A/C

There is no way to know if there is enough refrigerant in the system. Only Chrysler uses a sight glass to allow you to see if the system is low. Ford started to use that too, but their systems still show vapor bubbles even when the system is fully-charged, so they do not do any good. With all other systems, the only way to know how much refrigerant is in the system is to drain it empty, pump the system into a vacuum for at least half an hour, then pump in the exact measured amount called for.

All AC systems work on the principle of a liquid becoming very cold where it goes from a liquid to a vapor, and from a high-temperature vapor back to a liquid. Turning to a vapor needs to occur in the middle of the evaporator in the dash. That is where the incoming air blows over it and gets cooled. If the system is low, the pressures will still be very close to normal, but the change in state takes place under the hood, and that is where it gets real cold. In misguided attempts at solving a problem that result in over-charging the system, there is too much liquid, so it sloshes out of the evaporator and turns to a vapor again under the hood, not in the dash. That also results in too little cooling in the passenger compartment, but you will find frost on the outside of the hose that goes from the firewall to the compressor. When the system is under-charged, you will find frost forming on the hose going from the condenser in front of the radiator to the firewall. The frost is showing the point at which the liquid is turning to a vapor. You may see a little of that on the hoses when everything is working properly, but a big blob of ice on a hose is showing the "cold" is being produced in the wrong place.