My 1990 W126 (560 SEC) still has its original R12 aircon system. But it has a TINY leak. The gas is low - indeed almost empty; and a nitrogen pressure-test confirmed that the circuit was very s-l-o-w-l-y losing pressure. Can't see or hear where the leak might be. It must be a pinhole, or possibly an ageing O-ring or seal.

Now...I have a limited quantity of R12, which I'd like to use to refill/top-up. But I don't want to use or lose any of this supply in trying to trace the leak (eg with dye). Is there any other way of tracing the leak without using up precious R12, and equally important, without polluting the aircon system with anything which might be incompatible with an eventual refill - once the leak is cured - of R12?

br1anstorm

Usually leaks are detected with a UV dye...

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

Dye is the best way to go. You can buy a kit that includes the dye, an injection syringe, a UV light and glasses. Or just take it to a shop.

You might find the leak by visual inspection. Usually some compressor oil will leak with the R12. Check fittings with your fingers for oil. Look for dirt that has accumulated on the oil.

Forgive my lack of expertise (I'm not a technician). I realise that dye is probably the best way of tracing a leak. But my question is - what can you use to carry the dye around the system? The gas in my system (R12) has dropped so low while the car was unused that there isn't now enough pressure to kick in the compressor clutch. I don't want to hot wire the compressor clutch, because I can't be sure there's enough oil and gas circulating in the system to protect it from expensive damage.

So the question is - what gas can I put into the system, with the dye, to get the aircon running and the dye around the circuit to trace the leak, without spoiling it all for an eventual refill of R12 when the leak is solved?

You might want to look at Duracool. It is a drop in replacement for R12 and I have been using it for about 5 yrs. They also have a great sealant that will stop most small leaks. www.duracool.com

I detected two smalll leaks in my system by observing a slight oily residue.
One was around the coupling between the condenser and the hose for the last couple of years, then last week I found an oily area on the condenser itself - bummer.
These are easy to spot if they have been seeping for awhile, and the under- hood area is reasonably clean. Just start at one end and work through.
You may have to unclip the radiator and ease it back to see the whole condenser, or remove the front fans.
Of course, it may well be the evaporator; in that case, you're toast!

Oh, after observing the little oil residue on the hose coupler, I took it to the best AC shop in town. They dyed it, sniffed it right at the coupler, and found no problem. So, I cleaned the coupler to operating room specs, and sure enough, a month later, there was that little oily residue again. Must be pretty small.

I'm like you, I have 4 cans of R12 rusting away in the cabinet. Since I think I know both leak points, I'm gonna let it run until it gets low, then replace the condenser and accumulator, and use my stash of R12 to charge it up, and have one can left for later top-off. Then pray the evaporator or the hose or the compressor don't blow.

DG

Use the UV dye, did that on my 420 sel and found out the o rings are bad.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Thanks to all recent posters...

As regards Duracool, thanks but no thanks. I've read some epic threads here and elsewhere about the pros and cons of this and other alternatives. I'm with the purists: I don't like mixing different refrigerants, so I want to keep to "clean" R12; or if i really have to, flush the system, drain and change oil, change lots of parts, and go over to straight R134a.

Thanks also S-class Guru for pointing me in some directions to look for the leak. Only clue I have is slight oily residue on the garage floor under the compressor. Hard to tell where it might have come from - and as my system is in fact now virtually empty, there's now nothing to sniff unless the circuit is re-gassed with something. And that's still the key question.. Can I re-gas with something now that won't affect what is at present an R12 system (and if so what?), and add dye to check for leaks. Or do I have to get it all converted for R134a, first, with new drier etc, then go looking and testing for the leak...?

It would probably be cheapest to fill with R134 for testing purposes (ONLY) and then vacuum it all out, repair the leak, and refill with R12 and the proper compressor oil.

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

Well, it's a while since I started this thread. Some progress, some bad news, and now a request for more help.

I got some help from an aircon techie friend with diagnostic kit. The system is virtually empty. A pressure test confirms that there is a small (=slow) leak. A dye test confirms that it's not the compressor or any of the hoses/connections in the engine bay. Now for the bad news. A sniffer test reveals that the leak is somewhere behind the dash, as it picks up the leaking R12 in the driver's side footwell.

That's narrowed things down a bit. Now I need advice - and even better, some diagrams or pictures - which show
(a) where the hoses run, and especially where the joints are, under/behind the dash - since the leak is more likely to be at a joint or connection rather than inside a component of the system; and
(b) what panels and parts to remove in order to see and check the hoses, joints, and connections under there for the leak.

Why do I ask? Because I know that dismantling the dash/console etc is horribly complicated, and I don't want to mess with it any more than I have to. It would be crazy to take the whole interior apart only to find that a single hose-clip just behind a kick-panel is slightly loose! So I just want to gain enough access to see, sniff and check the most likely suspect areas. Can anyone offer help in showing me (a pic is worth a thousand words) what exactly to remove in order to pinpoint, and maybe cure, the leak?

br1anstorm

Well, the good news is that we've traced the leak to the expansion valve. So the problem is not in the evaporator (sigh of relief).

In case anyone is following this thread and has a similar problem, the best guide I could find to how to identify and access the expansion valve is at http://dieselgiant.com/repairyourac.htm . The pics show a W123, but the valve setup is virtually the same. On the W126 you remove the panel below the steering column, and the panel above the pedals - and the expansion valve is on the bulkhead above the accelerator pedal.

Now all I have to do is install new valve and O rings - and maybe new receiver/drier. Could have been a lot worse...

Definitely install a new dryer. It should not be expensive.

If it were mine, I would take the rest apart and flush the system completely. Actually, that's just what I did with the system in my W210, which needed a new dryer due to a restriction in the old unit. Lots of very fine debris came out of the evaporator.

It sounds like your TXV is a lot easier to access than mine. I had to remove the wiper system and some other stuff below it.