Need help with A/C pressure and discharge

So the compressor clutch is not coming on at all? Are you running the AC on MAX AC, full blast? Is it blowing hot air?

It just lost its refrigerant. You need to take it to an AC shop. Evacuating the system by yourself is dangerous and terrible for the air. Just have them find the leak, fix it, and fill it with fresh refrigerant. It could have a leak from the valves, compressor seals, or piping. AC systems are complicated, and you are better off taking it to a pro.

the98stang said:

It just lost its refrigerant. You need to take it to an AC shop. Evacuating the system by yourself is dangerous and terrible for the air. Just have them find the leak, fix it, and fill it with fresh refrigerant. It could have a leak from the valves, compressor seals, or piping. AC systems are complicated, and you are better off taking it to a pro.

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That is utterly incorrect.

If you have hooked up a guage and your low side pressure is 110 PSI, than discharging and refilling the rerigerant won't do anything. there are two switches on your system that control the A/C clutch. One low side pressure switch which opens below 25PSI and is closed with anything greater than 45PSI, this one is in the low side line and is there to cycle the clutch and make sure you have enough refrigerant in the system to operate.

The other switch is your high pressure cutout/fan controller switch. it's located by the radiator fan. it turns off the clutch if the high side pressures get too high, and also controls the speed of the cooling fan accordingly.

I'd start by checking your fuses, if nothing there, get out a wiring diagram and start running the problem down.

by the way, 110 PSI is about normal pressure at rest if you have a hot engine bay or it's a really hot day out. With refrigerant pressure=temperature.

oh yea, that yellow stuff, it's dye. it's put in there by the factory to find leaks.

I agree with the98stang on one aspect, if you don't have any knowledge of how an a/c system operates and all the fuses are good, take it to a pro. A/C systems are complicated and you really need a basic understanding of how they work to fix them, i know, i do it for a living. Plus, being as you have more than adequate system pressure, it's probably just a simple electrical problem that can easily be fixed and probably won't cost that much if you take it in. If you jack with it and make it worse, it'll only cost more in the end.

I didn't realize the pressure was fine. Sorry for the mixup...

the98stang said:

I didn't realize the pressure was fine. Sorry for the mixup...

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it happens brotha, didn't mean to sound like an as*hole, sorry if I did.

85 Coupe 5.0 said:

it happens brotha, didn't mean to sound like an as*hole, sorry if I did.

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Whats a good place to take your car to have the AC checked? Any national shops that you would recommend (Firestone, etc....)? Just wondering because I need to take my car in to have the AC checked as well, as it is no longer coming out cold after being down for awhile. Sorry for the hijack, but this might be helpful for the original poster as well...

I'm not big on anyone but the dealers, as that's were I work. I have seen first hand what can happen when people that aren't trained work on A/C systems. Sure it may work for a while, but when it fails because of some shortcut that was taken to get it working cheaply, it's going to cost you big.

I would personally recommend that you take it to a Ford dealer. I know it's going to be expensive, but then again you have some piece of mind that it's going to be done right. It is true, dealers sometimes have people that don't know jack, again, i've seen that too, and in some situations you may be better off taking it a jiffy lube.

My recommendation, look around, ask questions, find out as much as possible about who you take it to. If you take it to a dealer, ask that whoever does all their a/c work gets your car. Working at a dealer, i know that we things get really busy sometimes jobs get pushed off onto tech's that don't normally get them, and this can lead to some issues. Don't mind waiting, if they are busy and only have one or two a/c certified guys it may take a few weeks to get it in.

If you aren't big on dealers, find a specialty shop that seems to only concentrate on a/c work. There are some great independent shops out there, and the ones that advertise being a/c specialists usually have trained personnel in that area.

A/C work is kinda hit and miss expense wise. If it's not kicking in, but has a good charge level it's most likely a simple electrical problem and won't be too expensive to fix. it's when you get into the refrigerant side of the system that it gets up there in cost. the average cost for a compressor is around $400, plus flushing $200, plus a drier $150, plus labor $400. you get the idea.

If you determine that your a/c has lost it's refrigerant and just needs recharged, it's not that simple. It lost it for a reason, it doesn't just get used up. It had to leak somewhere, and buying those cans of refrigerant at the parts store that look like fix a flat cans is a waste of time. Sure it'll probably get you going for now, but will eventually do damage to your system that will be some big $ in the end. Don't ever use STOP LEAK, EVER! this stuff is nasty. It works by coming in a can that you inject into your system. It works by hardening when exposed to air, that stuff gets rock hard. So, when your system eventually crashes, which it will as the stop leak messes it up over time, and the poor tech that comes along next to open up your system and replace components, the minute he cracks that system open all that sealer hardens and now the whole system is trashed.

Sorry bout that long rant, but this is just some of the things i've seen people do.

Basically, please, take it to a professional if you don't know how it works. You may fix it for now, it may work for 2 or 3 years, but in the end it's just not worth it guys.

