R129 AC compressor won’t disengage

[Paul Bee777]

Hi guys, I’m new to the forum but I’ve owned my 1991 500SL for about 10 years and I’ve got an issue that I can’t fix which I’m hoping somebody might be able to assist with. Basically the AC compressor is continuously engaged unless I pull the fuse that powers it....in which case the clutch disengages immediately. This means that the after about 30 minutes driving the condenser ices up and blocks air entry to the heating system....which isn’t ideal! I’ve installed an “interrupter” switch in the car which operates across the fuse to switch the compressor off as a “fix” but I can’t work out what’s causing the problem. I’ve replaced the interior air temp sensors by the air intake and the one in the mirror, the heater control panel, and even swapped the NAS unit but unless I mechanically wedge the Siemens relay in the NAS open (opened the NAS unit so I could check the relay wasn’t arced closed...but it seems to work fine) when you start the car the relay operates and the compressor remains engaged even with the EC switch on, the blower off etc etc. It’s doing my head in....any ideas? Thanks

 

[mattkh]

Without too much hassle, can you give some details of the compressor.
Basically, one would expect the clutch to disengage when
1. the pressure goes low
2. the required coolness has been achieved
How do you set the coolness demand on your car ?

 

[mattkh]

Afterthought!
A/c Relay. Have you checked it ?

 

[Paul Bee777]

Without too much hassle, can you give some details of the compressor.
Basically, one would expect the clutch to disengage when
1. the pressure goes low
2. the required coolness has been achieved
How do you set the coolness demand on your car ?

I’ve tried every setting on the heater control system from hot to cold, with blower on/off EC switch on or off......the heater system works fine but the compressor keeps running constantly which eventually ices the condenser/air entry to the car and also makes the “cold” air coming from the centre vents (when the blue button is selected) extremely cold (sometimes ice forms on the vents!)

 

[Paul Bee777]

Afterthought!
A/c Relay. Have you checked it ?

Where’s the AC relay......I know there’s one in the NAS unit.....is there another one somewhere?

 

[Paul Bee777]

Couple of terminology corrections......I meant the evaporator ie the matrix that does the cooling rather than the condenser...and I also got NAS wrong.....should have said MAS control module.....doh! Apologies

 

[mattkh]

MAS (N16) 3E 9A 22A seem to be relevant .
Try cleaning those contacts if you can get to MAS (N16).

Sorry, I have just read your original post again. You may ignore the above.

I guess that leaves the compressor, which you have tested OK in terms of the clutch engaging/disengaging.

 

[Botus]

sometime after 2000 most went to a variable compressor, where is always on but its load varies controlled via a swash plate

earlier cars had one rate that must switch the clutch in and out.

(worth remembering at this time of year checking what's happing will be interesting as they should all cut out below 3C to prevent condenser freeze up due to wind chill)

with those thoughts in mind I would think either:

A) the control circuit is not switching with something like a temp senor fault
B) faulty relay means its always on

either of, is keeping the clutch in regardless - it ought to be something simple as at least its all working

 

[Paul Bee777]

Thanks Botus, it’s really doing my head in. I tried another head unit (the thing with the controls in the cabin) a few years ago...but no effect. I have replaced the cabin air temp sensor in the mirror housing and also the temp sensor just after evaporator......with no effect......are there any others that influence this system? Also recently tried another MAS (from a breaker) which stopped the compressor so I thought I’d cracked it.....but after some checks it was clear that with this “new” MAS the compressor never engaged. So I stripped the MAS box down and the small Siemens relay that switches power to the compressor had obviously been arcing/burnt out so wasn’t engaging at all.....ie just like taking the fuse out I guess! I’m reasonably handy with a soldering iron so have located a replacement relay and when that arrives next week I’ll install it in the “new” MAS...the hope being that my old MAS has a working relay but a defect in the control circuit so if the control circuit is ok in the “new” one then maybe with a new relay I’ll have an operational compressor that only engages when it’s needed rather that running continuously and freezing up the whole heating AC systemHowever after the hours that have been spent trying to sort this I have to say I’m not 100% confident so all suggestions gratefully considered.

 

[Botus]

contactors (big relays) can fail burnt On in big boy stuff, not really seen it on cars, but possible I guess

 

[mattkh]

.....................small Siemens relay that switches power to the compressor....... all suggestions gratefully considered.

May I have the code/number/spec of this Siemens relay please.

 

[Paul Bee777]

Here you go; I’ve included the spec of the relay and the details of the vendor in Poland. They only took a few days to arrive.

 

[Paul Bee777]

Update to previous Post. I’ve now stripped the old relay out of the circuit board it’s a Siemens V23072-A1059-A208 5 pin relay, and it appears they contact when energised. The ones I got from the electronics guy in Poland are physically ALMOST Identical, reference number V23072-A1059-X002, also 5 pin, same dimensions etc but unfortunately they appear to be set up to disconnect on energising....so aren’t suitable. It may be possible to strip and rebuild the contactors to operate in the correct way but might just wreck the whole thing so better if we can find the correct units

 

[Wearsafoxhat]

Doubt re building the relay would work however it may be a simple case of changing one lead on the pcb.
Is it possible to take the lead off the N/C and put it on the N/O leg of the relay, or visa versa. The common would remain where it is.


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[Paul Bee777]

I agree that it’s not ideal and don’t want to mess around with the pcb wiring, fortunately after more googling I’ve found some of the correct type (see link) so waiting for these to arrive, I’ll report back when I’ve rebuilt the module and tried it in the car. https://electronic-studio.com/Relay-12VDC-on-2x-on-2-x-6A-A23072-A1059-A208

 

[Paul Bee777]

Finally solved the problem…..Long story but replacing the failed relay in the replacement used MAS unit didn’t make any difference….that box wouldn’t energise the AC compressor under any circumstances. Feeling rather dejected I decided I’d have to live with my original MAS that never disengaged the compressor and use my “interupter” switch (equivalent to pulling the main AC fuse out) to manually switch off the compressor when the evaporator started to ice up……however recent circumstances meant I had an opportunity to test another MAS unit from a car with working AC at my local Mercedes specialist and that worked in mine Consequently I got authority from the boss to buy yet another used MAS unit from eBay (this time from the USA), this arrived last week and I fitted it this morning….and BINGO…..I now have fully operational AC with a compressor that switches on and off automatically and also can be switched off manually using the EC button on the AC/Heater controls. I’ve now taken out my manual switch “bodge” and also swapped out all the old original 30 year old aluminium fuses……some were looking very badly oxidised and were so fragile they actually disintegrated as I removed them…….so I think a very worthwhile few £ spent. Car now 100% operational and AC very much in use in this heatwave! Finally a very big thanks to everyone for your assistance and support. Cheers Paul

 

[mattkh]

Very happy for you.
What would be different in the MAS unit from USA.??

 

[Paul Bee777]

nothing, except it was much cheaper than the ones in the UK and Europe even with import duty and freight