How often should an AC compressor cycle?

[badpeanut]

I'm working on a '77 with AC that was coverted to R134a two years ago by a professional. It was so low on refrigerant this spring the compressor wouldn't kick in and cool. After I added two cans of R134a it blows acceptably cold air and reads in the recommended pressure range while it is running, but the compressor cycles every minute or so.

Am I correct to assume it is still low?

How often should the compressor cycle if it is properly charged?

Is cycling the compressor like this bad?

[AceX]

I have the same deal in my '85. I would talk with the shop that did the conversion if at all possible...

[chevymotocross]

Sounds like it's cycling too much. Also, as you probably know, the A/C system is a self contained system..... nothing should have to be added, unless it leaked out. For you to add two cans of refrigerant since it was last charged, there is obviously a good leak. I would have dye ran through the system to check where the leak is, have it recovered, vacuumed, and recharged with proper amount (I think it's 3 lbs). With a fresh charge and leak repaired, the excessive cycling should stop when AC is on. Speaking of cycling, it should only be cycling on Defrost, or, when the a/c is on it might cycle some because temperature inside the cab is nice and cold, the compressor must now cycle to prevent the evaporator from turning into a block of ice, restricting airflow. You have a charge problem for sure.

[badpeanut]

I double checked pressures yesterday, and it was low (I was mistaken earlier). I added a third can of R134a to get into the recommened pressure range; now it no longer cycles and it cools very nicely.

Our long cold winters are tough on AC systems (they cause a lot of the older systems to leak).

[75sierraclassic]

what are the recommended pressure ranges?

[badpeanut]

Originally Posted by 75sierraclassic

what are the recommended pressure ranges?

25-45psi on the low pressure port. I'm on the low end of this now.

[ElGracho]

Originally Posted by badpeanut

25-45psi on the low pressure port. I'm on the low end of this now.

That's good. Ideally you want the evaporator pressure to be as low as possible without the compressor cycling excessively. Lower pressure means lower temperature and then colder A/C. Adding any more refrigerant will raise the pressure (and temperature) of the whole system.

The only trouble you might run in to is the fact that your system was pretty much empty and you never drew a vacuum on it to get rid of air, non-condensable gases and moisture. They add pressure to the system and again cause it to get run at a higher temperature. Since you didn't fix the leak, you'll be doing this again sometime too...

[badpeanut]

Originally Posted by ElGracho

The only trouble you might run in to is the fact that your system was pretty much empty and you never drew a vacuum on it to get rid of air, non-condensable gases and moisture. They add pressure to the system and again cause it to get run at a higher temperature. Since you didn't fix the leak, you'll be doing this again sometime too...

The previous owner had it in to the dealer every year for at least five years prior to me buying it when it was an R12 system and they couldn't fix the leak that occurs over the winter. I believe an R12 system converted to R134a (using the R12 hoses) like mine is, are even more prone to leak over the winter. I'm sure it will leak down again this coming winter and I'm not sure how to stop it.

Two years ago when it was converted to R134a they pulled a vacuum. I was feeling good since I haven't opened the system and I was thinking the R134a molecules are smaller than air or water molecules (hence making it more difficult for the air or water molecules to enter the system), but what you are saying makes sense that my system likely has some contamination.