Richard tends to assume you know more. Unless you know that the compressor actually works (runs) all by itself then you can connect a gauge set. Unless your unit has a dryer with a spyglass you will need a correctly calibrated refrigerant gauge set to measure and see what is happening.
- - When the system is running with a proper gauge set and refrigerant refill bottle/tank attached - then the "low side" pressures Richard lists will determine whether you need to refill the unit with refrigerant or not. Sometimes you will need to let some out if it has been "overfilled."
- - That "low side" pressure will set the temperature at the evaporator plates. This is where an experienced person with refrigeration
is important - and - having the proper gauge set and fittings to service
- - Basically, too much refrigerant and the unit will not cool or get cold, too little and the same result. But in that narrow range of low side pressures you will go from no cooling
to proper cooling
to frosting over (too much cooling) and finally no cooling again.