By Glenn Goodspeed (August, 1998)
This article is intended as an introduction to auto air conditioning service on the Volvo P1800 with stock York reciprocating compressor, and it covers only a few of the most common problems and solutions. For more detailed procedures, check your local library or bring the car to a professional.
Ever wonder how your car's air conditioner works? It relies on the fact that a liquid absorbs heat as it evaporates. Yes, the evaporator in your car does just what its name suggests. The evaporator is the part inside the passenger compartment where the liquid refrigerant is allowed to evaporate into metal tubing. A fan blows air over the tubing, cooling the air and blowing it into the passenger compartment.
The refrigerant, now gaseous, heads back to the compressor, where it is compressed, and then proceeds on to the condenser, which is located in front of the radiator. When you remove enough heat from a compressed gas, it turns into a liquid. In the case of your refrigerant, it takes a good breeze blowing through the condenser to remove enough heat to turn it back into a liquid. The liquid is then filtered in the receiver-drier and sent back to the evaporator. A special valve lets just enough liquid into the evaporator to produce maximum cooling.
Moisture is the arch enemy of automotive refrigeration systems. The smallest amount of moisture mixed with the refrigerant will turn to ice and stop the flow of refrigerant through the system. This is why an a/c system that has been opened to the atmosphere must be vacuum-pumped before adding a new charge of refrigerant. The air that has entered the open system contains moisture in the form of water vapor. The vacuum pump removes the air and water vapor.
The receiver-dryer, a soda can-shaped object, contains a compound that absorbs any moisture left in the system after a thorough vacuum pumping. The compound can absorb only a small amount of moisture before it is saturated, so a system that remains open for more than a few minutes must have a new receiver-drier installed before pumping and recharging.
If your system only needs recharging, it is not necessary to buy gauges or a vacuum pump, but you will need a can tap and a charging hose. If more extensive service is required, you will need some expensive tools. A cheap vacuum pump will set you back two hundred U.S. dollars, and a gauge manifold for R-134a with all the necessary hoses and adapters costs about another hundred fifty. But compared with the price of air conditioner service, the tools are a bargain, and they will pay for themselves after a few uses.
Air conditioning work, like most automotive work, can be hazardous. The most obvious dangers are pressure, freezing and asphixiation. When the a/c system has a full charge of refrigerant, the pressure inside the components is enough to send hoses and parts hurtling through the air at knockout speeds. Never disconnect any part of the system without first slowly relieving the pressure. R-12 and R-134a are not toxic, but if released in a small area, such as inside a closed car, they can displace enough oxygen to make breathing fruitless. Make sure you have good ventilation before starting work. Also be aware that liquid refrigerant can cause severe frostbite if spilled on your skin.
R-12 is being phased out because it causes damage to the earth's ozone layer. It is against the law to vent R-12 into the atmosphere in the United States and other countries. If you decide to convert to R-134a, the first step is to take the car to a shop where they can remove the R-12 safely from the system.
If your air conditioner stops cooling, the most likely cause is a lack of refrigerant. All auto air conditioning systems leak, some faster than others. If your system needs recharging more than once a year, you should try to find the leak and repair it. If it holds a charge for two years or more, you're in good shape.
To check the refrigerant level in the system, start the engine and run the air conditioner full blast for five minutes. With everything still running, look at the sight glass, which is a tiny window on top of the receiver-drier. You might have to hold a light on it and wipe it clean to see through it. If the system is o.k., you should see a clear liquid with a few bubbles passing by now and then. If you're low on refrigerant, you'll see a foamy froth, and if you're very low, you'll see nothing at all. This can be deceptive. A slightly overfilled system can look the same because it won't have any bubbles in it. To determine if the system is overfilled, have someone turn off the a/c while you're looking at the sight glass. You should be able to see the liquid draining from the sight glass.
Of course, you can use a gauge set to detemine the level of refrigerant and to troubleshoot mechanical problems in the system. Consult the instructions that came with the gauges or check out a recently-published textbook from your local library. Automotive Air Conditioning, by Boyce H. Dwiggins, is a good one.
