My '90 740 GL has 200,000 miles and my A/C is slowly giving up the ghost. From 10-50 MPH it blows freezing cold, but when sitting at stoplights or over 50 MPH, it gets warm and stays warm. Baseline is the temp selector all the way to cold, max AC and blower on 4, exterior temp at 85F.

The upper row selector for climate control no longer works: there's no difference between high, low and floor A/C, there's no difference in the vent settings and there's no defrost. Basically, it's AC on or off. The blower motor works in all positions, however, it's still blowing when set to 0. I've also noticed an odd new noise from under the hood, which I suspect is the compressor.

I had R-12 added by my mechanic about 4 months ago when I initially noticed the AC being weak. He noted that the blower motor would need replacement, although it still does a passable job. In the last two weeks I replaced the climate control check valve, checked the hoses for leaks and found none, the fan over the condenser functions normally and I shorted out the low pressure switch connector to check freon level, which appears to be OK. The last was suggested by the UK Volvo Club on their 740 Maintenance FAQ. I also noticed that the accumulator has a sticker that says "This product is compatible with R-12 and R-134a," so I'm assuming that's somewhat newish.

I think I have two issues: the A/C system itself and then the vacuum lines behind the climate control selector. I don't mind tearing into either, but I'm at a loss as to a starting point. Suggestions?

 

after you drive do you have an extremely large puddle under the car?

 

I've noticed a puddle, but nothing huge. Maybe 12" or so.

 

Sounds like you are a little low on freon and the condenser is not getting cooled enough. Make sure that your electric fan in front of the condenser is working. You might also want to jumper it so it runs with the AC on. Get a set of AC gages on it and see what it says.

 

The fan over the condenser appears to be working. It comes on with the A/C and goes off with the A/C.

As I said before, I jumped the low pressure switch to check the level and it seemed good. I'll see about gauges so I can get a better idea.

 

OK I guess I need a little clarification here. You say that you jumpered the low pressure switch?? If you have a sufficient charge in the system you do not have to jumper the switch to take a pressure reading. You need a set of AC gages and when you attach them start the car and run at 1500 rpm and look at the low and high pressure. The compressor should turn on by itself. The low pressure gage should be above 30PSI. If the system is low on freon it will cycle on and off. Look up auto AC systems on the web and it will give you correct pressures based on outside temperatures along with vent temps.

 

I should have clarified. I had read about jumping the low pressure switch on the UK Volvo board and it seemed like the quickest way to establish low freon. This was without gauges. Their FAQ stated:

"To determine whether your poor A/C is due to low Freon, start the engine and turn a/c and the fan to "on" . Open the hood and locate the receiver dryer (the big aluminum can up near the firewall on the passenger side in LHD cars. Push the spring
clip on the low pressure switch connector, mounted on the side of the dryer, and remove the connector. Using a paper clip, short the two connectors. If the compressor engages, your Freon is low."

It sounds like I may have been chasing something down a rabbit hole. I'll get gauges and see what is actually in my system.

 

My first thought is that your belt is slipping at high rpm's

 

Thanks for the tip, jpravi8tor. Made me think of another question: the compressor seems to be making some noise and I read something about filling the compressor with PAG or POE oil. Possible culprit as well?

 

Unless you have a leak the compressor oil should not be an issue. Look and feel around the compressor pulley/clutch area and see if you have any oil. That can be a sign of the front bearing being bad. By the way when you jumper out the low pressure switch the compressor will run even without any freon in the system. That can also blow the compressor because the compressor oil flows with the freon through the system.

 

Thanks, jagtoes. Checked around the pulley/clutch area while I was swapping out the belt, per jpravi's suggestion. No leaks, but I did find a lot of rust colored dust and I've noticed a pretty bad bearing related sound coming from the compressor while running-kind of a clunking.

1. Assuming it is the bearing, can the compressor be rebuilt?
2. If not, who is a good source?

Will be pricing it out at Rock Auto, IDP and others. Any hits or misses would be appreciated.

 

Usually not worth rebuilding . Just go to ebay or search for volvo AC compressors. You'll come up with a bunch of them so just pick the one you want. If you do have a blown compressor you should be out of freon. If this is the case you will need to flush the system with AC flush , replace the accumulator or receiver/drier and replace the orifice valve. I'd try to make sure that the compressor is bad but if it is just the clutch you can replace it.

