Someone please tell me I am wrong. A/C Compressor Issue

[chengny]

Pull the system down as deep as your vacuum pump can get it. If its a good pump - like a JB Platinum - you should be able to pull at least 28" of vacuum. Let it run continuously for at least 1/2 hour. When the vacuum doesn't seem to be getting any deeper, valve the system off from the pump. Kill the pump and observe your gauges.

The vacuum should remain in that 28" range for a long time. When building a new AC system, I pull it down as low as it can go. Then - provided it doesn't immediately start to lose vacuum - I just walk away. After letting it sit for at least 12 hours, check system vacuum. There should be no noticeable rise in pressure.

Don't stress if - at first - the vacuum creeps up. Since you have already charged the system, there will be some refrigerant still entrained in the oil. When system pressure is brought down into a deep vacuum, that refrigerant will slowly boil out. But, I just read that your intention was to flush the system before re-evacuating, so that should not be an issue.

Also, keep in mind that a rise in ambient temperature will cause a slight change in vacuum. So, if you evacuate on a cold night and then check the residual vacuum at 3 the next afternoon (when it is sunny and 70 F outside), don't be surprised if you only show 27" Hg.

If you can pump down to 28" and hold that vacuum overnight (or even 6 hours), consider the system tight. Add your refrigerant, button up and return the vacuum pump to the rental store. There is no need to conduct a pressure test.


BTW - you are overcharging your system.

I was putting my 134 back in and after the expected 5 cans

The design capacity of these trucks (when using R-12) is 56 ounces. When retrofitting to 134a - in a system that originally used R-12, the amount of the charge is reduced by 10%.

56 X .9 = 50

There are 12 ounces in a can of 134a.

50 / 12 = 4.167

Start off with 4 cans of SUVA (DuPont's proprietary name for 134a). If you think you are short on charge, just add a little bit more. Drive around for a day or so and see if it helped. If necessary add another quick shot. If you don't see any improvement and you aren't getting about 45 F air coming out of the vents, look elsewhere for a problem. Resist the temptation to keep adding refrigerant after say 4 1/2 cans - excess charge is just as bad as an undercharge.

The factory performance specs for various ambient temps:

You must be registered for see images attach

 

[itscaleb]

56 X .9 = 50

There are 12 ounces in a can of 134a.

50 / 12 = 4.167

Makes me feel kinda dumb now... I didn't even pay attention to the amount in each can. I was told by someone that I need to add about 5 cans and as soon as it stops cycling, I am full and not to add any more.

 

[smoothandlow84]

When I charged my new system (now 134a ), it took 4 cans. Having a few extra cans is good, especially if you discover a leak. When we first charged my new system, we assumed it was holding, but it leaked overnight. We discovered a small leak at the back of the compressor on the low/high side connections. The new compressor came with 4 sets of o-ring seals, so it was a crap shoot as to which thickness to use for the proper seal. Lesson learned.

 

[chengny]

I was told by someone that I need to add about 5 cans and as soon as it stops cycling, I am full and not to add any more.

That advice is not exactly correct.

Start with an evacuated (and oil charged) system - engine off.

Connect your hose to the low side service port. Open the valve on the charge hose (or gauge manifold) and allow the residual vacuum to pull in whatever quantity of refrigerant it is able to - from the can.

Jump the low side control switch and start the engine. Observe that the compressor clutch is pulled in. If the compressor is pumping, start adding refrigerant. As before, do this through the low side service port.

Always keep the can of refrigerant in the vertical position with the valve on the top. With it held this way, the 134a exits the can in the form of a gas. The compressor valves/pistons are not designed to pump liquid refrigerant - only gas. Liquid impingement can cause permanent damage. This is mostly only an issue during the initial charging stage. Later on, when the system is near fully charged, it isn't so critical to keep the can upright.

Anyway, with the control switch still jumped, add 3 cans to the system. At that point there should be sufficient pressure in the low side piping to keep the control switch closed. Remove your jumper, replace it with the harness plug and set the dash controls to MAX AC. The control system will then take over and the compressor will probably run continuously. It will run either most of the time - or continuously - because the system is still undercharged.

Now, add that 4th can. Depending on ambient temperature this can take a while. The refrigerant will only be drawn from the can when the compressor is cycled on. You can speed the rate of charge by shaking the can, warming it on the engine or even putting the jumper back across the control switch.

Anyway, here is where the advice you got is wrong:

At some point around 4 cans (again depending on OAT), the compressor will settle out into a nice on/off cycle rhythm. Iv'e never really timed it - and it always depends on the BTU load in the evaporator coil, but it should most certainly cycle on/off.

The control switch closes (and supplies power the comp clutch) when the pressure in the low side piping rises to the cut-in point. Then the compressor starts pumping and begins to pull the low side pressure down. It will run until the pressure is reduced to the cut-out point. Then, when the cut-out is reached, the switch opens and shuts off power to the clutch.

GM explains the function of the control switch much better (and in way fewer words) than I can:

Control of the refrigeration cycle (on and off operation of the compressor) is done with a switch that senses low side pressure as an indicator of evaporator pressure. During air temperatures of 60-80°F, the equalized pressures within the charged air conditioning system will close contacts of the pressure switch.

When an air conditioning mode is selected, voltage is supplied to the compressor clutch coil. As the compressor reduces the evaporator pressure to approximately 25 psi , the pressure switch will open, de-energizing the compressor clutch. As system equalizes and the pressure reaches approximately 46 psi , the pressure switch contacts close, re-energizing the clutch coil.

If your compressor isn't cycling something is wrong with the charge quantity:

1. If it runs continuously, you have overcharged the system. There is too much refrigerant in the piping to ever allow the compressor to pull the low side down to the 25 psi cut-out point.

2. If the comp doesn't run at all - or only starts for a moment and then it immediately kicks out - it is said to be "short cycling". Short cycling is almost always due to an undercharged system. It can be caused by an oversized compressor. But if you are using a standard R-4 unit, that isn't an issue.

Without the use of gauges to determine proper charge (which can get involved), a good method is to just watch/feel the low side piping where it exits the evaporator housing:

You must be registered for see images attach

With an extremely undercharged system, that section of pipe will be warm. As the charge is increased it will begin to get a little cooler. At some point - as gas is added - it will get really cold. It may even get cold enough to become coated with frost on high humidity days. But a frosted low side isn't good. It means that the charge is nearing the proper quantity but the degree of superheat is still too low. Keep adding gas, but only in tiny quantities now. Soon the frost should begin to melt back towards the evap casing. Eventually it will all be gone and the piping will just have a coating of condensation. At that point you are nearly fully charged. Add more gas in small quantities until the condensation stops forming. Then, the system is performing at peak efficiency. You can add another 1/4 can or so for a reserve. This extra charge will be accommodated by the volume of the accumulator.

 

[smoothandlow84]

Dont forget to add oil to a new compressor. Most compressors have a small amount of oil added from the factory. I disagree about jumping the switch to engage the compressor. Thats not a good idea since you can damage the compressor while charging the system. Unless you add the proper compressor oil to the system, you can damage the it by running it dry. Let the switch work as it should to control the cycling of the compressor.

Like Chengny stated above, if the compressor runs continuously, the system is overcharged.

 

[itscaleb]

So something weird going on. I have stopped using my AC till I can get a new compressor to keep the system with a positive charge so no air gets into the system and ruins the new accumulator. I noticed even when I have the system off the compressor clutch still engages and cycles. The only way I can get it to stop is to unplug the low pressure switch connector. Is that normal? I know some systems will run even when not on AC to remove humidity but I thought that was a newer system type of thing.