I agree with taking it to a dealer. They know the cars and the a/c systems better than anyone. you might get a cheaper quote somewhere else but next year you'll have to have it fixed again.

stylin99 said:

You guys have been very helpful with this!

I did some more testing. Put a paperclip in the low pressure switch (Haynes manual calls it a compressor clutch cycling switch). It's the sensor on top of the drier right?

The compressor kicked on just fine, and stayed on, the air at the vents got nice and cold for about 30 seconds before I shut it off for fear of hurting something. The lines around the drier were also nice and cold.

So what's potentially wrong here? Compressor works, and has enough juice to flow cold air.

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Yes, the haynes is right, that's the cycling switch. IF you indeed have hooked up a guage and got a reading of 110 PSI then it sounds like your charge level is probably o.k. and you have found a defective low pressure/cycling switch.

However, you need a guage set hooked up to see what is going on when you short that switch across, that way you see how the refrigerant and system are working, and this will also tell you if the switch is indeed bad. Running the system with a low charge level will ruin the compressor as the oil in the system is carried by the refrigerant, so if the system is low no oil gets circulated.

My best advice, look in your haynes manual and see if you can un-screw that low pressure switch from the drier without having to evacuate the system, most have a schrader valve like a tire that lets you remove the switch with the system charged. from here we have two options. Get a fitting to adapt an air hose to the screw on end of the switch and hook it up to a regulated air supply, set to about 80 psi, once you apply air pressure to the switch use an ohm meter to measure resistance across the contacts and see if it's closed, as it should be.

or just get a new switch and try it, it's like $30 probably.

Sounds like you've rooted out the problem, either we have a low charge situation and the switch is doing it's job protecting the system from operation due to low refrigerant, or the switch is failed.

Whatever you do, DONT run the system with it jumpered for more than 10 or 15 secs, until you find out if it's low on charge. doing so a few times really won't hurt anything, but don't leave it jumpered, otherwise we'll get some real damage.

Aghhh, i thought we went through the can thing? Yes, by your pressure readings, your system is full. The cans are pressurized by the refrigerant itself, like I said, with refrigerant temperature=pressure. So if you have 110PSI in your system, the can would have to be more than 110 degrees to overcome the pressure in your system. They can't use propellent in the can to push the refrigerant in because it would mix with the refrigerant. Those cans really only work if your system is low, because if it was the pressure wouldn't be 110 PSI and the can could fill the system.

O.K, another potentially fatal flaw. NEVER charge a running A/C system with the can upside down, or with the LIQUID refrigerant. Here's the deal. the compressor works just like any other compressor, it has some sort of piston and valving. You can't run a liquid through a compressor, it will hydrolock and break. Here's where the don't do it if you don't know stuff comes in. When the system is in working order, the refrigerant is in a GAS state when it hits the compressor, so all is well. It's in a Liquid state when it leaves the condensor (that thing in front of your radiator) and changes to a gas when it goes through the evap core in your dash. The way it sounds you didn't get any in, and it apparrently still works so all is probably still well.


60 PSI on the low side with a jumpered switch is probably around normal, depending on the temperature/humidity of that day, but would have to have high side readings to tell you for sure.

O.k. all mustangs use what's called a CCOT system, that stands for Clutch Cycling Orifice Tube. This means that it has a low pressure cycling switch to kill the A/C clutch. Reason being, on a CCOT system the compressor only runs till it gets the evaporator (the heater core like thing in your dash) down to around 32 deg, then it shuts off the compressor till it gets back up to around 45 deg. If it didn't shut the clutch off it would let the evaporator core get below freezing and it would ice over and you would have no air coming out your vents because ice would block the core.

It IS ABSOLUTELY NORMAL for this system to kick in and out when it's not being fully loaded, fully loaded being a hot enough day to use the full potential of the A/C system. YES YOUR A/C IS SUPPOSSED TO OPERATE IN HEATER MODE TOO. On the Mustang, the A/C runs in the defrost, mix, normal a/c and MAX a/c setting. Why? becuase the moisture in the air is attracted to the evaporator core like a cold beer attracts it when you let it set. so the only thing coming out of your dash is dry warm air that will take the moisture (your breath) off the windshild so you can see, in the winter time or on a humid morning.

I'm sorry to say, that even if you get your system fixed, your car will still surge and will still run the a/c even in heater mode. Although, if your cycling switch was going out it could have been cycling more than it normally should have, so if this is truly the problem, then it wouldn't cycle as much with a good switch. This is normal operation of a CCOT system and there is nothing you can do about it. You won't notice it as much as the days get hotter because your a/c will be loaded more with the heat and won't cycle as much, it is the worst in the spring and fall because of the in-between temperatures.

I would try the low pressure switch, and if it still won't work when you turn the knob to MAX A/C and blower to high, take it to a professional. But it sounds as if the low pressure switch is bad.

ahhhh just refill it and be done. works everytime!

jcint said:

ahhhh just refill it and be done. works everytime!

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We all wish it was that easy....

awesome! glad to hear it all went well.