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Unread 06-02-2011   #1 (permalink)
PeteG is offline Rookie

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Default 1996 Mustang GT Air Conditioning clutch cycling

ok the clutch for my A/C is cycling on and off. I have added a can of Refrigerant and replaced the A/C cycle switch yet the clutch still cycles. I have reviewed other threads and posts here and it appears that I might need more refrigerant in the lines. right now it reads around 25 psi while the clutch is engaged but then jumps all the way up to 55 psi when it is not.

I am concerned about adding another can for two reasons. one is that when I added the first can it did not empty the can making me think its is somehow full despite shaking the can repeatedly and wraping it in a warm wet towel to keep it going. after ten minutes the can still had some left in it and the gauge was borderline on the red when the clutch cycled off and I am afraid I will over pressurize the system. Plus after all that it did not change the temperature of the air in the system.

The second reason I am hesitant to add the second can is that the clutch is cycling on and off and I am concerned that there is something wrong elsewhere and I do not want to waste the can. I read on here that the A/C cycling switch needed to be replaced so I got the replacement switch. this helped somewhat, the temperature of the air coming out of the fan is significantly cooler but definately not cold, yet the clutch is still cycling.

now if I add the second can and bring up the low side pressure of the gauge from 25 PSI to 50 psi will this stop the clutch from cycling? and if not will the disengaged pressure (now 55 psi) increase above the safety limit and hurt the A/C lines? another thread mentiond that the high pressure is caused a suction gap in the line, if so does this mean I can discount the high side readings?

any feedback will be appreciated.

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Unread 06-02-2011   #2 (permalink)
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If the clutch is turning off at 25 PSI, then the low pressure switch is doing it's job. Why? Becasue R134a has a state change temperature of 29 degress F. What so special about that temperature? Water freezes.

So. If the AC cycling switch did not cut off, then the evaporator would FREEZE into a solid block of ice.

It is not possible to correctly charge a AC system without first confirming air flow over the inside and outside coils.

Anybody that gives you a set pressure number to indicate the system is full is giving bad advice. The pressure has to be corrected for the air temperature over the coil.

But assuming that the rest of the system is in good shape, then a low side pressure reading above 27.8 PSI is OK. Remember that it's better to be undercharged rather than over charged.

Refer to the following chart.
http://seit.unsw.adfa.edu.au/coursew...frigerants.pdf
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Unread 06-02-2011   #3 (permalink)
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The pressure numbers are comming from the gauge manufactureer and based on the ambient temperature.

the ambient temperature was 85 degrees when I was doing the refill so the the pressure reading needed to be around 50 psi yet because the clutch was cycling it was bouncing all over the place and into the red so i wanted to make sure I wasn't making a bad situation worse by adding more freon.

so what your saying is that the clutch is acting normally and that the pressure I should be concerned with is the low pressure reading and not the high?
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Unread 06-02-2011   #4 (permalink)
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how quickly is it cycling on and off? if it's every couple of seconds, then you're low. if it runs for 10-20 seconds or more before cycling off, that's normal function. Ford A/C (unlike GM and some others) doesn't have a variable clutch which stays engaged. It's supposed to turn on & off, just not too rapidly (which would mean the pressure switch is shutting it down).
 
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Unread 06-02-2011   #5 (permalink)
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it engages for 2-3 seconds then shuts off for about 5-6 seconds then rengages.
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Unread 06-03-2011   #6 (permalink)
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The truth of the matter is that BOTH high and low has to be looked at. The pressures have to be corrected for air temperature.

For example, let's say that the high side reads 281 PSI. If we look at the table, that gives a state change temperature of 155 degrees F.

Here where the outside temperature gets important. For this example, let's assume it's a very mild 50 degrees outside. This give a delta T of 105 degrees. This is way too HIGH and almost certaintly indicates a problem.

Now let's say its 100 degrees outside. Add say 10-15 degrees for engine heat and now the Delta-T is 40-45 degrees. This is a more reasonable Delta-T.

The high side pressures have to be monitored to protect against overcharging. However, the "normal" pressures are going to depend upon what the outside temperature and air flow is.

That is why it is vital to clean the coils and make sure that all fans are working correctly. Reduced air flow will skew the pressure readings!

If you absolutely can not monitor high side pressures, then monitor the low side but do not let the low side get above say 45 PSI. Add Freon until cold air starts coming out. If you get to 45 PSI and no cold air is coming out, best to stop and find out why. It's just not wise to go higher unless the pressures are crossed checked against temperature or you understand WHY the pressures are elevated.

