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Short version: the truck had been losing coolness to the AC for about 2 years, slowly... to the point where it does nothing now. I assumed there must be a small leak and wanted to recharge first to give it a shot. Here's where I'm at...

PSI on the low side with engine off is around 100psi.

Turning on, AC on max, the compressor runs for around 2 seconds and cuts out.

When the compressor is on... the low side quickly drops from 100psi to around 10psi and cuts off. This takes about 2 seconds. I'm assuming it is so low on refrigerant that the low side goes too-low and the system shuts the compressor off.

Once the compressor kicks off.... the pressure slowly comes back up to 100psi again... where the compressor will cycle back on. The pressure quickly drops again, and cuts off.

I slowly put in 1/2 can (20oz) and the compressor seems to be running longer. It takes about 10 seconds now for it to drop to around 10-15psi before cutting off. The dash air is also starting to blow colder now.

Here's the actual question: should I be filling this (max of 3lbs as calculated by can size) until the compressor stays on, and hovers around 30-40lbs of pressure? (Assuming it's not on long enough, minutes, for the evaporator to get too cold and cycle the compressor for a different reason.)

Just wondering if I'm going about this the right way.

Thanks for any insight!
 

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Aimless Wanderer
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929 Posts
Short version: the truck had been losing coolness to the AC for about 2 years, slowly... to the point where it does nothing now. I assumed there must be a small leak and wanted to recharge first to give it a shot. Here's where I'm at...

PSI on the low side with engine off is around 100psi.

Turning on, AC on max, the compressor runs for around 2 seconds and cuts out.

When the compressor is on... the low side quickly drops from 100psi to around 10psi and cuts off. This takes about 2 seconds. I'm assuming it is so low on refrigerant that the low side goes too-low and the system shuts the compressor off.

Once the compressor kicks off.... the pressure slowly comes back up to 100psi again... where the compressor will cycle back on. The pressure quickly drops again, and cuts off.

I slowly put in 1/2 can (20oz) and the compressor seems to be running longer. It takes about 10 seconds now for it to drop to around 10-15psi before cutting off. The dash air is also starting to blow colder now.

Here's the actual question: should I be filling this (max of 3lbs as calculated by can size) until the compressor stays on, and hovers around 30-40lbs of pressure? (Assuming it's not on long enough, minutes, for the evaporator to get too cold and cycle the compressor for a different reason.)

Just wondering if I'm going about this the right way.

Thanks for any insight!
You are correct in the system is low and the low pressure cutout is interrupting the electic clutch current to shut the compressor off. Before you go much further get a small amount of refrigerant dye into the system to help you troubleshoot the system in the near future for the culprit leak. The being said, clamp your guages onto the suction and discharge ports of the system; while monitoring the low pressure side, (blue guage) administer further charge until the reading is about 30 degrees on the scale, (R-134 scale about 35 psi) increase idle speed to about 1500 rpm, drop the windows in the cab and turn the unit on high without using the recycling mode, (max) with a fairly strong fan blowing air into the condensor, allow the system to stabilize and then bring your charge temperature/pressure to 37 on the guage which will be about 35 degrees evaporator temperature. Using a non contact thermometer gun measure both the input, and outlet of the evaporator core pipes, (right at the mounting box) and get as close to 11 degrees as possible. This is superheat as mentioned in other posts. Basically you want whatever temperature is entering the evaporator core to be about 11 degrees cooler then the return. When you have the proper amount of superheat the refrigerant has completely become "flash gas" meaning the evaporator has completely evaporated liquid refrigerant from the high temperature liquid of compresssion, to a low temperature gas within the evaporator. The superheat of the gas is temperature absorbed from the passenger compartment. 11 degrees give a good balance of efficiency and comfort in HVAC. Charging an automotive A/C system with the windows up and on recirculate gives a false reading which usually shows up in high humidity coupled with hot climates as customer complaints. You want the evaporator to work at the highest efficiency possible and it needs fresh hot, humid air to do this from a service standpoint.

The compressor is cycled as the system cannot oil itself if low or devoid of refrigerant. This doesn't much matter to a compressor with an oil sump with the exception of the valves which are lubricated by atomized oil vapor. They usually don't last long without oil.
 
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