low side dont go over 20 psi

[Titemotte]

Hi all
im working on an 2001 ford f53 chassis motorhome with 6.8l
r134a system with 2.75 pound
a/c stop working so ive put gauge on it and ive see 6inch vacuum on low side and 75 psi on high
ive found that the lowside/cycling swicht was loose and leaked so ive repaired it and add an 12oz can on system
after that low side was about 12 and high side about 110psi outdoor temp were around 70f
so ive added another can and pressure goes to 21 low side and 125 high side
temperature on the vent was 52f
so ive added one more can and low side pressure become unstable and go from 18 to 22 psi and high side was stable at 130 the compressor always engaged never cycling
suction line is frosting
all of this testing was done at idle
after i test at 1800 rpm so the low side drop to 11psi and high side increase to 150
i know im a newbie so any help will be appreciate
what should a do next ? already added 2.25 pound on a 2.75 pound system
does ive overcharged it? someting clogged?
can i run it with this pressure without damaging it ?
thanks for your help and have a nice day

[JohnHere]

If you're sure the system is now leak free and all the components are good, start by charging exactly 2.75 pounds into the system and then performance-test it again.

You'll need to recover the refrigerant that's in there now (or have it done), evacuate, and recharge by weight using an accurate scale and a Manifold Gauge Set.

An ambient temperature of 70°F is rather low for diagnosing an MVAC system. Suggest waiting until the temperature hits 80°F or above before attempting additional work.

Are all of the components in this system the originals (except for the cycling switch)?

[tony1963]

The low side dropping below 20 psi (you observed 11 psi) is a problem. Any low pressure below 20 psi will begin to freeze the evaporator core and basically reduce the cooling ability. The low pressure switches are usually adjustable.

With a system of that size, 2.75 lbs, 44 ozs, you need to have the charge close to the specified amount. At 2.25 lbs, you are only about 80% charged and hard to properly diagnose the system.

[tbirdtbird]

As others have already said, the system will not perform adequately without the specified charge. It makes all the difference

[Titemotte]

thanks for your response
yes i know that 2.25 lbs not enought for an 2.75 system
but as i start with an 75 psi on high side this should had some refrigérant left inside ?
and the last 120z can ive added not increase pressure
so im guessing ive already overcharged
its my third car ive recharge and the first 2 the pressure increased as ive added refrigerant
so its sound weird to me to continue add refrigerant while both pressure not increasing

[tbirdtbird]

You are over-thinking it.
"after i test at 1800 rpm so the low side drop to 11psi and high side increase to 150"
This shows you are undercharged. Which is what we been saying.
Glad you got pressures at 1800 RPM, pressures at idle have no meaning.
Re-read the advice you have been given, and note the comment about waiting for at least an 80*F day to charge/test pressures.

[OMW John]

If you're seeing vacuum on the low side or extremely low pressure then you most likely have a bad expansion valve or orifice tube but you really do need to start with a proper, complete metered charge. After a full recovery and vacuum. You may also have a low side hose collapsing but that's rare.