Disclaimer. Working on AC systems can be dangerous. Serious personal injury can result from improper operation.
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Unread 06-04-2011   #7 (permalink)
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The other gentleman (wmburns) seems very knowledgeable about A/C systems, I'm more of a shadetree kind of guy. Throw in another can and see if it helps. I usually keep the cans in upside-down (works better that way if it doesn't want to take the charge rightside-up). You'd have to ask yourself where it leaked out that much, however, so you might be in for some more repairs.

I'm not an expert but this has never caused a problem with any of my cars (either with overfill or by filling with the can upside-down). I overfilled one 20 years ago and it caused excessive windshield fogging when using defrost (it blew really, really cold). Since yours is kind of cool and keeps shutting down, you're probably still too low.
 
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Unread 06-05-2011   #8 (permalink)
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To anyone reading this post, it's not a good idea to ever turn the Freon can upside down during re-charging. Why? Because this will allow LIQUID Freon to enter the suction line. If the liquid Freon makes it to the compressor, then the compressor could be hydro-locked.

It's pure luck that all of the liguid has turned to gas before entering the AC compressor.

If the Freon is charging too slowy for your tastes, then set the upright can on something hot.

OBTW, it's normal for the low side reading to go high when the compressor kicks off. So don't pay any attention to the gauge readings when the compressor is not running.

Try jumping the low pressure cut off swich to force the compressor to run during re-charge.
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Unread 06-05-2011   #9 (permalink)
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^ Thankfully I've been lucky that way, good to learn something new. I looked at the A/C lines on my 95 to see if the low-side port was before the accumulator and it's right before the compressor so I could have had a problem.

wmburns - if I could ask... I've typically used the all-in-one charge (with oil and refrigerant) and the cans say to flip them upside-down (checked one of the cans in the garage). Do you think that presents a similar risk? Also, there's some bulbous things on the A/C lines directly before the compressor. What are those - filters perhaps? (wondering if they have anything to do with my "luck.") I don't think they are orifice tubes or anything like that (they're also sealed).

http://www.partswebsite.com//stores/...sJ/JC94090.gif (part 7 on the image)
 
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Unread 06-05-2011   #10 (permalink)
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It's OK to liquid charge when the AC compressor is not working. In fact, when re-filling an empty system, the fastest/easiest why is with liquid Freon through the High and low side at the same time. The vacuum inside will pull the liquid in faster.

With regards to the oil charge, the accumlator will "trap" the liquid oil.

Bottom line, it's a question of luck and the specifics of how your car is built. Consider two guys. One guy charges gas until it's almost full and then switches to liquid.

The 2nd guy charges using liquid from the get go.

The 1st guy will likely be OK because the liquid will enter slower and have more time to gas.

The 2nd guy is pushing far more liquid because of the pressure difference between the can and the system.

Clearly there's also a difference between the guy that turns the can upside down for a few seconds vs the guy that pushes a whole can of liquid.

So. which is easier advice to give (and follow) and which would you rather get? Would you like to be the guy with a hydro-locked compressor?
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Unread 06-05-2011   #11 (permalink)
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Much appreciated, thanks for the advice!
 
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Unread 06-06-2011   #12 (permalink)
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thank you to all who replied.

I put in another can in the system on sunday. as the can emptied the compressor cycling started to minimize and the readings on the gauge calmed down to a more resonable level. it was 65 degrees ambient and I was getting readings from 30psi to 50psi. it was blowing cold inside the car and the compressor would stay on for 8 to 10 seconds before shuting off for 4-5.

after I shut the car off I could hear a hiss but not sure if it was the A/C or not the next day it was still blowing cold. I will know more by the end of the week if there is a leak and then start on finding that and correcting that problem.
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Unread 06-06-2011   #13 (permalink)
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The hiss is normal for a bit after you shut it off - - just de-pressurizing the remaining compressed refrigerant. If it's still hissing an hour later that's not so good. Good luck hope it all works out!
 
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Unread 06-16-2011   #14 (permalink)
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On my 2000 GT my A/C blows cold enough air but cycles the compressor on and off too much when at cruising rpms. At idle the compressor does not cycle off, reading 24 to 25 psi with compressor engaged. When I rev the engine the pressure drops slightly and compressor cycles, sometimes on and off every second. I try adding freon but it only took about 1/3 of the can. Any suggestions? The car is blowing ice cold air when idling and cold enough at speed.
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