PatG

I have been working on the AC for my 79 here recently. I am still running R12 in this system. I had the system apart about 7 years ago when I did my engine install. At that time I installed a new dryer and replaced a bubba'ed orifice with the correct factory part. In addition I have recently sealed up a few air leaks around the evaporator box. I have also replaced the vacuum operated heater valve in the engine compartment and it is working. So I have done what I can to insure I have max airflow and no unwanted heat entering the system. All of the vacuum operated actuators in the ductwork appear to be functioning properly.


Despite this the system still has trouble cooling the interior. I borrowed a set of hoses and went to work. The compressor was cycling excessively so I suspected I had leaked a bit over the past 7 years. After hooking up the hoses I found just that. When the compressor would cycle on the suction pressure would come down to 10-15 psi in short order and the compressor would cycle off. Discharge pressure at the point was about 160-170 psi. So I charged the system with 1 can of R12. At the finish the compressor will stay on much longer with ~22 psi suction and ~250 psi discharge(is this high?). I really did not want to charge anymore as I was told that I should be running 35 suction and 175 discharge from an earlier post. Is this correct?

After charging I take the car out for a spin. It is kinda cool today, high 80's and overcast. While driving with AC on MAX and blower on high the duct outlet temperature will only get down to about 73F. I notice that if I idle at a stop sign the temp will drop a bit to 68 or 69 F. I tested a little more idling on the street and when I turn the blower down one notch I got the air temp out of the duct to get down to 58 F. This seems good but the air volume at this point is not that great and wont help much on a mid 90s sunny day. I also put my temperature probe on the orifice discharge line and get a reading of about 39 F when the compressor is running.

So with all of this the interior car temp was never much less that 80F. Is this all I can expect from the factory setup? Does anyone see something glaringly wrong with the compressor pressures? Why is the air temp not getting a better approach to the 39 F at the evaporator inlet when the fan is on high? I don't know, there are so many questions I could ask but basically I am looking for pointers on what might be wrong with the system and what I might try next.

Thanks for reading.

mrvette

Prop the throttle open to a 2000 tach rpm speed, then charge the system...

sounds like you still a bit lo....

GENE

azswede1960

I am not even to that point yet, but what way does the Orifice go down into the tube? Which end is down, the short or long end of orifice ?

Big Fish

Sounds to me like your system may be contaminated with air. A check for this would be to measure the temperature at the condenser coil after it has been sitting for a day or so with a very accurate thermometer. Hook up the high side gauge and read the pressure, then using a pressure/temperature chart for R-12 convert the gauge pressure to temperature (you could also use the temperature scale for R-12 on the high side gauge). If the corresponding temperature is the same as the temperature you actually measured at the coil the refrigerant is okay. Air in the system will cause the gauge pressure to be higher. If you find the refrigerant is contaminated you will need to remove the charge, evacuate to 500 microns, and re-charge the system with virgin R-12.

To have abnormally high (high side) operating pressure and low (low side) is very typical of air or incorrect refrigerant. The problem could also be a restricted orifice and an overcharge of refrigerant, but I would try the temperature/pressure check first. :cheers:

zwede

Also make sure that the air is not going through the heater core! If you air control door as adjusted wrong or leaking you may be heating the air back up.

And no, you should expect much more from the factory setup. My system uses all factory parts except it is converted to R134a. I get 42-45F vent temps on a 90F day.

Ben Taylor

When it's 80°F outside the high side pressure should be 160-210 psi and the low side should be 25-40 psi and the temp coming out of the dash should be 30° - 50°F.

How long does the compressor stay on? How long does the compresssor stay off? What are the pressures when the compressor cycles off? Does the low side ever go into vacuum (below 0 psi)? What is the pressure when the high and low side level out? How long after you turn the A/C off does it take for the pressures to level out?

Air in the system would most likely cause the low side to be higher than 22 psi, it would be more like 35-55 psi.

I suspect that there is moisture in the system, the system is overcharged or you have insufficient airflow through your condenser.

As far as your duct outlet temp not matching your evaporator temp, there may be some leaves blocking your evaporator, or your temperature door is not working correctly. You also need to make certain that the temperature door cable activates the vacuum switch for the heater core valve.

mark79,80

Would suggest adding a manual water shut off valve to the heater hose
above the factory valve. Purchase a 1/2" brass ball valve and 5/8"
hose barbs from any hardware/ plumbing store. Did this on several
corvettes and found that the vent air temperature is cooler with the
valve in the closed position even with a functioning factory heater control
valve.

