First, the cheap deal. Some of you may be aware of Groupon, where you buy a voucher which gives you goods or services at a discounted price, but their latest one is definitely worth having if you need your AC regassing. £37.99, which I suspect barely covers the cost of the refrigerant for a P38. See https://www.groupon.co.uk/deals/ats-euromaster-ltd-21 Unfortunately the voucher is only valid for 31 days so you can't buy it now and keep it until you need it doing at some point in the future.

Now to the question. I bought the voucher to get Dina's SLK regassed. They pressure tested it, no problems, they then regassed it only to find that the AC comporessor didn't kick in so it still didn't work. So I wasn't charged for the regas. Guy told me to either apply for a refund or if I had another car that needed doing, use the voucher on that. Turned out that the blower motor brushes were buggered so the blower wasn't turning, so no feedback to tell it the blower wasn't turning, so no compressor. Pulled the blower apart, sorted it with brushes nicked from a P38 blower with duff bearings and lots of icy cold AC.

Last week in France I noticed my AC, while working, didn't seem to work as well as it had a couple of years ago when I'd replaced the condensor. Fine in the UK but once the temperature got the wrong side of about 28 degrees, it struggled. So I've just taken it in to use the voucher on that. AC working fine on the way there, nice cold air coming out but outside was only 22 degrees and it was coping fine. Got there and they connected the Nitrogen bottle to pressure test it. Pressure was dropping so they declared a leak and said there's no point in gassing it as it will all fall out. Now it's working and had been working when I got there (and, thankfully, is still working). They connected to the high pressure side (or at least the larger of the two ports which I think is the high pressure side) but it would have had pressure in it anyway, it had just been working, so surely, once the engine was turned off the high pressure will drop anyway? Or won't it? I've no idea. I've had a look with my UV torch and there is a glowing area on one of the pipe connections on the compressor but I remember taking that off so I could remove the old condenser with the pipe attached and undo the union with it on the bench where I could get a bit more grunt on it. So I suspect that may well be a leftover from that or I need to replace the O ring. However, as the system is currently working, I won't be doing that until it stops......

I thought to nitrogen test it, they first had to suck out the refrigerant from the system, so they could fill the entire system with nitrogen?

So did I but the guy doing it did seem to have to go and ask someone else a number of times. He seemed puzzled when he first connected the Nitrogen as it was showing pressure, which it would on a system still working although maybe a little down on gas, unlike one that was completely empty.

They were going to pump nitrogen into a gassed system to pressure test it??? Yikes!
When the car is off, the high and low sides will equalise, and as long as there is any liquid R-134 in the system, it will show the same pressure.
Pressure will be vary, and is determined by ambient temp.
As soon as the system is active, it will suck the vapor through the compressor, and condenser will cool it into liquid, then through the expansion valve in the
evaporator, and cold happens! This is when the high side versus low side pressures will tell you if additional gas is needed.
If there is not enough gas in the system, things will still work, but not as well. When it gets down too low, the trinary switch kills the
power to the compressor so as not to damage it. Thus, when things nudge above 35 or so, it just blows coolish air at you as it is short cycling.
Here in the US of A, we are still allowed to re gas our own vehicles. In OZ, they practically panicked and called the cops when I inquired about purchasing a can of R-134.....
I am guessing that over there in Borris land, like OZ you are not allowed to do your own..........Possibly after Brexit? ;)

You've just confirmed what I was thinking and no, they weren't just going to do it, they actually did do it so God knows how much pressure I've got in there now. When they told me it wasn't holding pressure the first thing I said was that the pressure on the high side would be dropping as it cooled down and the high and low sides equalised but they insisted that the only way the pressure could drop would be if there was a leak. But, at least it is still working, I was concerned that I'd go in with a working, albeit not as well as it could, and come out with an empty and not working system. I found it hard to believe it has a leak as it has been working fine for at least 2, if not 3, years.

It isn't just UK, it's an EU thing, you can no longer buy R134a to refill yourself. You used to be able to get a can of the stuff and a hose to fill with, now all you can buy is the hose. I've got one from when you could buy the gas and one to fit older systems that used R12 (but I can't get that now either). I've found a supplier in Italy that internittently, and illegally, will sell 500g cans for export only but no regular supplier.

I have owned AC regas gear albeit not professional gear for years and recently bought another set including little refillable/exchangeable bottle and charging 'gun' (pipe with LP connector, connector to exchange bottle, trigger and pressure gauge) from Halfords.