 

second thought is clutch/compressor failing noise from the clutch/comp is bad
third is low freon
you really need a set of gauges to diagnose and a good understanding of a/c operation to effectively troubleshoot

The ac system is actually a simple system protected by a high pressure switch and a low pressure switch. Pressure too low? clutch wont engage
pressure to high clutch disengages.
The system is designed to operate in a balanced condition, The proper charge is actually measured by weight of the freon not by volume. The compressor does just that, compresses the low pressure gas into a high pressure gas wich is than sent to the condensor wich converts the high pressure gas(giving up heat) into a high pressure liquid. This liquid then is routed to the reciever dryer, which does just that( recieves the liquid and removes any accumulated moisture) and acts as a well or storage area for high pressure liquid to feed the demand of the evaporator/expansion valve.
Once the high pressure liquid is allowed to expand across the expansion valve into the evaporator (Bernoulli Principle) the high pressure gas expands turning into a low pressure gas giving up heat wich is used to convert the air from the blower motor into cool/dry air (moisture condenses on the evaporator due to a lowered dew point)
then returns to the compressor as a low pressure gas and starts the process all over again. So the addition of air or water (a compressable gas and a non compressable liquid) into the system disturbs the efficency by altering the operating pressures and temps of the conversion process. so hers the charging process. So guess what not enough freon poor cooling! too much freon poor cooling! Good luck!
Basic Charging Procedures


1. Make sure what is the required amount of gas. From factory, all systems have a decal under the hood that give the data. If the decal is missing or you are not sure, please specify make, model, and the type of refrigerant used and whether your system is a factory or an after market unit here. It is very important to know if your system has oil. Oil starvation is the main reason of compressor failure. Oil can be added to the system in two ways: with oil injectors or through the low side port under vacuum. The procedure to add oil through vacuum is described here. Some refrigerant charge and other useful specifications are provided here.
2. Connect both blue and red gauge hoses to the system's service ports. The discharge port (red hose) is located somewhere between the compressor and expansion device, either before the condenser or after it. In R134a systems, the port is the thicker of both, while in R12 systems is the thinner one. In some aftermarket systems, the port is located in the back of the compressor. If your ports are located in the compressor, the low side is marked by an "S" and the high side by an "H". Port caps have an "L" for low or "H" for high.
3. Open both gauge valves. Connect the common hose (yellow one) to the vacuum pump. Make sure both gauge needles are zeroed down. Needle is adjusted by turning a fine screw inside the dial. You must remove the plastic lens to do this.
4. Turn the pump on. You'll notice that both gauge readings begin to drop. The blue gauge's needle will even dip into negative values. The desired low side reading should be less than -25, while the high side will remain at 0.
5. After approximately 5 minutes, close both gauge valves. Turn the pump off, and observe the needles. Any movement will indicate vacuum loss. The faster the movement, the greater the loss. If after two minutes you don't see any needle movement, open both valves, turn the pump on, and continue vacuum for not less than 30 minutes. Close both valves then turn the pump off.
6. Get ready to charge. Have the necessary refrigerant amount handy. Cans make charging easier and more precise unless you have a charging cylinder or a precision scale.
7. Disconnect the yellow hose from the vacuum pump and connect it to the can tap or charging cylinder valve. You can charge in two ways: liquid (can upside down) or gas. Liquid charging is a lot faster but not recommended unless you extreme care or have experience. Traditional (gas) charging is slower but safer.
8. Once you've decided, and with the can or container connected, unscrew the top yellow hose connection (at the gauges) allowing refrigerant to escape for a couple of seconds. This will purge the air out of the hoses so you make sure that all you dispense is refrigerant. You can do this venting in liquid form so you can see when a fine, steady stream of refrigerant escapes indicating that all the air is gone from the hose. Don't breathe refrigerant and don't charge in a flammable environment.
9. "Flood" the system with liquid refrigerant (can upside down) by opening the red valve (high side) until it won't take anymore. Close the red valve. Jump the low pressure cycling switch (if so equipped). If you don't know what this is or where it is, ask for help here.
10. Turn the engine on. Turn the A/C into MAX on its third or higher blower speed. You'll note that both gauge readings are now positive. The red gauge should read between 100 and 150, while the blue gauge between 5 and 15. If you are charging with cans, don't forget to purge air out the hose after hooking a new can as described in step 8 above. Now, charge will continue through the low (blue) side (see note below) . If you haven't jumpered any switch (you don't have to), you'll notice the compressor cycling frequently on and off. That is normal. The cycling will disappear as you dispense more refrigerant. The only disadvantage of frequent compressor cycling is that charge will take longer. Needles will move up and down with every cycle. This is normal. Do not jump any switch unless you are absolutely certain!.