PatG

First off I have a nice K type thermocouple hooked to a digital readout that I believe is pretty accurate.

Second, the pressures were a little different when I was not charging the system. It seems to settle out at about 200 psig discharge and 25-30 psig when the system is running. Then the suction pressure will slowly drop until the compressor cycles off at about 15 psig.

I measured the temperature and pressure
92.4 °F , 92 psig low side, 100 psig high side (some gauge mismatch)

TP charts shows the following for R12
90 °F 99.8 psig

I'm thinking that this is pretty close and does not indicate air in the system.

Idling the compressor will stay on for a long time. When I rev to 2000 RPM it is cycling about every 15 – 30 seconds.

The compressor will shut off when the suction pressure drops to about 15 psig. Low side climbs from 15 to 65. High side maybe drops 10 psi.

No.

90-100 psig, there is some gauge mismatch.

1 minute with everything off, 15 seconds if I leave the blower on.

Is there anyway to see the inlet side of the evaporator without out opening the evap box completely? I have looked at the outlet side of the evaporator and everything looks clean in there.

Vacuum is on the line to the valve and the heater hoses stay cool so I know that there is no water flow.

Big Fish

"Second, the pressures were a little different when I was not charging the system. It seems to settle out at about 200 psig discharge and 25-30 psig when the system is running. Then the suction pressure will slowly drop until the compressor cycles off at about 15 psig."

Based on this I believe you have moisture. 25 PSIG low side is below freezing, moisture will collect at the orifice and gradually close it off as ice forms until your pressure drops and the compressor shuts down on low pressure. This cycle would repeat each time it thaws and restarts. The repair would be to change the filter/drier, evacuate to 500 microns, and recharge with virgin R12.

PatG

As stated in the first post the system was open about 7 years ago and a new dryer was installed then. The system was thuroughly evacuated then and it held a full vacuum. It has been under pressure ever since. Don't see how any water could have made its way into the system.

Big Fish

Venturi effect through one of your connectors, most likely low side. How many microns did the system hold when evacuated before?

Ben Taylor

When the compressor cycles on and off the pressure of both the high and low side should only change by about 15 psi. The fact that it is changing by 50 psi on the low side only indicates a restriction on the low side, which 75% of the time is moisture freezing at the expansion device, but there could also be a restriction in the evaporator causing the same problem. This is also reinforced by the by the fact that your compressor cycles quickly at higher RPMs.

The easiest way to check your evaporator is to check the temperature of the inlet and outlet, the maximum difference should be 15°F.

The fact that your pressures level out so quickly indicates either a bad expansion device, or a compressor with an internal leak. If you use the car as a daily driver the compressor probably only has 3-5 years left.

Moisture can enter the system anywhere that refrigerant can escape. The most common place for leaks is the valves. The valve cores should be changed any time that you service the system.

PatG

Don't know what to tell you. I took it to a co-worker who had a commercial recovery and charging unit. We pulled what ever vacuum it would pull but the system did not have a torr gauge on it. It read in inches of HG not mm HG. I had other cars done by him and the systems worked fine.

As far as the connectors go there is nothing on the stock system as far as I know from the orifice outlet until the dryer unless there is trash in the system somewhere. I do not know the complete history of the system but the compressor does have a remanufactured sticker on it. I found no evidence of metal shavings in the system when I took it apart; not in the old orifice or dryer that I cut apart to inspect.

Big Fish

Pat, I don't believe the restriction is caused by debris, based on what you said it starts out okay, then pumps down after running a few minutes. If you are going to take the system apart for a drier change it might not hurt to flush the system anyway.
Not having a vacuum gauge is just taking a chance and by luck the system might be completely dehydrated. 1,000 microns (moisture pressure) looks the same on a service gauge as 500 microns (adequate dehydration) For what it's worth, most car mechanics don't have vacuum gauges.

PatG

Thanks for the replies folks, but you know what? There is another active thread on 134A pressures and system operation. Some of the info there is in direct contradiction with information posted in this thread. I am finding it difficult to discern the opinions from the facts.

http://forums.corvetteforum.com/zerothread?id=599453