My car, not a P38, supposedly takes 0.99kg of refrigerant. I recently fitted a replacement rear evaporator and some new lines on it then took it to a garage for re-gassing. It worked fine after re-gassing but I regularly drive many other examples of the same model vehicle and some of the others seem to get just a bit colder. With my paranoid head on I wondered if the 0.99kg should really be 1.05kg but the vehicle manufacturer avoided stating over 1kg as some refill garages would charge a lot more for a regas over a 1kg threshold, also wondered if the garage machine was set to over-read the amount of gas it was putting in (to short charge systems to save the garage some refrigerant). So I put a bit more in myself... quite a bit more, and it seems I overdid it! Because it worked perfectly after the garage regas but now on a particularly hot day (for the UK) if the engine has already been run and the condenser is heatsoaked, as soon as the compressor clutch engages it disengages and this continues until I've driven some distance until maybe the condensor has cooled a bit and some refrigerant has been shifted. It was even worse until I let some refrigerant vent to atmosphere.. I could probably improve it by venting some more refrigerant but don't want to risk breaking it altogether while I'm on holiday.

Wouldn't think you could have that much pressure in there Gilbert or wouldn't some high pressure switch prevent the AC compressor engaging? As seems to be the case on mine since I overfilled it.

Well, seeing as the only thing worse than an undercharged ac system, which is merely annoying, is an Overcharged one.......
You may well do yourself a favor by letting a bit more out.
The primary issue with overfilling is having liquid refrigerant on the suction side of the system. This will cause compressor "Slugging" which is when the compressor sucks up liquid, which, of course is a bit tough to compress........it can cause a pretty catastrophic failure of the compressor.
The field method of checking this without guages is to look for the suction line (Fat one on the compressor) to be sweating, or worse, frosted up to the compressor.
In normal operation it will be relatively cool to the touch, but not so cold as to give you frostbite! Actually, as it is not jacketed, you can follow the suction line as far back as possible and at no point should it be frosty. If this is happening with yours, let a bit of gas out at a time until, with it on High with doors and windows closed, you no longer get frost, or if it is dry out (Low humidity), it should never be uncomfortable to hold for an extended time at any exposed portion.

That's interesting as I've always confirmed the system is working by looking for condensation on the fat pipe. On a reasonably hot day, with standard UK type humidity, when the compressor is running the fat pipe will be covered in condensation and too cold to touch, while the thinner one will be too hot to touch. It's always been like that. I also get quite an impressive stream of condensed water coming out from under the car, to the point where I've stopped at a filling station and had people tell me I've got a water leak. The hose I have for filling from a can fits on the low pressure side and has a pressure guage so I'll connect that and see what sort of pressure is in there before starting it up and after it's been running with the AC on. As my partners Merc has recently been gassed and is working fine, I can check the pressures on that to compare them and see if mine is reading way different.

As already said... the pressure should equalise when the compressor stops. I guess the guy presumed there was nothing in it, connected the gauges up and saw the falling pressure hopefully before he put any nitrogen in. Still doesn't sound like they knew what they were doing.

The P38 system does remove a huge amount of humidity, though both my BMWs have still been better at actually cooling at idle. The P38 definitely likes the engine revving a bit higher, and a lot of airflow over the condenser. That's been my only issue going to an electric fan - on a hot day from start up, I'll often get the two front fans going at high speed for a bit.

If you find the pressure isn't stupidly high (it can't be too bad or the trinary switch would kill it off), I'd clean any leftover traces of oil/dye on the pipework and see if it reappears. As and when it stops working, I'd get a specialist to do a decent nitrogen test on it. I really should sort my regulator out... of course I won't remember to before I next have a problem myself!

He didn't even bother connecting the gauges, he just connected the Nitrogen bottle to the larger of the two ports. I couldn't see what the gauge on the bottle was reading as I was standing on the other side of the car, but he did look puzzled. I suspect he was seeing pressure before he'd opened the valve and couldn't understand where it was coming from. I had told them that it was working, just not as well as it had in the past so he should have realised there would be pressure in it. They did the same on Dina's car but that was completely empty so it wouldn't have mattered. Bunged some Nitrogen in to pressurise the system, closed the valve and left it for a few minutes to see if the gauge dropped and when it didn't, took the Nitrogen hose off and connected their machine to vac it out and refill it. That was at the same place but a different person, the guy that did that one didn't seem to be about when I took mine in.

