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what pressure should aircon low pressure side read


the goat
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hi folks

my lad had his fiesta regassed a few weeks ago at the garage, and said its working (I never checked it) this week he said its now working, so I thought there must be slight leak, I looked around but could not see any tell tale signs of a leak, so thinking its a pin hole size hole, I got the recharge & sealer kit, checking the gas it was low on the gauge, so I topped up with gas, (between 25-40 psi "filled" part on gauge) still the A/C was not working, so I tested the Low pressure switch for continuity (switch closed= right pressure) but switch shows open, with the pressure gauge reading filled, so I jumpered out the low pressure connector and A/C runs, in the ford ETIS it says pressure should be between 3-10 bar, but my gauge reads psi and KPA, as I said gauge needle is in the "filled" section = 25-40 psi and 172-275 kpa 

anyone know if that pressure is correct (I'm sure it must be) and so I'm thinking it must be the pressure switch has gone, as well as a slight leak, but would the low pressure switch just fail in the last few weeks ?

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The pressure changes depending on the ambient temperature.  Don't just rely on the 'green part' of the gauge.

2012-07-08_213803_air_conditioning_press 

 

Sounds like it's just a leaky system though. 

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thanks, the outside temperature is around 26-28c, so by the chart the pressure is low, so maybe that is why the pressure switch is not kicking in, I'll retest later tonight

I've run the A/C for around 5 mins but the air blowing from the vents is still warm, what sort of temperature still the high & low pressure pipes be ? (the car is at standstill) At the moment I'm getting low 30's on low pressure side & 40-50's on high pressure side is that about right ?  I thought I would be getting 10c or so on low side

I'm only using a raytek minitemp laser to get the temperatures

 

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On 23/07/2016 at 11:22 AM, the goat said:

what sort of temperature still the high & low pressure pipes be ? (the car is at standstill) At the moment I'm getting low 30's on low pressure side

My a/c gets down to under 5C on the cold side, within 30 sec or so of turning on the a/c.

Unfortunately there is no good way to tell how much gas in in the a/c system. The table above is very approximate. It is absolutely essential not to overfill the system, it will wreck the compressor if it draws in liquid.

Weigh the gas refill bottle before and after each fill attempt. Then you will know how much is going in. There should be a label on the car, and in the handbook, giving the a/c capacity fluid. Do not exceed this amount, ideally stop before 50% of the amount. Fill in small stages, and test between fills.

Once there is some liquid refrigerant in the system, then adding more gas does not raise the pressure, it just increases the amount of liquid. The temperature of the liquid will determine the pressure. That is the danger of using pressure to determine the amount in the system.

Also be careful not to get dirt, water or water vapour, or air, into the system. Flush out the fill hose and the fitting, with a tiny squirt of gas before fully connecting the fill hose.

I added a tiny bit of gas from a diy kit a year ago, and it has worked well since. But large leaks or other faults can not be fixed so easily.

Peter.

 

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first thing is why did your lad get air con gassed up in the first place? was it not working? if that was the case, how long hàs it not worked for .

if after regassing to the correct amount by the garage it then worked and now stopped , then it appears there is a significant leak, just putting more gas in is only just going to add to the global warming. lol

r134a is dangerous stuff in inexperiened hands,

i would just take back to garage for them to recheck if theres any gas left or an electrical issue. ( im betting you have a leak from the condensor)

they a quickly check for leaks via uv dye if they injected some in on original fill, or nitrogen , or forming gas.

its not good practice to refill with r134a and check for leaks by garage as they are governed by rules/regs . the tech and or garage owner are open to £1000's in fines and risk imprisonment too.

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we only got the car in March, its 07 plate, from the auction

when we tried the A/C it didn't work, so electrically I checked that I had power to pressure switch & compressor worked on test leads, then took it to garage to have A/C vacuumed and refilled (the normal thing they do) they could not find any leaks on their test (independent garage) my lad said it was work ok,

it was ok for about 4 weeks, then stopped working, I've looked round the system the best I can but can't see any tell tale signs of a leak (dirty marks etc) so because it run for 4 weeks think it has to be a pin hole leak some where, so I was just going to put in leak sealer as even with UV it would be hard to find a pin hole leak

when I checked the gas level it was around 7 psi on the gauge, so its not a big leak,

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had it running yesterday and I can see one of the pipes on the cabin bulkhead is frosting so its working, but there is not much cooling air inside the car

if I turn the heater control from cold to hot there is lots of hot air coming though, so the heater control is working, I changed the Pollen filter and checked it was still ok yesterday, I've read about a blend door on the heater system by from what I've read its normally fails by not supplying hot air

is there anything else to check to find out why once the pipes are frosting the inside of the car is not getting cold

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8 hours ago, the goat said:

I can see one of the pipes on the cabin bulkhead is frosting so its working, but there is not much cooling air inside the car

Is it now working with the pressure switches operative? Normally the pipes should get very cold, but not ice up. The low pressure (cold) side pressure switch should stop it icing up too much.

There have been a lot of discussions lately about Focus a/c effectiveness! See

How much this also applies to the Fiesta, I am not sure, but it is quite an interesting discussion.

An idea there was that the evaporator was undersized for the system. But reports are very variable, and I do now wonder if faults with the air control flaps are common, allowing some heat from the heater matrix through, perhaps. But since it all very difficult to get at, it is hard to tell.

