aircon troubleshooting - compressor clutch?

[br1anstorm]

I have a 1990 560SEC, with - original - R12 aircon system. It has the rotary dial controls, not the pushbutton ones.

Until recently the aircon 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; (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 quantity/pressure is low? And if it is just a compressor clutch failure, I've heard that it is possible to replace the clutch without draining the system. I'm not even sure if it's possible to find an aircon service technician or garage who will test, check and top-up an (old) R12 system. I desperately want to retain and preserve the R12 setup if I can, so I'm hoping that the 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 some pictures, would be even better!

br1anstorm

 

[KLP 92]

Probably not engaging due to low pressure. You need to check the pressure in the AC system and find out how much gas you have left.

There are plenty of small seals in the W126 AC system that leak air over time. Best bet is to check the comdensor doesn't leak and then renew the seals and convert it to R134a wheile you have the chance now.

 

[br1anstorm]

Thanks - I agree that low pressure is one of the more likely explanations. But how can I check or test (does this need an aircon specialist)?. In some other cars I've owned, it was possible to look into a little glass window in one part of the system and see whether there was froth/bubbles (which meant it needed recharge). But I can't see whether or where the 560 SEC system has any kind of inspection window...

br1anstorm

 

[Dieselman]

Behind the n/s headlamp is the drier which has a bubble window on top of it.

ITEM 35

 

[fredfloggle]

Thanks - I agree that low pressure is one of the more likely explanations. But how can I check or test (does this need an aircon specialist)?. In some other cars I've owned, it was possible to look into a little glass window in one part of the system and see whether there was froth/bubbles (which meant it needed recharge). But I can't see whether or where the 560 SEC system has any kind of inspection window...

br1anstorm

Won't really help you unless you cheat the compressor clutch.

 

[br1anstorm]

Thanks for the directions, Dieselman. But I fear my own brain wasn't engaged properly. I realised, as Fred has just pointed out, that unless the compressor is circulating the refrigerant, looking in the sight glass reveals nothing!

Looks as if I'll have to find someone with all the kit, gauges and fittings to check an R12 system. Could be difficult: it seems that many so-called aircon service people, and even my nearest MB official dealer, only have the equipment to deal with R134a systems(the connectors for testing or charging R12 and R134a are different).

 

[br1anstorm]

An update. Good news and bad news. The good news - I found a local a/c technician with all the gauges and equipment. The bad news - my car's system was virtually empty of refrigerant. Not really a huge surprise in an 18 year old car. Pressure test suggested there was a TINY leak somewhere: pressurised to 14 bar with nitrogen, it slipped down very slightly (13.5 after 10 minutes).

So now I have short term problem - how to find the leak? And longer term decision - to convert to R134a, which seems unavoidable. Tracing the leak is difficult because the logical step, of refilling with R12 plus dye, isn't possible without a supply of R12. And converting to R134a (change accumulator, drain compressor, etc - expensive business) without knowing where the leak is - and there are lots of hoses, seals, and joints! - seems to be moving to a solution before fully identifying the problem. Catch 22?

br1anstorm

 

[fredfloggle]

Did your guy not have a sniffer? Obviously this is only good where you can get access, but you may have some luck and find a leak in an accessable place.

 

[GRAV888]

More likely a pinhole in the condensor.
Have changed these on both my cars due to this.

 

[br1anstorm]

Sniffer didn't pick anything up, though access is a bit of a problem. Tried squirty stuff around all the visible joints and seals - again no sign of bubbles/leaks. Had a look behind the fans at the condenser for any traces of pinhole oozing fluid and/or oil. No sign (and it hasn't got any visible corrosion or flying-stone damage). Wherever the leak is, it is so slow, so small, or so inaccessible, that I fear dye may be the only way of tracing it...

 

[ventech]

The seals in the compressor will harden & leak unless the air con is switched on once a week during winter. Probably first thing is to get the compressor seals reapled before looking for other leaks

 

[jamesmc]

br1anstorm

Did you get this sorted? My car has developed the same problem. One day it was OK the next time of use (about 24hrs later).... air-con compressor clutch not engaging. It's quite useful here in Portugal too!

This week I'll try and get the car to a vehicle air-con specialist (one without £££MB in the name) and see what the outcome is.

 

[television]

br1anstorm

Did you get this sorted? My car has developed the same problem. One day it was OK the next time of use (about 24hrs later).... air-con compressor clutch not engaging. It's quite useful here in Portugal too!

This week I'll try and get the car to a vehicle air-con specialist (one without £££MB in the name) and see what the outcome is.

This may help a little


http://www.mercedesshop.com/Wikka/AcDiags

 

[jaymanek]

my car had the same issue, dont do what i did and spent £500 on a compressor as once id gone to fit my shiny new compressor i found the two rubber O rings where the pipes joing to the compressor were split... i could have saved about £499.98 if id just taken it apart before buying a new compressor.

The leak under the area is the oil in the system, so i would bet its the seals and nothing more.

Bad news is you will have to convert to R134a now... actually did the guy fill up with R12? thats illegal!

 

[br1anstorm]

br1anstorm

Did you get this sorted? My car has developed the same problem. One day it was OK the next time of use (about 24hrs later).... air-con compressor clutch not engaging. [edit] .

James

Haven't done any more on my system yet (I've been distracted by work on another of my cars...). If the circumstantial evidence of a leak somewhere around the compressor is any clue, I'm hoping the problem is simply leaky seals at the joints between the hoses and compressor, rather than the shaft seal of the compressor itself (which I fear would mean an expensive new compressor).

I'm still hoping someone will come up with a bright idea for how to leak-test the circuit of an R12 system without filling it with some snake-oil substitute. I want to pin down the exact location of the leak, and cure it, before I take the major step of changing the Acc/drier, seals, valves etc etc over to run on R134a...