Gilbertd wrote:

You've just confirmed what I was thinking and no, they weren't just going to do it, they actually did do it so God knows how much pressure I've got in there now. When they told me it wasn't holding pressure the first thing I said was that the pressure on the high side would be dropping as it cooled down and the high and low sides equalised but they insisted that the only way the pressure could drop would be if there was a leak. But, at least it is still working, I was concerned that I'd go in with a working, albeit not as well as it could, and come out with an empty and not working system. I found it hard to believe it has a leak as it has been working fine for at least 2, if not 3, years.

It isn't just UK, it's an EU thing, you can no longer buy R134a to refill yourself. You used to be able to get a can of the stuff and a hose to fill with, now all you can buy is the hose. I've got one from when you could buy the gas and one to fit older systems that used R12 (but I can't get that now either). I've found a supplier in Italy that internittently, and illegally, will sell 500g cans for export only but no regular supplier.

Ahem

https://www.halfords.com/webapp/wcs/stores/servlet/ProductMobileDisplay?catalogId=10151&langId=-1&categoryId=165632&productId=812353&storeId=10001

Although they describe it as R134a, it's actually an organic R134a substitute. But at £60 a pop, you'd be better off getting the system evacuated and filled properly rather than dumping an unspecified amount of something in there.

Indeed you would. I would rather it be done right too.

Not really sure just what dry nitrogen would do in the system being rather stable temp wise, but it probably does not help.
I used to put these things on all the systems I built waaaaayyyy back when. They take all of the guess work out of AC charging as you simply run the system and keep on adding small sips of gas till the liquid stream is bubble free. As a bonus, it will show moisture in the system.
In olden times, virtually all US cars had a sight glass on the top of the drier. No more, alas.
I may just add one of these buggers to mine when I get my service kit where I am.
I even found an Eurocentric link for you all!

https://www.parker.com/literature/RAC%20Division%20-%20Europe/RAC%20Division%20-%20Europe%20-%20Literature%20Documents/Catalogues/Sight%20Glasses/RACE%20Catalogue%20Sight%20Glasses%2070-10.pdf

Oh, yea, I have played with R-600 (Dry Isobutane) and plain old LPG as R-134 replacements.(Use a LOT less) They actually outperform the Dupont patented, legislated products!
Disclaimer: Do not try this at home, I used a disposable car for the testing...........And it is still just fine 13 years on............

I think it was the old, pre-94 R12, cars that had sight glasses, I've not noticed them on anything more modern. I've got a 91 Maserati lurking in the garage that, now I've got rid of one P38, Dina's daughter's Nissan Micra and the motorcycle that was blocking it in the garage, I've the space to get stuck into that. The high pressure hose from the compressor nees to be replaced as a previous owner had neatly tie wrapped lots of hoses and wiring but unfortunately this hose had been tie wrapped in such a way that the back of the PAS pump belt wore through it. Before I found out why it was leaking, I filled that a couple of times. I used to be able to get 1kg cans of Isceon R413A which was a drop in replacement for R12. I would sit the can on a pair of postal scales and open the valve with the engine running until the required 750g had gone in. Unfortunately, R413A is no longer manufactured as it is alledgedly ozone layer depleting, so the replacement for it now is R437A butI've not been able to find it in anything smaller than 13 kg bottles, a bit overkill when all I need is 750g. I suspect that might end up getting LPG as I've got an awful lot of that available......

Here is a pretty good description of how to do it:
http://www.possumliving.com/2012/08/propane-refrigerant-for-automotive-air.html

I have used the Enviro-safe product as well, and found it to work just fine.
Incidentally, R-12, though it does really screw with ozone in the upper atmosphere, was finally "Banned" when Dupont could no longer renew the patent on it, thus could no longer charge their world wide licensees lots of money to make it.
Guess who instantly had a less harmful R-12 replacement? Yep! Those clever little monkeys at Dupont!
Golly, what a coincidence!
As this guy says, the Germans have been using R-600 (Isobutane) for ages.
Lowers the energy consumption on a home fridge by about 20% versus R-134a (From my own testing with Enviro-safe)

I used Johnsens Freeze 12 in my 93 Classic and it worked perfectly, but, like most things these days, I can no longer find a supplier.

OK, so I've just put my cheapo charging hose with a built in pressure gauge on it. Connected to the low pressure side I've got 4 bar with engine running but the compressor not engaged, with compressor clutch engaged at idle it drops to 3 bar (which, the colour coded scale on the gauge says, Full) and if I rev the engine up to 2,000 rpm it drops further down to around 1.5 bar. Is that about right?

Thanks for the reply om 31st July Bolt, yes that's what I suspected. Now I'm back from holiday I'll keep letting some out until the surging stops, then let a bit more out.