To find a slow leak in a system, use a thick mixture of dishwashing liquid with a little water. Brush the mixture on the hose connections. If the connections are leaky, the mixture will bubble. (You must have at least 30 psi pressure in the system to do leak testing.) Leaks at hose connections usually can be stopped simply by tightening the connections with a pair of large wrenches. Often you can pinpoint a fast leak by listening to the various connections and seals with a mechanic's stethoscope. This is the best way to check the compressor shaft seal located behind the pulley.
If you've determined that your air conditioner needs refrigerant, and you have not opened any part of the system to the outside air, you can add refrigerant from 14-ounce cans using only a short charging hose. If your system is still using the original Refrigerant 12, you will not be able to find this refrigerant in small cans. If you don't want to go to the trouble of converting to R-134a, it would be best to take the car to a professional for a recharge of R-12. Under no circumstances should you mix different types of refrigerant, since this could cause expensive mechanical failure.
A system that has been converted for use with R-134a should have quick-connect adapters on the compressor service ports. These adapters allow the use of standard equipment designed for handling R-134a. The quick-connect adapters are not threaded like the old-style 1/4-inch flare fittings used for R-12. Also, the high-pressure quick-connect adapter is larger than the low-pressure adapter. To be sure your quick-connect adapters have been fitted to the correct service ports, check the top of the compressor. There should be a large "S" embossed on the aluminum head near the low-pressure (or suction) port.
To recharge an R-134a system low on refrigerant, you'll need a charging hose, a can tap, and three cans of R-134a, available at your local auto supply store. If possible, purchase a hose that has a right-angle quick-connect fitting rather than the straight kind, because the right-angle type is easier to attach. Unscrew the valve on the can tap until the handle is at its highest position, then screw the tap tightly by hand onto a can of R-134a. Screw the valve all the way down to open the can. The refrigerant will not be released until you open the valve again.
Prepare a pot of lukewarm water that you can use to dip the can of R-134a in while charging. Do not heat the water to more than 100 degrees Fahrenheit, or your refrigerant container may explode. Holding the can upright, attach the screw-on end of the charging hose to the can tap outlet and open the valve just until you start to hear the gas hissing. This will clear out air and moisture from the charging hose. Remove the blue cap from the compressor low side service port and attach the charging hose to it. As with all quick-connect fittings, you have to pull back the collar on the fitting, push the fitting onto the port, and then release the collar.
Some early P1800s had manual valves on the compressor service ports instead of the Schrader valves used on the later models. If you have manual valves, you should back out the low-pressure valve as far as possible after attaching the charging hose, and then turn it all the way in when finished charging.* The operating shaft for the valve is opposite the port opening, and should have a dust cover over it.
Once the hose is attached, open the can tap valve all the way (being careful not to unscrew the can tap from the can) and place the R-134a can upright in the pot of water. Start the engine and turn on the air conditioner full blast. Watch the sight glass while gently moving the refrigerant can around in the water. Try not to get water on the can tap. Occasionally pull the can out of the water and feel it. It should get lighter as the refrigerant flows into the system. If it cools rapidly in your hand, you still have some refrigerant left in it. If it doesn't cool at all, it's time for another can.
To start a new can, first turn off the air conditioner and remove the charging hose from the port. Remove the can tap from the can. Wipe off the can tap if it is wet before starting the next can. Using the same procedure as for the first can, attach the hose and continue charging. When the sight glass shows a clear liquid with occasional bubbles, you are finished.
If you attempt to charge the system following the procedure above, and the refrigerant does not flow out of the can, you may have a mechanical problem inside the compressor or a blockage somewhere in the system.
Never attempt to connect a can of refrigerant to the high pressure port. A normal R-134a charging hose will not fit the high pressure port, but it is easy to do this accidentally using a gauge manifold.
Never turn the refrigerant container upside down while charging. This may force liquid refrigerant into the compressor and cause severe mechanical damage. If you are foolish enough to have tried this and survived, it is because enough refrigerant was able to evaporate before being compressed. You might not be so lucky next time.
*A good R-134a port adapter will have a Schrader valve installed in it. There are some cheap adapters on the market that have only a pin in the middle that is supposed to operate an existing Schrader valve in the port. If these have been installed on a compressor with manual valves, the procedure for changing refrigerant cans during charging will be complicated by the need to close the manual valve before the charging hose or refrigerant can is disconnected from the port.