aircon troubleshooting - compressor clutch

[br1anstorm]

It makes me nervous when the aircon doesn't work - both because of the warnings that work on MB aircon systems ain't cheap, and because I still have, and don't want to abandon, the original R12 system in my 1990 560 SEC.

Until recently the aircon in my car worked fine, but the car has been garaged over the winter. I've just started running it again, and the aircon doesn't work. The compressor clutch isn't engaging.

As there are few cars with aircon where I live (the north of Scotland) and even fewer aircon service technicians, I need to do some initial diagnosis if I can. What should I check, and how, to establish why the compressor clutch isn't engaging? I assume that the most likely explanations are

(a) some electrical problem - switch, fuse, or wiring (incidentally my car has the two rotary controls, not the row of pushbuttons);
(b) low gas pressure - so a safety cutout is preventing the clutch from activating; or
(c) some mechanical fault with the compressor-clutch itself.

Apart from checking the fuses, how can I test and in what order for possible electrical problems? And is there any way I can check - I have no specialist tools or gauges - on whether the refrigerant is low? I desperately want to retain and preserve the R12 setup if I can, so I'm hoping that this problem is not one which requires complete conversion of the system to R134a.

One other possible clue (but it might be a red herring...) . There are traces of an oily leak on the ground under the car in the area (front left) under the aircon compressor and the power steering and other hydraulic reservoirs. I had assumed that this was indeed most likely to be power steering fluid. But could it be aircon related? I had always assumed that any leaking refrigerant would simply vaporise and disappear into the atmosphere (hence the ozone-layer problem!)

Any advice on basic troubleshooting using essentially eyes, hands screwdriver and a multimeter would be appreciated. Links to tech advice sites, or even better, some pictures, would be even better!

br1anstorm

 

[brandwooddixon]

I'll admit that I don't know the prticulars for your system but the causes that you raised are valid.

Having stood for a year it's most likely a refrigerant loss due to a leak around the compressor seals.

The oily patch could be an indication of this, as there is a lubricant added to the refrigerant I believe.

Most MB garages don't perform the aircon service themselves. They tend to call in outside contractors as the H&S people have made handling these gases an expensive undertaking.

Best bet is to call several dealers in your area and ask them if they use outside contractors for aircon and if so who.

As for fluid level I think that most systems of this era were fitted with a sight glass. If the system is full you should see a fluid level.

 

[Richard Moakes]

I think the problem is most likely to be (b) i.e. you have lost refrigerant pressure and the low pressure safety cut-out switch has stopped the compressor from engaging.

The oil may well be from the compressor, a certain amount of oil is always in the system to lubricate the compressor and all other parts of the system.

I regret to say that you will probably now have to convert to R134a, which will mean the system needs flushing, new and different oil adding, and new refrigerant gas, and in the worst case new seals may be required. There are alternatives to R12 and R134a, but they are controversial, possibly illegal, and many aircon service people don't deal with them any more.

The sight glass is only of use when the system is running, the appearance of bubbles or steady fluid stream helps in diagnosing faults.

Good Luck,

 

[br1anstorm]

Thanks for the replies so far - which tend to confirm my suspicion that the cause is low pressure and/or possible refrigerant leak.

Does the 560 SEC have a sight glass, and if so where is it? I'm used to checking other cars' systems this way for signs of froth/bubbles in that little window, signalling the need for a recharge. But I can't see where that little viewer is on the SEC - if indeed it has one...

[edit - I've just re-read the earlier replies and realised that of course the sight-glass check won't help if the compressor isn't running and circulating the refrigerant!). So I guess the next step has to be finding someone with the necessary pressure gauges to test the circuit.

I was trying to do initial diagnostics because aircon technicians are a bit thin on the ground this far north - so i'd like to be able to identify the problem as precisely as I can before I ask someone else to solve it!

br1anstorm

 

[wireman]

Connect a lamp across the clutch coil that will indicate if there is a signal to turn on the compressor. I doubt that you will get any service people to use R12 its no longer legal to do so throughout europe. Its not the H&S mob but the green brigde that have made it so.

 

[Richard Moakes]

Sorry, don't know my way around a W126, but the sight glass is usually part of (visible from the top) the receiver dryer unit, which looks like this...

 

[br1anstorm]

Connect a lamp across the clutch coil that will indicate if there is a signal to turn on the compressor. I doubt that you will get any service people to use R12 its no longer legal to do so throughout europe. Its not the H&S mob but the green brigde that have made it so.

I know that R12 systems are no longer made. But as far as I know it's not illegal to own, run or maintain an existing R12 system. So I'm applying common sense: if I can keep and continue to use my R12 setup, and ensure it doesn't leak, I'm not contributing to ozone layer destruction nor global warming. Surely a service person can check/test/top-up an existing system?

Meanwhile, I'm getting out my meter to check relays and wiring...

 

[Richard Moakes]

I know that R12 systems are no longer made. But as far as I know it's not illegal to own, run or maintain an existing R12 system. So I'm applying common sense: if I can keep and continue to use my R12 setup, and ensure it doesn't leak, I'm not contributing to ozone layer destruction nor global warming. Surely a service person can check/test/top-up an existing system?

Meanwhile, I'm getting out my meter to check relays and wiring...

You are absolutely right, if the system is gas tight, then running an existing R12 system is environmentally sound and perfectly legal.

