you hear when you turn your car off and the AC has been running). If you've had a leak and there is a very low (or no) static pressure, the compressor clutch will not engage due to the low pressure switch failing to close.

Higher ambient temperatures will result in higher system pressures. And the vice versa. Keep in mind that when the system is running (and ambient conditions are constant) high side pressures are the same throughout the entire high side of the system. And pressures on the low side of the system are the same throughout the entire low side of the system. The changes that occur in the condenser and evaporator are temperature changes, and changes of state only.

System pressures (and temperatures, if you have a strap-on thermocouple) can be an excellent indication of what is happening inside your system, and how efficiently it is running. Unfortunately, my 87 only has one connection to attach gauges (or charge) to, which happens to be a 1/4" male flare connection on the suction accumulator, which makes it difficult to quickly diagnose problems.

Just keep in mind that the compressor discharge should always be hot or uncomfortably warm. The metering device should be much colder on its outlet than it is on its inlet.

Vehicles 92' and older will probably use R134A refrigerant (non ozone depleting) instead of R12. If you are doing a retrofit from R12, R134A 'technically' is not supposed to be compatible with the oil used with R12 (PAG for R134A and mineral for R12). Some will say to drain all oil out of the compressor and flush the system before adding the new oil, because R134A will react with any mineral oil to form an acid. In my experience, I've seen AC rooftop units and other equipment 'retrofitted' for R134A and the oil was never changed. They ran for years without problems. I've spoken to manufacturers that state that it really doesn't matter. However, if you have PAG oil on hand, it can't hurt to drain the old oil from the compressor, measure it, and add the same quantity of new oil.

Although R134A is very similar to R12 in terms of the pressure/temperature relationship, they are not the same. R134A equipment is generally a bit bigger than R12 equipment due to the fact that it is NOT as good as a refrigerant as R12, and requires more surface area to exchange its heat. So keep in mind that if you do retrofit from R12, you are still using R12 equipment - your AC will not work as well as it did with R12, although the change may not be obvious.

I would not use POE oil in an automotive AC system due to the fact that it loves to absorb moisture, and will quickly break down into an acidic substance once it comes into contact with water.

Vehicles using R12 will most likely have 1/4" male flare connections. Newer vehicles running R134A will probably have a 10mil and 13mil quick coupler connection. I know there are some Ford and GM vehicles that use a retarded size (something like 5/16 and 7/32 flare) for their connections. Seriously. Thank the desk engineer for thinking of that one. Pass me the 11/17ths wrench.

Whether you use R12 or R134A, you will most likely need to recharge the system every couple of years. This is due to the compressor being an open drive and containing a mechanical shaft seal... which is prime for leakage over time.

An R12 TXV can be used with R134A. The valve itself does not care what it is metering, all it does is adjust for superheat. The orifice/port of the valve depends on the tonnage/capacity of the system, and since you are not up or downsizing components, it will not need to change. If one is available for your vehicle and is suitable for R134A, buy it if you want. Both will work.

Using the defrost fan will cycle the AC compressor on. Air is routed over the evaporator coil in order to remove humidity (the moisture will condense on the coil) before it is heated and blown onto your windshield.

If you're opening up the system to atmosphere and replacing parts, it is a good idea to replace the suction accumulator (drier) when you're done. It's a thirty dollar part, and moisture in a small AC system is extremely detrimental. If possible, pressure test with dry nitrogen, and proceed to pull a good vacuum for at least an hour before introducing a refrigerant charge.

Keep in mind that there is no way to tell how much refrigerant a system contains based on the pressure. You cannot charge a system based off of pressures. People may say things like "suction pressure should be around 40psi, and discharge around 120psi," but it all really depends on ambient conditions, and the condition of the equipment in your car. A dirty condenser/evap, dying/overheating compressor, restriction, or contaminants etc (such as air) in the system can really screw pressures up, and therefore totally throw off your charge.

Overcharging could result in liquid refrigerant flooding back to the compressor. Undercharging could lead to blowing warm air, or short cycling (which can be hard on the compressor). The best way to accurately charge a system is to evacuate and then use a scale to weigh the cylinder you're charging from. There should be a sticker under the hood that designates the refrigerant charge. In most cases, weight of R134A added in a retrofit will be 10-20% less than the original weight of R12.



That's really all I can throw out there for now.... I was thinking of writing up an AC 103 topic that will go over the most common car AC problems, the symptoms, and how to correctly diagnose and fix them. If anyone's interested, let me know. I'm not on the forums religiously - usually trolling around when there's something wrong with my car. So if anyone has any specific questions, send me a PM and hopefully it'll notify me via email.

cheers, stay cool.

[Money pit Beretta]

Ok the new Chilton's has nothing on any of this. I would be a good idea to sticky this and the other topic.

[indyyellow]

I am cold just from reading that.