NEVER OPEN THE RED VALVE WHILE CHARGING. The red valve is to be opened only during pre-charge, vacuum, or when a system is evacuated. Its function is keeping an eye on the high side only. It must be closed at all times. If you have any doubts, wait until you receive information. Severe injury or death may occur. Remember: ALWAYS wear safety goggles. NEVER charge your system in a closed environment.

There are two ways to charge: gas (can or cylinder up), or liquid (can or cylinder upside down). A compressor is designed to compress gas. Direct liquid charge will harm the compressor if suction pressure is not controlled. Liquid charging is faster, yet is riskier. Liquid charging should be made by professionals or under supervision. Liquid charging can be done if the suction port is away from the compressor (some compressors have the suction valve in its body).
Liquid charging can be done never exceeding a 50 PSI suction pressure. If you can't control it, charge in gas form. In cooler weather, cans may freeze. You may immerse them in water and shake them while charging.
11. Once about 2/3 of the charge has been dispensed into the system, spray water in the condenser to optimize heat exchange and speed the process. When you get the condenser wet, vent temperature is likely to raise. This is normal. You'll also note pressure drop in both gauges.
12. Once the specified amount has been dispensed, close the blue valve. Let the system run for a minute. Turn the A/C off and then the engine. Wait another minute and disconnect the couplers from the service ports. Disconnect the low side first. If you jumpered a switch, reconnect it too.
Optimum cooling performance is attained after 10 minutes of operation. Our own acceptance criteria is at least 50°F in the center vent to the driver side at idle after 10 minutes or less. Remember: cooling increases while the vehicle is in motion.

Due to the physical properties and chemical composition, R134a and R12 charging amounts are different. Never, if you are retrofitting, charge the same or specified amount of R12 with R134a. If in doubt, please e-mail us here. Do not use this procedure if you are using any other refrigerant. This is just for R12 or R134a. Although procedures and parameters may be similar, we do not use nor recommend alternative refrigerants.

Every vehicle has its own operation parameters specification. Depending on whether or not the vehicle has a factory or an after market system, and whether it was retrofitted or is still original, among others. There is no established calculation to determine the exact operating pressures.
The low pressure (blue gauge) should be 35 or less at idle, regardless of the type of gas. Only if you're running a dual system, the low pressure may be between 45 and 50 at idle.
The high pressure, for starters, is directly related to ambient temperature: the higher the temperature, the higher the higher the pressure.
Bear in mind that dispensing a complete charge without lowering the pressures with water will result in higher readings. This is normal, and that is why you should spray water in the condenser at 2/3rds of the charge or once you have completed it. You'll note an immediate pressure drop when you spray water in the condenser.
To obtain a ballpark high side value, multiply ambient temperature by 2.2 if you have an R12 system. If you have a factory R134a system, use 2.3. To convert °C to °F, use this formula: °C X 1.8 + 32.
Remember: this is only a ballpark. If in doubt, we have factory charts to help you determine the correct pressures. Please have your vehicle's make, model, refrigerant type, and year and click here.
Factors like a an obstructed or very dirty radiator and condenser, weak or inoperative fan clutch, weak or inoperative radiator fan(s), either electric or mechanical, will make pressures go up and impair cooling, even in mild days.

 

And it died. I replaced the belt, was driving to the airport and the whole thing seized! Melted rubber, smoke, etc. It was quite a show. I'm assuming the whole unit needs to be replaced now?

 

Slipping clutch eventually led to complete seizing of compressor. This occurred while on a road trip, in addition to ECT going out and heater hose bursting. Not knowing what had happened, I thought I had grenaded my engine. Luckily everything works now. I got a new compressor and dryer and had the system converted to R134a for $565.

 

Compressor seized! Problem solved by replacing compressor and dryer and converting system to R134a. Cost $565, but worth it. A/C is almost too cold....

 

Glad to hear it. Though that's a lot of money, here in Dallas it would have been closer to $800 for the same service. By the way, having just now read the entire thread, there is NO high pressure switch in your 90 740...only the low pressure switch on the accumulator you found already. High pressure is controlled by two system blow off valves. One is on the compressor itself while the other is mounted on a small line muffler under the car at the passenger front corner near the condenser. You should be good to go for quite some time now!