Sadly, I think you have two problems: A drop in system pressure to 7psi in 4 weeks is not a small leak by a/c standards. Ian (iantt) said a 50% loss in 3 years could be considered normal. And Ian is usually right, he has massive experience. The dye test should find that leak quite easily. Adding gas from kits on a frequent basis is not a good idea, it will imbalance the oil to gas ratio for a start. Too much oil or too little are both serious problems.

Then there is the effectiveness of the heater / a/c air control system. Testing or fixing that may be easy once you have access, but that access is a fairly big job. The question really then is: How much is the a/c system worth? And is it worth having it fully repaired?

Anyway, keep monitoring it, you might be lucky and the kit will fix the leak, or it was a one off, perhaps only just enough gas in it to start with. You may be able to nurse it through the little hot weather we get in the summer. But be aware that compressors in particular, are easily damaged, and very expensive to replace. Also R134a is still a significant pollutant, even if not as bad as the old CFCs.

Peter.

PS: If that leak was there when the garage did the vac & refill, they should have noticed it. You would have a good warranty claim there. But they will, of course, claim the leak happened after, like a stone hitting the condenser perhaps. But if the leak is elsewhere, or any damage to the condenser clearly looks old, then they might be on sticky ground. Worth a try?

 

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low pressure switch was jumpered out when the a/c pipe was frosty, I'm getting a replacement switch this week, the garage didn't detect any leaks when they did a vacuum test,

I've not got a diagram showing the layout of the heater, to see where and how the A/C fits in & the control of it,

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4 hours ago, the goat said:

I'm getting a replacement switch this week, the garage didn't detect any leaks when they did a vacuum test,

I am fairly sure that replacing the switch will need the system to be emptied, evacuated & re-filled. The vacuum stage is not just a test. It is essential to remove all air, and especially, all water vapour, from the system. Air will badly degrade performance, and water is said to react with the refrigerant or oil to produce an acidic product which damages the compressor & other internals.

I assume that having measured the pressure in the LP side with the gauge, and tested the switch, you are sure that it is broken. It is an expensive operation, so it might be worth double checking the existing switch.

For the heater / a/c evaporator & air flaps, I have not seen any drawings or views of it either!

------------

An informative test is to measure the system pressure with the engine cold, and off. So that the whole system is at about ambient temperature. This may be the best guide to the amount of gas in the system. If the pressure is below the SVP curve, then there will be no liquid present, only pressurised gas. If on the curve, then there will be liquid and gas. If above the curve then air is present. I would expect this resting system pressure to be on or just below the curve.

For r134a, the SVP curve is:

20C: 470 kPa, 68 psi.

22C: 506 kPa, 73 psi

24C: 544 kPa, 79 psi

26C: 584 kPa, 85 psi

28C: 625 kPa, 91 psi

30C: 669 kPa, 97 psi

All these pressures are "gauge" not absolute, based on 102kPa Atmospheric pressure. So they will match pressures read by a normal gauge that reads 0 when open to atmosphere.

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Where do these SVP curve pressures come from ? in Tomsfocus post the pressures on the low side start at 25-35 psi which is what I'm reading on other webs is the right sort of pressure or is that wrong

I tested the pressure switch after I topped up the air con to 30 psi and again after I topped it up to nearly 40 psi, with the ambient temp, it had not closed so I thought it must be broken, I would have thought that the A/C should be emptied before changing the Low Pressure switch but from other web sites it says the dehydrator has a valve that shuts off when the pressure switch is removed

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4 hours ago, the goat said:

Where do these SVP curve pressures come from ?

From the Du-Pont data sheet for the r134a refrigerant. They make or market it.

Tom's table is with the engine (and a/c) running, so the evaporator will be cold (lower pressure), and the condenser will be warm (higher pressure). The trouble is, these temperatures are poorly related to ambient temperature, and can be affected by many other things than amount of gas in the system. So there are quite wide bands of pressure given, as you can see.

If the whole system is at a uniform temperature (ambient, and engine stopped), then the SVP curve can be used. But even so, once up to the curve, there is no change in pressure as gas is added. Just the amount of liquid changes.

Think of a kettle or saucepan. Once up to 100C, the temperature stays constant (and the pressure stays at atmospheric pressure), until it boils dry. The SVP curve for water at 100C is 100kPa (14.7psi) absolute. This is why it boils.

That is why I say there is no reliable way to judge if a system is getting overfilled, and you should add small amounts, weighing to check the total amount, and testing between.

It is possible there is some sort of seal valve inside the pressure switch port, a bit like the valves on the fill points, that allows the switch to be removed. Seems a rather expensive thing for Ford to put in. If you do unscrew the switch, take great care as it may release pressurised refrigerant!

 

 

 

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Thats a focus pressure switch port, I would assume there all the same, valve in middle to stop escaping gas

2016-07-26 13.20.33.jpg

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48 minutes ago, iantt said:

valve in middle to stop escaping gas

I like that!

Either Ford have been very helpful about being able to service these items, or more likely I suspect, they think the pressure switches will be so bloomin' unreliable they will get loads of warranty claims, and put the valve in to reduce their costs!

But still be careful if removing a switch, there will be some release of gas, and the valve could stick open or leak, since it does not get a lot of use!

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Ive never replaced a pressure switch. 

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not replaced the low pressure switch yet, but to give you heads up, it looks as if it have a probe inside which pushes down a valve needle and there is a small hole which looks as is if that is where the pressure sensor is

P1050918.JPG

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