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Originally Posted by Ieatcamaros
Is there anything else that would make that low-pressure cut out switch de-energize the compressor besides low refrigerant? All the visual inspections I did checked out ok. While we are on this type of subject, what would make a car only blow on defrost? I don't think it's the switch cause it has done this ever since I converted it to carbed. And it was working fine beforehand. This is on a 89 mustang.
**Could you tell me what the hell an orifice tube does for the a/c system?
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The blend door doesn't necessarily have to be stuck fully open, maybe it didn't fully close. Even a small opening from a blend door that's not tightly closed will allow enough hot air into the duct to warm up the air that has been refrigerated.
There are a several reasons why the clutch would not engage:
1) Low Pressure because of a low refrigerant level
2) The Low-Pressure Cut-Out Switch is out of calibration (with R-134a, it should shut off the compressor at 19psig and with R-12 it should cut off the compressor at 25psig.
3) Defective Low-Pressure Cut-Out Switch. It's screwed into the accumulator and has a (two wire) plug connected to it. (BTW, if you unplug the plug from the switch, there is a small screw recessed in there; this is the adjustment for setting the pressure cut-out point)
4) Compressor clutch gap is too wide
5) Compressor clutch coil is defective (weak or shorted)
6) Compressor relay is defective
7) Wide Open Throttle Switch is defective (at wide open throttle, this switch de-energizes the compressor clutch so the engine has more power)
8) On R-134a systems, there is also a High-Pressure Cut-Out switch that shuts off the compressor if the high side pressure exceeds about 375psig. If your radiator cooling fan is not working properly, this could cause the high side pressure to shoot up and cut off the compressor. Also, a dirty condenser or a missing air deflector could cause the same problem. R-12 cars don't have a High-Pressure Cut-Out Switch.
On a side note, I like to seal the parameter gap between the condenser and the radiator with foam so that the air being pulled by the fan has to get sucked through the condenser first. Without the seal, air can be pulled only through the radiator (through the gap) so that cool air doesn’t pass through the condenser. That’s one reason why some cars cool good while driving but not when their stopped at a red light. Also, (if your car has a belt driven fan) it’s important to insure that fan clutch is working properly so that there is sufficient air flow through the condenser. If you want you’re a/C to work, the fan clutch should be replaced if the car has more than 35 or 40 thousand miles on the odometer. As far as the A/C goes, the fan clutch doesn’t last much longer than that; even though the car is not overheating!
If your car has an electric radiator fan, there is a low and high speed, make sure it goes into high speed when the A/C is turned on.
As for your problem about only blowing on defrost…. I assume you mean, air only comes out of the defrost vent regardless of where the selector is set? Again, this is a vacuum problem. The diverter that controls where the air goes is vacuum operated. Maybe you’ve got a leak or maybe you don’t have sufficient vacuum. Might need to add a vacuum can.
You asked what an orifice tube does…. The orifice tube (or O-Tube) introduces the pressure drop between the high side of the system and the low side. It’s basically a controlled restriction in the system. At the high side of the O-Tube, the refrigerant is in a High Pressure liquid state. After passing through the O-Tube, the refrigerant is in a Low Pressure liquid state. This low pressure, liquid refrigerant then goes to the evaporator where is picks up heat (from inside the car) and boils back into a vapor. That’s how it cools.
Hope this wasn’t too much for you to digest but now you know why A/C work isn’t really a do-it-yourself job; unless you’ve got a lot of knowledge and experience in these systems.
Phil