ac compressor keeps cycling on/off?

chengny said:

If a compressor is stopping/starting at a frequency that appears to be way faster than would be expected - it is said to be "short-cycling".

Short cycling (in automotive air conditioning applications) is almost always due to an undercharged system.

Without getting too involved, here is why a compressor short cycles – skip it if you want and go to the bottom:

The compressor clutch is energized/de-energized by a pressure control switch which is located in the low side piping. The switch is closed (and supplying power to the clutch) at higher pressure. When the pressure falls below a design set point the switch opens and shuts the compressor down.

Since the compressor pumps at a constant rate, the amount of refrigerant flowing out of the evaporator coil (and into the low side of the system) is the determining factor for pressure in the low side. The metering device controls the flow of refrigerant into the evap.

If there is not a sufficient charge in the system, there will be limited flow through the evap and into the low side. This limited flow will not be enough to keep the pressure up for very long. Soon the control switch will open and stop the compressor. The metering device keeps allowing what little liquid refrigerant it can into the evap and the pressure rises. But since the supply of liquid refrigerant is limited, the compressor quickly pumps the low side down and the control switch shuts it down again.

This happens repeatedly and quickly.

If everything else seems to be normal, try adding refrigerant in small doses and note whether the compressor stays on longer. If it does, continue to add refrigerant slowly. The system is full when the low side piping cold. If it doesn't seem to be getting any colder with more refrigerant being added - STOP. An over charged system is just as useless as an undercharged system.

I use this charge gun. It has a little gauge that gives a pretty good indication of the state of charge. Screw it on to a can of 134a, connect to the low side service port fitting and pull the trigger. Only $17 at your local Walmart:

You must be registered for see images attach

As far as your lack of vent control; do you have the HVAC vacuum supply line connected to the intake manifold or an unported nipple on the carburetor? If so, check the tube (especially on the engine side) for breaks/disconnects. Check where it runs over to the vacuum tank too.

Click to expand...

chengny said:

If a compressor is stopping/starting at a frequency that appears to be way faster than would be expected - it is said to be "short-cycling".

Short cycling (in automotive air conditioning applications) is almost always due to an undercharged system.

Without getting too involved, here is why a compressor short cycles – skip it if you want and go to the bottom:

The compressor clutch is energized/de-energized by a pressure control switch which is located in the low side piping. The switch is closed (and supplying power to the clutch) at higher pressure. When the pressure falls below a design set point the switch opens and shuts the compressor down.

Since the compressor pumps at a constant rate, the amount of refrigerant flowing out of the evaporator coil (and into the low side of the system) is the determining factor for pressure in the low side. The metering device controls the flow of refrigerant into the evap.

If there is not a sufficient charge in the system, there will be limited flow through the evap and into the low side. This limited flow will not be enough to keep the pressure up for very long. Soon the control switch will open and stop the compressor. The metering device keeps allowing what little liquid refrigerant it can into the evap and the pressure rises. But since the supply of liquid refrigerant is limited, the compressor quickly pumps the low side down and the control switch shuts it down again.

This happens repeatedly and quickly.

If everything else seems to be normal, try adding refrigerant in small doses and note whether the compressor stays on longer. If it does, continue to add refrigerant slowly. The system is full when the low side piping cold. If it doesn't seem to be getting any colder with more refrigerant being added - STOP. An over charged system is just as useless as an undercharged system.

I use this charge gun. It has a little gauge that gives a pretty good indication of the state of charge. Screw it on to a can of 134a, connect to the low side service port fitting and pull the trigger. Only $17 at your local Walmart:

You must be registered for see images attach

As far as your lack of vent control; do you have the HVAC vacuum supply line connected to the intake manifold or an unported nipple on the carburetor? If so, check the tube (especially on the engine side) for breaks/disconnects. Check where it runs over to the vacuum tank too.

Click to expand...

squaredeal91 said:

I'm not an a/c guy but I've filled a few systems with cans. Put a meat thermometer in your vent to watch it. I learned that if I slightly overfilled it the air got warmer and it cycled more frequently. I blead off some extra a little at a time till I got it the coldest. Had to add and dump a couple times to get it right.

Click to expand...

Doppleganger said:

Do these trucks have orifice tubes/valves in them? 134a should have them. They are usually inline and about the size of a cigarette. If you have one, check it first - they are a very fine mesh screen and if anyone in the history of that truck ever used a leak sealer (or refrigerant with leak sealer) its guaranteed plugged.

Click to expand...