A/C problems

I'm having some air conditioning problems on my 1998 3.8L Mustang. When the car is moving, the A/C blows cold air, but at low speeds (and even worse when the car isn't moving), the A/C doesn't cool down the air at
all.
I checked the A/C pressure with a gauge while the A/C was on (and set to max). When the compressor is on, the gauge pressure is between 45 and 65 psi (this area is labeled "Alert"). When the compressor turns off, the needle moves to the area labeled "Danger". This corresponds to 65 psi or more.
Do you know what the issue might be?
Any feedback is greatly appreciated.
Thanks.
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snipped-for-privacy@yahoo.com wrote:

The answer's the same as I posted on the mustang group.
Sorry if you dont like it.. but kudo's for figuring out where it should have been posted, in the first place. Paste: I assume the rad fan is running and the condenser coil is clean. If not, fix that first.
See if you can find a hobbyist with a set of full gauges to help you. There being something wrong, the gauge you are using is only an indicator not a tool. The reading you are getting with compressor on indicates either a bad compressor or a damaged orifice tube (the little 'jet' that restricts flow so as to produce cooling) IOW, that pressure is too high, indicating either not enough flow through the compressor.. or too much through the orifice. A hint as to which would be the temperature of the vapor line at the compressor. If it's near ice cold, likely the orifice. Likewise the line OUT of the compressor to condenser being less than 'too hot to touch' would point to bad compressor valves.
That last being too hot to touch, along with the pressure you're reading would possibly indicate someone just 'dumped a can in' to see if it would fix the problem.
The pressure you're seeing with compressor off means nothing, as equalized pressure (no flow) relates to ambient temperature and is likely twice '65' (look up a temp chart for 134a). This is ALSO why you cant tell if there's too much or too little refrigerant just by reading some pressure without the system running.
Summary: The gauge you're using assumes fully operational system with the only variables being the switch-point of the low-pressure cutout and the amount of refrigerant in system
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Backyard Mechanic wrote:

Yep. Make sure the fan is running, I'll bet it isn't. And clean the crap out of the condenser. I'll bet the head pressure is over 300psi at idle, that's going to kill your compressor very soon.
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Tom Adkins wrote:

yep, Most likely no fan. I have found all the dirt ends up in the radiator, not the condenser. I guess it goes through the condensers larger "holes" and gets stuck in the radiator. A garden hose does the trick nicely. I have seen Crown Vics with radiators so plugged up you would think they would overheat in a minute, but they just run a little hotter than normal. Man I love a clean condenser and radiator! :)
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Hi Backyard Mechanic et al and thanks for your feedback. My A/C problems were indeed caused by a bad radiatior fan, just as you indicated. I just had the fan replaced for $319 (parts + labor).
The A/C is now blowing cold air again. However, the fan is noisier than the previous one --when it was working. It emits a humming sound when it's on, so I'm going to have my mechanic take a look at it. Do you know if the hum is normal? Will it go away over a few days?
As you can see I'm no car expert (although I'm very willing to learn) so any input is very much appreciated here. Thanks again.
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