The problem is the availability of the gas for a top-up, most A/C service people don't top-up any more, and would need to completely remove all the gas, put the system under vacuum for leak testing, pressurize with Nitrogen for leak testing, and then if no leaks are detected, then refill with the correct weight of gas. Now, a few years back, this was done with a separate vacuum pump, recovery cylinders, new gas cylinders etc..., these days it tends to be done by an all-in-1 machine which is only equipped to deal with R134a as you cannot mix the refrigerants, and you need different gauges and oil for R12 and R134a.

What you need is an 'old-school' aircon engineer, not one of these guys who have done City and Guilds training, bought all the modern automated equipment and travel round in a van.

I WILL get flamed for this, but there are R12 substitutes, even talking about them gets some people hot under the collar, take a look here http://www.autofrost.com/ I make no recommendation as to their use, and have never used them myself.

 

[bigasotonuk]

I know that R12 systems are no longer made. But as far as I know it's not illegal to own, run or maintain an existing R12 system. So I'm applying common sense: if I can keep and continue to use my R12 setup, and ensure it doesn't leak, I'm not contributing to ozone layer destruction nor global warming. Surely a service person can check/test/top-up an existing system?

Meanwhile, I'm getting out my meter to check relays and wiring...

Richard is quite correct, I believe it is actually illegal to supply this refrigerant any longer.

 

[br1anstorm]

But just for the sake of argument (or clarity): if perchance I happened already to have a spare can or two of R-12 for topping-up, then surely no reason why I can't use it - or have a technician check the system and put it in if required?

 

[Richard Moakes]

But just for the sake of argument (or clarity): if perchance I happened already to have a spare can or two of R-12 for topping-up, then surely no reason why I can't use it - or have a technician check the system and put it in if required?

If all that has happened is a gradual loss of pressure, rather than a seal failure, then yes you could top-up with R12, but you will need to judge the gas charge the old fashioned way by using hands on the suction and high pressure hoses, and feeling the temperature differences etc...

Good luck with it

 

[br1anstorm]

An update. Some progress. Found a local a/c technician with experience and all the gauges and kit (an ex RAF engineer now with his own business).

Turns out the system is virtually empty of refrigerant, so topping up not an option. Pressure test (using nitrogen) suggests the tiniest of leaks somewhere. Difficult to trace, as the obvious method - refill with R12 plus a dye for leaktracing - isn't an option without a supply of R12.

So it looks as if I'm forced into having to go for a conversion to R134a (new accumulator/drier, drain the compressor, vacuum out the system - expensive even without changing all the hoses!). But yet it seems unwise to go ahead with conversion without nailing the location of the leak. Catch 22...

br1anstorm

 

[br1anstorm]

aircon leak - progress?

Some time since I started this thread. More testing reveals good news and bad. Now I seek more advice.

The good news is that with the help of my tech friend's equipment we have narrowed down the source of the leak, by using a sniffer. The bad news is that it isn't anywhere in the engine bay, but somewhere behind the dash. My SEC is a LHD euro-spec, and the leak is in the vicinity of the driver's side dash/footwell.

Before I start trying to dismantle the dash and console, which is a daunting prospect, can anyone describe or explain where the pipes run and the connections and expansion valve might be located on an LHD vehicle? I'd like to know what panels or fittings to remove in order to see and check on the most likely places for the leak - which may be a hose-connection rather than within a system component.

br1anstorm

 

[Andy Wr]

Just picked up on this thread, sounds like it could be a leaking evaporator coil, I thought the expansion device was on the bulkhead in the engine bay on these. Is the air con system an original factory fitted item or an aftermarket item as evaporator coils may not be avvailable off the shelf but could be manufactured to the pattern of the old.

Regarding gas charging, dont go the R134a route, recharge with R49, that is a "drop in" replacement without changing the oil in the compressor or flushing the system.

 

[br1anstorm]

Andy - I have no intention of changing over to R134a. Problem is, before the system is recharged with anything, I need to pinpoint the source of the existing tiny leak. I'm hoping it's in a hose leading to the expansion valve and/or evaporator (or in a connection at one ot the other of these). Both components are, I believe, on the passenger side of the bulkhead/firewall. Oh and by the way the a/c system is original factory-fit when new - and the car is LHD.

At this stage, all I need to know is what else I have to remove in order simply to see (or stick a sniffer close to) the relevant hoses and connections. Ideally, the sort of diagrams, pics or advice that you get in a Haynes manual which show how to access the parts you want to look at!

 

[star]

from what you say, the leak is more likely in the evaporator, in the heater box. stick you sniffer down one of the outlets to see, or look at the drains above the gearbox if a dye has been added. good luck with the R12, i dont think its legal to supply anymore.

 

[br1anstorm]

aircon troubleshooting - it's the expansion valve

Thanks for advice/comments so far. The good news (!) is that the leak in my system seems NOT to be the evaporator (sigh of relief) but at the expansion valve. We've manged with the help of a sniffer to locate it precisely without tearing the dashboard to pieces. So the problem is fix-able.

Now for the next question. Can anyone suggest a supplier/source for new 'O' rings for the expansion valve connections? Do I have to go to a dealer, a specialist aircon supplier, or eBay? And - though the rings are obviously the same size - are those for R12 systems made of a different material to those for R134a systems? I've seen both black and green 'O' ring seals: how do they differ?