I am having a hard time charging my r12 ac system after replacing the compressor, orifice tube and accumulator in my 79 vette. I flushed the evaporator and condensor, vacuumed the system and added 4 oz of oil to the new compressor and accumulator before trying to charge. I rotated the compressor 12-14 times to clear any oil. I connected my gauge lines at the port at the compressor and the port at the cut off switch above the accumulator. I let the system absorb as much r12 as it could before cycling the compressor. I had to jump the low pressure switch to get the compressor to engage. OK..now here's what is happening: The high and low sides show 50ibs of pressure at rest. When I bypass the low pressure switch the compressor engages for about 5-8 seconds and then cuts off. During that time, the high pressure rises to 75 lbs and the low pressure remains at 50. 10 seconds later the compressor goes on again and the cycle repeats itself. Note that I have bypassed the low pressure switch so something else is causing the compressor to cycle off. Additionally, the pressure on the low side does not change at all when the compressor engages.
Does anyone have any suggestions?? Thanks in advance.

 

I am no AC tech but Ive worked on quite a few.

I also don't know the 79 but maybe it has an integral control in the controls for managing the compressor? Seems odd that with the low cut out jumper-ed it will run for a short time then stop. Must be another control somewhere.

What happens if you hot wire the compressor to 12v with the engine running? With 50psi static in the system you aren't too low to run the compressor for a short time. I remember that most r12 systems at about 80-90 degrees ambient temp would have about 90 psi static on both the high an low with the system at rest.

I also don't know if 5-8 seconds is enough to get the suction side to start to show a differential. If you do hot wire the compressor only leave it like that for a short time if you don't see the pressure differential growing. I seem to remember on a 90 degree day you would have about 30-35 psi on the low side at idle with about 5 times on the high side. I just don't remember off hand.

How is the receiver dryer? Was the system left open to the elements for a long period of time? Maybe the silica is full of water and the dryer needs to be replaced? Was the system ever running for you or are you resurrecting something that was dead before you owned it?

 

you probably also need to jumper out the thermostatic switch, to keep the comp. running while you add additional freon. thats the only other thing i know that will cause the comp. to cycle. i jumpered around both switches when i charged mine (134A). keep the rpms up, and i also put a fan in front of the radiator / condensing core. it should show the exact freon amount needed (oz.) on the evaporator sticker.

http://www.corvetteforum.net/c3/tune.../index16.shtml

 

Speedreed8 found the other control I was thinking might be in there.

The odd thing is that its meant to keep the evaporator from freezing up and opens up bellow 33 degrees. Maybe its bad?

Does the compressor have any other electrical connectors on it other than for the clutch? I seem to remember some later compressors had a built in cut out switch.

 

i had to replace mine (thermostat switch) when i rebuilt the system, it was bad. the new compressor we installed for 134, has an internal switch connector on the outside of it, but i didnt connect it into the system. but the ac works good.

 

Thanks for the suggestions! I checked the voltage at the plug that plugs into the compressor and it goes from 12 v to 0 when the compressor cycles off. So it is not any kind of internal compressor switch. Does anyone know where the thermostatic switch is so I can bypass that?

 

Also, to answer the other questions: the receiver/dryer is new, but the first time I tried to hook up the compressor I used a bad vacuum and I'm pretty sure it didn't properly evacuated the system. Is it possible that by improperly vacuuming the system the first time that the accumulator has already gone bad? (note that I used a different new compressor after I improperly charged the system so i shouldn't have any compressor issues).I also put 4 oz of mineral oil directly in one the ports on the accumulator. Would it matter which side of the accumulator I put it in?

 

I did my '79 system in the spring so it is still fairly fresh in my mind.

Do not flush any of these components accumulator/drier, compressors, expansion valves/orifice tubes and any hose assembles the have either a muffler or filter attached to them. Flush the evaporator and condenser individually without all the other components attached. ALL the flush fluid must be pressure blasted out of the condenser and evaporator and dried. If you leave any fluid in the system it will ruin the accumulator/dryer the first time. If you have any flush fluid in the system the system won't charge and the compressor will cycle off. I know this from first hand experience. I ruined my accumulator/dryer and tried to charge the system over and over and it wouldn't charge. In order to get the flush fluid out I had to build up pressure using my shopair compressor in the evaporator, hold it and then release it rapidly out the other end. Same with the condenser. Just blowing air through it didn't work. My compressor was kicking off because the refrigerant was not cycling completely through the system. It was being blocked by the flush fluid.


The thermo switch if I remember correctly is at the evaporator pipe that has the thermocouple attached to the switch and is insulated by the black tar like stuff around the pipe.

Purge the system with a good vacuum pump capable of about 27" Hg for at least an hour. Wait 15 minutes to see if the vacuum holds. If it doesn't hold the vacuum it won't hold the charge.

Add the first can of R134a upside down with the A/C not on.

Put the rest in with the A/C running, fan on max and maybe a fan in front of the radiator to help "pull" it in.

If you live up North and there is a blizzard going on with the ambient temp below freezing you will have a hard time. Do the charge in a heated garage.

Here is a link to some help and to convert to R134a if you choose to do so in the future:

http://www.corvetteforum.net/c3/zwed.../ac/acmain.htm

 

You can have up to 30% mineral oil in system and be just fine with poe oil as well. I work with Copeland compresses company and test have shown its just fine. But back to you problem It sounds like you have non condensates in the system from either not vacuum properly or when you added refrigerant you had air in your hoses and its raising your head pressure when the compressor is on as 134a plus whatever non condesables are in the system

 

you can charge with can upside down as long as you charge on the high side cause the liquid with go through the metering device vaporizing the liquid as it flows into the evap.

 

It's possible you got a bad switch. The high pressure might be kicking it off if it's faulty. Jump it and see what happens but don't take your eyes off the gauges. Blowing a line is not pleasant ...