Compressor cycling frequently

[71RALLYEDART]

I have a 71 Dart. I did a R134 a/c conversion on an original a/c car. The a/c compressor cycles about every 30 seconds. I had it checked by a few shops they drained the refrigerant checked for leaks and refilled it and I have the same issue. What should I be checking next? I also jumped the dryer switch and the issue continued. Any help would be greatly appreciated I’m stumped

 

[Jeremiah (jd)]

Compressor is turned on or off by a pressure switch. You need to figure out why the pressure is spiking every 30 seconds or you have a 12v supply issue (switch, etc).

 

[slickchick]

there is also a switch on the evaporator that cycles when the evap freezes

 

[George Jets]

Overfilled freon system??? Not room for the liquid freon to turn into a gas, so it goes over pressure and it kicks out.

Look up your system's R-12 freon capacity.

Evacuate the whole system clean and empty, this allows the vacuum pressure to boil out any remaining moisture in the system and then evacuates it out.

Then reinstall the proper amount of 134 Freon (from the R-12 specs) without over filling it.

Put a meat thermometer in the AC duct work as you are filling the system back up to keep and eye on the temperature dropping, as the engine is running and AC on Full Cool. Filling the system from the low pressure side only. When you get it down to 60° (f) you are getting close and your system should be fully working.

Keep adding 134 freon until the duct temp gets to 55 - 50° then the system will be full. Should be close to the same amount of freon in the specs.

Happy Motoring

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[Northern Grit]

I have a 71 Dart. I did a R134 a/c conversion on an original a/c car. The a/c compressor cycles about every 30 seconds. I had it checked by a few shops they drained the refrigerant checked for leaks and refilled it and I have the same issue. What should I be checking next? I also jumped the dryer switch and the issue continued. Any help would be greatly appreciated I’m stumped

What kind of duty cycle are you expecting to see? 30 seconds doesn't really sound abnormal to me. Most of the R134a systems will engage the clutch for 5-15 seconds (depending on heat load, humidity ect) and then disengage for about 15-40 seconds until the pressure in the accumulator drops back down then the cycle starts over again. Is it blowing cold?

 

[67Dart273]

Overfilled freon system??? Not room for the liquid freon to turn into a gas, so it goes over pressure and it kicks out.

Look up your system's R-12 freon capacity.

Evacuate the whole system clean and empty, this allows the vacuum pressure to boil out any remaining moisture in the system and then evacuates it out.

Then reinstall the proper amount of 134 Freon (from the R-12 specs) without over filling it.

Put a meat thermometer in the AC duct work as you are filling the system back up to keep and eye on the temperature dropping, as the engine is running and AC on Full Cool. Filling the system from the low pressure side only. When you get it down to 60° (f) you are getting close and your system should be fully working.

Keep adding 134 freon until the duct temp gets to 55 - 50° then the system will be full. Should be close to the same amount of freon in the specs.

Happy Motoring

☆☆☆☆☆

These systems usually DO NOT HAVE a high pressure cutout

 

[67Dart273]

What kind of duty cycle are you expecting to see? 30 seconds doesn't really sound abnormal to me. Most of the R134a systems will engage the clutch for 5-15 seconds (depending on heat load, humidity ect) and then disengage for about 15-40 seconds until the pressure in the accumulator drops back down then the cycle starts over again. Is it blowing cold?

No, not loaded on a hot day. NO

 

[67Dart273]

If it is cycling on the low pressure switch, and you can tell with a set of gauges, or by just bypassing the switch, either the system is not loaded (cold day) or the refrigerant is low, or there is a restriction in the system such as a bad TXV, moisture plugging the metering device, or some other "plug" / restriction.

 

[Demonic]

These systems usually DO NOT HAVE a high pressure cutout

That used to be true. Many of the aftermarket systems have a "double throw" switch - it has three positions, opens on a low set and a high setting. The OP needs to find what exactly is interrupting power and what the system pressures are. Proper charge is about 80% of R12, or if you're really smart, the goal is a constant 38* evaporator.

 

[71RALLYEDART]

Ok thanks the 1 thing I forgot to add is that when I first start driving it will just about coolenough but after about a half an hour it’s not very cool at all

 

[George Jets]

Ok thanks the 1 thing I forgot to add is that when I first start driving it will just about coolenough but after about a half an hour it’s not very cool at all

OK
Then you low one 12oz. can of 134 freon.

You have just enough freon in it right now to cycle the compressor clutch. When it kicks in and starts pumping the freon, it sucks what is left in the freon well dry and it throws out the low freon switch and disengages the compressor clutch.

Then it sits idle for a bit, the freon equalizes out and goes back into the low side reserve. Then the compressor turns back on again and pumps it dry and shuts down the compressor again. Repeat, Repeat, Repeat...

Put another can of 134 in it, an watch the Meat Thermometer in the cooling vent come into the 50° range. Then stop adding freon.

Lots of early AC systems have a site glass on the Reciever Dryer. If you see bubbles in there, that tells you your system is low on freon.

Add a Can

Happy Motoring

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[RustyRatRod]

What are the operating pressures and ambient temperature when you check them?

 

[bcschief]

Rule of thumb on R12 to R134 Conversions is 80% of the R12 charge.

 

[slantsixdan]

Look up your system's R-12 freon capacity. Then reinstall the proper amount of 134 Freon (from the R-12 specs)

No, you need to multiply the R12 capacity spec by 0.75 to get the correct amount of R134a. Nonstandard components can throw off this calculation—a new filter-dryer that's a different size than the original, for example, or a parallel-flow condenser instead of the original serpentine type so the system has a prayer of doing a good job for you.

Put a meat thermometer in the AC duct Keep adding 134 freon until the duct temp gets to 55 - 50° then the system will be full. Should be close to the same amount of freon in the specs.

Lookit-a-thermometer is fine for a quick, general evaluation of A/C system performance, but 50° isn't some universal magic number; the temperature you want to see varies depending on ambient temperature and other variables.

You charge an A/C system by weight, not by guessing based on what a thermometer says. Also, there are no meat thermometers that read anywhere near low enough for this job. Second preference is to charge the system by reference to the manifold gauge readings—but you have to know how to intepret them, and these readings also vary with ambient temperature and other prevailing conditions, so you also have to know what you're doing.

A/C work by guess and by gosh is a pretty dependable way of breaking stuff and wasting money and endangering yourself and ending up still sweating. It's better to do it right.

 

[George Jets]

No, you need to multiply the R12 capacity spec by 0.75 to get the correct amount of R134a. Nonstandard components can throw off this calculation—a new filter-dryer that's a different size than the original, for example, or a parallel-flow condenser instead of the original serpentine type so the system has a prayer of doing a good job for you.



Lookit-a-thermometer is fine for a quick, general evaluation of A/C system performance, but 50° isn't some universal magic number; the temperature you want to see varies depending on ambient temperature and other variables.

You charge an A/C system by weight, not by guessing based on what a thermometer says. Also, there are no meat thermometers that read anywhere near low enough for this job.

A/C work by guess and by gosh is a pretty dependable way of breaking stuff and wasting money and ending up still sweating. It's better to do it right.

Read the Peterbilt Truck manual, that explains AC systems as good as any info out there.

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[George Jets]

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