ac compressor keeps cycling on/off?

[guitarfreak235]

Hello fellow squarebody enthusiasts, long time no see!

Im having an issue with my ac, more specifically the compressor. When I switch the ac on the compressor will switch on then right back off. After a few seconds it will do the same over and over again until I just turn it off. Also, I don't seem to have functioning flaps for vent selection. The air just blows out of all vents.

Sound familiar to anyone? Im thinking it might be an issue with the vacuum for HVAC. Does the compressor show these symptoms with insufficient or entirely lacking vaccum? Perhaps an electrical issue? It worked before my 350 swap so im thinking I messed something up...

 

[chengny]

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.

 

[guitarfreak235]

Yup that was the problem. wow I was checking for electrical/vacuum problems when it was literally the easiest fix possible. thanks!

 

[moorevisual]

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.

I'm dealing with this issue now, but it only happens once I get to about 2000-2500 rpms, and no matter what I charge it up to (I've tried 35, 40, 45, 55 psi), it continues to to cycle on and off every 5 seconds or so. When I have the guage on it, I can watch it drop as the RPMs increase and as soon as it drops below 25 psi (into the white on the gauge) the cycling starts.

You must be registered for see images attach

BTW - when I got the truck the compressor was dead, so I've put on a new compressor, dryer, evaporator, orifice tube, flushed and vacuumed the system, and charged with 134.

Someone mentioned to me that maybe the pressure switch should be changed to one with a lower cut off, since 134 runs at a lower pressure that r12?

Any advice?

Thanks

 

[moorevisual]

i resolved my issue...found out there's an adjustment screw on the low pressure switch. all i had to do was turn a couple times counter clockwise and it lowered the cut off point. problem solved.

 

[TheRealDylan]

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.

I am having the same issue and I have put an entire can of r134a into the system and nothing changed. Any tips?

 

[squaredeal91]

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.

 

[Finkaire]

A proper charge is critical in a 134a system, the only way to charge the system properly is to start from scratch. Only then can the system be diagnosed.

 

[TheRealDylan]

Ok thanks, I will be just starting from scratch and vacuum the system.

 

[TheRealDylan]

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.

Could I have over filled the system and caused the AC clutch to go ON and then Off repeatedly?

 

[bucket]

If there is a clog in the system it can have the same cycling issues. If it's still not a full charge and it's clogged, it could still be tripping the low pressure switch. If it's a full charge and clogged, it could be building enough pressure to trip the high pressure cutoff switch.

If it's just an issue at low speeds, it could be lack of airflow through the condenser, possibly from a failed fan clutch.

You really need to get a set of gauges on it to figure out what may be going on, otherwise it's just guessing.

 

[bucket]

Or, if you vacuum the system and refill with the proper amount and the AC works, then yay! Lol. But if you have a clog in the system, you'll have to evacuate the system again and fix it.

 

[TheRealDylan]

Okay, thanks! I will try this.

 

[Doppleganger]

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.

 

[bucket]

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.

Yes, they do have an orifice tube. You are 100% correct about the leak sealer.