A/C expansion valve

Or low charge. Have you checked that? Could also be moisture in the system -- freezes at the valve. Could be valve. Nothing strange about any of the above.

Don

I don't see how low charge would result in low side vacuum that persists for many minutes after engine off. Moisture could be a possibility, but the system has never been opened up and still has about 60psi at rest on both high and low sides. I don't see how moisture could get in. I still like the valve but, since I've never played with one, I don't know if the failure mode is open or closed. I'd think open, but that's just a guess. Thanks for the response. Art

Are you sure this thing even uses an expansion valve. Seems like most stuff these days uses an orifice tube with low pressure cycling for temp control. What you describe sometimes happens with low freon charge because it lets the evap coil freeze up and the air starts to feel warmer. It's also possible that the system is computer controlled and the computer is messing up.

Yep, it's got an expansion valve. It looks like an oblong block with a diaphram on the end--no thermal probe. Nope, it's not computer controlled.

I didn't think of the evap coil freezing up and I'm not clear how a low charge would cause that. Can you expand on that idea? Tks, Art

It's complicated. When the overall pressure in the system is too low, the compressor can suck too hard on the suction side. Really, the compressor can be thought of a pump that simply moves the refrigerant through the system. In the process it creates a low pressure on one side and a high pressure on the other side. At the high side, the heat is removed in the condensor which allows the refrigerant to condense to a liquid despite only a relatively small temperature change. This liquid is then stored in the drier unit, which acts like a reservoir. Finally it hits the expansion valve where it goes from high pressure to low pressure and when it hits the evaporator it has a chance to absorb heat which allows it to boil. It will boil at a low temp because of the low pressure created on the suction side of the pump (compressor). This low pressure, low temp refrigerant absorbs the heat from the passenger compartment and should in the process be heated to around room temperature.

The amount of refrigerant in the system controls the overall pressure, which in turn affects the temperatures at which the refrigerant will boil and condense. If the pressure is too low, the system will have low temps where it should have relatively high temps, which can cause ice to form in places you do not want, such as on the coils of the evaporator. If you see ice forming on your drier unit, it could mean the refrigerant at this point is too cold (due to the low pressure). No doubt I have simplified some things, but hopefully this flow description will be helpful.

I hope I explained this correctly...

Peace, Harry

Another way to explain it is that the temp of the evap coil (the one inside the car) will be approximately the same as the low side pressure. When the freon gets low it lets the system pressure get lower then it's supposed to so instead of, say, 30 psi on the low side with a 33 coil temp, the low side drops to 20 with a 24 degree coil temp and the condensation water freezes and blocks the air flow.

I recall way back when people started putting aftermarket underdash AC units in and the ones back then were allowed to run to fairly low pressures when you set the temp control on full cold. It was not at all unusual for them to spit out little chunks of ice.

On Mar 24, 7:43 pm, Ashton Crusher wrote: > Another way to explain it is that the temp of the evap coil (the one

That's a pretty good rule of thumb. I'll have to remember it.

I have such a system in a '74 Beetle I picked up last winter. Come summer I will be highly motivated to get the AC working. The PO scrapped the fresh air system and installed one piece windows. Talk about sweatbox...

Harry