A/C cycling on/off rapidly

[ia-k]

Getting my '84 Chrysler Lebaron "woody" convertible ready for spring/summer. 

 

Car was retrofitted to R134A about 8 years ago according to the sticker.

 

Turn on the A/C to test and works fine, cold air blowing out after a few minutes. Leave it running to charge the battery back up and open the hood to do some under the hood checks. Notice the A/C pump cycles on/off rather quickly, 5 seconds on, maybe 35-40 seconds off. Hooked up my hose/gauge that came with a R134A fill kit that I used for another vehicle and it shows in the middle of "green/full" while the pump is off. Pump kicks on and it drops down to the middle of the "white/add" section for the 5 seconds while the pump is on then back in to the green/full section when is shuts off. 

 

I always understood if the pump cycles on/off quickly it is low on refrigerant but my gauge tells me (part of the time anyways) it is at the correct level. Considering the air is getting cold is it just supposed to cycle this fast? 

 

I used to do some minor industrial/commercial refrigeration 30+ years ago and still have the manifold gauge set but they are for R12 and obsolete.

[padgett]

When standing still I have seen some cars cycle rapidly because high side is getting, well, too high. Try putting a reasonably big fan in front of the radiator. Do you have an electrical fan or a mechanical fan with clutch. Bad clutch can also cause an over pressure.

[1930 Kram66]

Without knowing pressures on both hi and low sides of the system and vent temps its very hard to accurately say what your problem is.

As padgett has just suggested, check if the condenser fan[ if fitted]or clutch fan is operating correctly. Too hi a pressure at the condenser will cause a pressure cut out switch [usually on the filter drier ] to cycle the compressor quickly to hopefully protect it from damage. Here in Australia when cars were being converted from R12 to R 134a a Hi/low pressure cut out switch was required to be fitted to the system as part of the retrofit. 

[meteor]

I say you are low on refrigerant. Short cycling is common when low. Gauge should be in the green when compressor is running.

[ia-k]

31 minutes ago, 1930 Kram66 said:

 and vent temps 

 

45 degrees at the vent with a 65 degrees outdoor air temp.

 

The radiator fan (electric) turns on and off as it should when the A/C pump is on and then after the car is warmed up if that makes a difference. 

[joe_padavano]

1 hour ago, ia-k said:

GHooked up my hose/gauge that came with a R134A fill kit that I used for another vehicle and it shows in the middle of "green/full" while the pump is off. Pump kicks on and it drops down to the middle of the "white/add" section for the 5 seconds while the pump is on then back in to the green/full section when is shuts off.

 

You're low on freon. That is EXACTLY how a clutch cycling orifice tube system (CCOT) behaves when it's low on freon. 

 

Edited March 29, 2021 by joe_padavano (see edit history)

[ia-k]

Where I am getting confused is the directions for use that came with the gauge repeatedly states "do not overfill" and "if the needle is in the green section do not add refrigerant". Nothing about if the needle moves from good to low back to good. I want to avoid adding refrigerant until it is good while the pump is running but then be overcharged when it shuts off. I am 99% sure this shouldn't happened but wanting to ask before I try it.

 

[1930 Kram66]

 

So I would advise you to to have the system completely evacuated of refrigerant then attached to a vacuum pump for at least 30mins [longer is better] to boil off any moisture in the system then add the correct amount of gas. You probably will need to see a A/C tech for this.

[padgett]

What is the gauge doing when running ? If pulls down real low (20-30psi )then pop's up when it cycles it is low. At 65F ambient a good system will be under 40F at MAX AC and not cycling. Actual pressure is temperature dependent - why you see a range.

 

Agree it needs some more. Life was a lot easier when we just had a sight glass and when the bubbles stopped it was full.

[joe_padavano]

14 hours ago, ia-k said:

Where I am getting confused is the directions for use that came with the gauge repeatedly states "do not overfill" and "if the needle is in the green section do not add refrigerant". Nothing about if the needle moves from good to low back to good. I want to avoid adding refrigerant until it is good while the pump is running but then be overcharged when it shuts off. I am 99% sure this shouldn't happened but wanting to ask before I try it.

 

 

The orifice tube causes a pressure spike on the inlet side, due to the restriction. Filling the low side increases pressure there, but the freon can't move past the compressor or orifice tube quickly. Once you operate the compressor, the freon distributes in the system, causing the pressure to go back down on the low side. Some people jumper across the pressure switch temporarily to keep the compressor running the whole time you are filling the system, which eliminates this phenomena. Personally I don't bother.  It's low on freon.

[Digger914]

"still have the manifold gauge set but they are for R12Z" 

Good tools never die.

Hose fittings need to update if removable fitting adapters weren't attached over top of factory connections when updated, but the gauge set itself is not obsolete and new 134 hoses are cheap. 

Basically you are a couple of fitting connectors away from being able to monitor high side while you add freon to the low.

[37_Roadmaster_C]

As said above. Buy a set of R134a adapters and you are back in business. The only other thing you will need is a pressure chart for T134a. Pressure is pressure. Just ignore the fancy color coded gauge sections and read the pressure in psi then use the chart.  Also, Joe is 100% correct. The pressure will vary as the system works. Many things effect system pressures and clutch cycling. However, if it is cycling like you describe, you are low on freon! I would wait for a warmer day, say 80+ and then add freon slowly until it keeps running and the gauge is in the green. Another thing you should do is either get a leak detector and sniff for the leak or add some of the leak detector dye and run it for a day or two and then look with a black light for the leak. Remember, an AC system is a SEALED system and does NOT leak if proper, period!!! If it is low, it has a leak and should be repaired. I will be honest and admit that I have topped off several of my systems when they had small leaks. However, those small leaks ALWAYS became bigger leaks and cost more in wasted freon and hot summer days before having time to repair. It is much nicer to repair is the spring when it is cooler than on a 100+ summer day!!!  Good Luck!!