A/c problems

[midwestLTD]

On my 91 Vic non climate control retro to 134 new compressor,evaporator,rings and all. Question when it's idling the air is very cold; but when I'm driving down the road it seems like the compressor is kicking on & off to much not blowing out as cold. Note it also is very cold at night even when rolling dwn the road. There are no apparent leaks in the lines, unit is fully charged.
Thanks Don

[sly]

the vacuum check valve at the firewall vaccum tree under the hood is bad. Someone else post in this thread and when I get home I'll post the part number. I have one sitting on my desk.

[gadget73]

the pressure switch may need adjusting, or maybe the charge is a little low. Check what pressure it cuts out at by revving the engine. R12 is right about 25 psi, R134 should be 20. If you unplug the electrical connector, there is a screw in between the contacts to adjust it. I forget which way it goes, but turn it 1/4 turn and see what it does. If it cuts out at a higher pressure, go the opposite way. Don't try and get it too low though, or the evaporator will freeze up and you won't get squat out of it. Marginally low charge will have very similar symptoms though so make sure its properly charged.

temperature regulation on a non-auto climate control won't be affected by vacuum.

[midwestLTD]

the pressure switch may need adjusting, or maybe the charge is a little low. Check what pressure it cuts out at by revving the engine. R12 is right about 25 psi, R134 should be 20. If you unplug the electrical connector, there is a screw in between the contacts to adjust it. I forget which way it goes, but turn it 1/4 turn and see what it does. If it cuts out at a higher pressure, go the opposite way. Don't try and get it too low though, or the evaporator will freeze up and you won't get squat out of it. Marginally low charge will have very similar symptoms though so make sure its properly charged.

temperature regulation on a non-auto climate control won't be affected by vacuum.

Ok NOOB ? Not very good at A/C stuff;but where is the pressure switch?? When I find it I will go adjust it now..

[sly]

pressure switch will be on the accumulator (big can just off the evap core near the heater core hoses) or on the line going to the top of it.

also, if the vents are dying and feeding the floor/defrost instead of the panel vents, it's the vacuum check valve. Part number is Motorcraft YG-337.

[gadget73]

Don't adjust it without hooking up a set of gauges to the system to watch what its doing. If you adjust it too low, the evaporator will freeze up and you could get liquid refrigerant back into the compressor. Liquid does not compress, so its possible for the compressor to blow up if this happens. If you don't have the right tools for checking this stuff out, I'd strongly advise not touching it.