Another A/C Mystery Diagnosis
08-06-2011, 10:02 PM
#1
Burning Brakes
Thread Starter
Another A/C Mystery Diagnosis
The A/C in my 993 is not very cold, and there is a tendency for the fan speed to increase and then decrease without my touching any of the controls. Searched the forum and came up with some excellent diagnostic write-ups. So far, I have not solved my problem, so I need a little bit of advice.
The A/C system cools the air down only about 15 degrees F. I'm measuring temp using a thermometer stuck in the center vent. Measured the suction and discharge pressures of the compressor, and had to expell a little bit of refrigerant since the system was running about 3-4 PSI above the specs. I also replaced the ballast resistor on the condenser and cleaned out some debris that was partially blocking about 10-15% of the condenser.
None of these steps has made a noticeable difference in the temperature of the air, though it appears that the cycling of the fan may have been solved (not sure why).
So, here is where I am at right now: With the motor revved up to 2000 rpm, my compressor suction/discharge pressures are 16 PSI and 250 PSI (exactly in the normal range according to the writeups). So, it appears that the compressor is working. I am also able to feel a flow of air blowing down over the condenser, so the low speed fan is apparently working fine. With the outside temp running at 85 degrees F, my center vent is measuring about 68 degrees F.
According to the write-ups, with the refrigerant circuit showing proper pressures, the only other area that I should check is the motor servo valves that allow heated air from the engine into the A/C system. So, I may be artificially heating up my cooled air. Is that the only thing that could be going on with the system?
The write-ups talk about removing the kick panels to gain access to these valves and then determining that both valves are shut off. I need a little more detail in how to gain access to the valves, and what to look for when I finally get in there. Is there an obvious way to determine if the valves are failed or leaking? How can I test the valves without removing them from the car? Is this a common mode of failure? And, how expensive are these valves? Or, can mine be repaired if broken? I've gone this far with the information contained in this forum.....I'd like not to have to give up and be forced to take the car to a professional.
Given the above, are there other things that I should check out?
The A/C system cools the air down only about 15 degrees F. I'm measuring temp using a thermometer stuck in the center vent. Measured the suction and discharge pressures of the compressor, and had to expell a little bit of refrigerant since the system was running about 3-4 PSI above the specs. I also replaced the ballast resistor on the condenser and cleaned out some debris that was partially blocking about 10-15% of the condenser.
None of these steps has made a noticeable difference in the temperature of the air, though it appears that the cycling of the fan may have been solved (not sure why).
So, here is where I am at right now: With the motor revved up to 2000 rpm, my compressor suction/discharge pressures are 16 PSI and 250 PSI (exactly in the normal range according to the writeups). So, it appears that the compressor is working. I am also able to feel a flow of air blowing down over the condenser, so the low speed fan is apparently working fine. With the outside temp running at 85 degrees F, my center vent is measuring about 68 degrees F.
According to the write-ups, with the refrigerant circuit showing proper pressures, the only other area that I should check is the motor servo valves that allow heated air from the engine into the A/C system. So, I may be artificially heating up my cooled air. Is that the only thing that could be going on with the system?
The write-ups talk about removing the kick panels to gain access to these valves and then determining that both valves are shut off. I need a little more detail in how to gain access to the valves, and what to look for when I finally get in there. Is there an obvious way to determine if the valves are failed or leaking? How can I test the valves without removing them from the car? Is this a common mode of failure? And, how expensive are these valves? Or, can mine be repaired if broken? I've gone this far with the information contained in this forum.....I'd like not to have to give up and be forced to take the car to a professional.
Given the above, are there other things that I should check out?
08-07-2011, 05:49 AM
#2
Rennlist Member
The air vent temp shuld be measured with A/C High button pressed, or manually settings: Resirc on, defrost vents off, footwell vents off, fan set to max and temp to minimum.
Interior fan speed will be intermittently high and low if you are having problems with the aux fan in the engine. This is a CCU fail safe feature.
Valve servo failures are fairly common. The two mixing servos should work simultaneously, open and close when setting temp **** to max and minimum. You may have problems spotting the movement without disconnecting the air valve plastic housing from the hoses. However, this is easy to do. The kick panels are fixed with some screws, you'll see them if you have a look down there.
Servos most often can't be fixed, new ones cost about 150 euros. See here for more details:
www.ccu.tore.bergvill.com
Cheers,
Tore
Interior fan speed will be intermittently high and low if you are having problems with the aux fan in the engine. This is a CCU fail safe feature.
Valve servo failures are fairly common. The two mixing servos should work simultaneously, open and close when setting temp **** to max and minimum. You may have problems spotting the movement without disconnecting the air valve plastic housing from the hoses. However, this is easy to do. The kick panels are fixed with some screws, you'll see them if you have a look down there.
Servos most often can't be fixed, new ones cost about 150 euros. See here for more details:
www.ccu.tore.bergvill.com
Cheers,
Tore
