Working on the wife's '91 740 Station Wagon, non-turbo, with updated 134a AC system.

Just replaced the leaking AC condensor, and was trying to put the 134a refrigerent in it, and it wouldn't take. I think the compressor, or perhaps the clutch on the compressor has gone bad. I think this because when the AC is running, the clutch kicks on and off, on and off, on and off, and never runs constantly when the AC is full blast. The pressure gauge on the can of 134a refrigerent drops in pressure every time the clutch engages.

Am I correct to assume the clutch on the compressor is shot? Can the clutch or clutch bearing be replaced without having to buy a $200+ compressor?

Thanks in advance. You all rock!

Marty

 

It is most likely cycling like that because it is still low on freon. When there is more freon in there it should run longer.

 

any particular reason why the system doesn't want to take any more refrigerent? I'm using those little cans you get from advanced auto....do they suck?

 

I do not like using them at all. I always have access to a machine to fill the car.
You might be able to jump the compressor for a few seconds to make it suck the freon out of the can.

 

what pressure does the gauge read when the engine is off and has sat for a bit?

 

55 psi

 

First question,
Did you evacuate the system utilizing a vacumn pump to remove moisture and air from the system?
next question,
With the system holding a vacumn did you introduce the freon?
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.

 

In the good old days (r12) The freon/temperature relationship was linear
If it was 100 degrees F the pressure in the system at rest would be close to 100 psi

with r134a the pressure/temp relationship is non linear so you must go here http://www.csgnetwork.com/r134apresstempconv.html

 

Also see Bubbas Post, here is a link he suggests
http://www.brickboard.com/FAQ/700-90...temMaintenance

 

your right about the non linearity, but it is still pretty close.

55psi is low. you need more refrigerant

 

What an informative post! Thanks!
I have a strange situation that maybe someone can make heads or tails of:
1991 940 Turbo, dealer converted system, "the new stuff R134a ester oil only".
Last summer I bought the cheapo filler hose and a cheapo gauge a few cans of refrigerant and a can of oil, filled the system 1 can of refrigerant 1 can of oil 1 can of refrigerant. the gauge read 35psi, "on the cheapo gauge it read over filled. I put a thermometer in the duct and it got down to 41 degrees. Worked fine all summer.
This summer, I checked and it needed some more refrigerant. So I added 1 more can got the system to 35ish psi and it was blowing cold at 41 degree's. everything was fine for a month, then as I was driving along at 60MPH the car just died, the dash lights came on and I coasted to the side of the road. cranked it for a few minutes and it started right up, but no ac.
So when I got home I thought maybe I had put to much refrigerant in , so I release enough till the cheapo gauge read "filled" around 22psi. Still no ac. I replace the low side sensor and "jumped" the high side so the fan would come on, no ac. I decided to put voltage to the compressor to see if it was a bad compressor, after running it straight of the battery the compressor was running and the clutch was engaged. No ac.
So I decided to check the pressure,

(here's the strange part) When the compressor is off, I am reading 25psi, when the compressor is hot wired it's reading 0psi, like it's blocked. I went as far as replacing the temp control unit in the dash, as this is the only ac relay I could find for my car, no luck, no ac.
I just walked in the house and washed up and logged on to research this and post this weird situation. Whats weird is when I filled the system it was reading pressure with the ac on. I charged the system with the car running and ac on and seeing the clutch spinning on the compressor.
Does anyone have any ideas? I'm stumped
PS, the car has since started dieing for no reason as I am driving at highway speeds. This only started when the ac died the first time. You sit on side of road for a few minutes and crank a bit and it starts just fine. runs like a champ no misses no hesitation turbo kicks in and your pushed to the back of your seat. Could these 2 items be ECU problem? I don't know I am at a loss......

 

OK, so I read on another forum that to low of freon and the compressor will not kick on. so I jumped all the sensors low and high, jumped the compressor and put a pound of freon in. Well, strangely it started cooling, got down to 62 degrees, but after I reconnected everything and turned the car back on, you guessed it, the compressor isn't turning, so no cool air. Once again I am suspect of 3 things. Either there is a relay somewhere or the temp switch in the dash or the control unit? I tried a number of con figs with the low side sensor jumped, the high side jumped but not the low, still no voltage to the compressor. Any help would be a great help.

 

The amount of 134A used to replace R12 is 85% of the amount required for R12 systems. I believe 45 pounds of pressure is the max for the system according to the product (Interdynamics) that I'm using or about 33 oz. for my 240DL. I'm not sure that that would be different dependant on the gauges you use, correct me if I'm wrong here.

 

Don't know if this will help at all....but I ended up taking the wife's volvo to a mechanic who had the equipment to vacuum the system, who then claimed that I had bad solder joints on the climate control unit, charged me $138 to fix the solder joints (on top of the $100 to vacuum and fill the system with freon) and now all is well.

If you replaced your temp control unit then this might be a moot point, but figured I would throw it out there.

Good luck.