300SL odd air con problem

[Jubag]

Hello, I have a very nice 1990 R129 300SL. I just had it regassed today and all went well, holds gas etc. Only problem is the compressor only works if you put a 12v feed onto the blue and green stripped wire on the compressor itself. The only other time the compressor will run is if you have left the car off for some time and you start it up, for about two seconds the clutch will engage and then trip out.

I think I'm being a bit hopeful, but does anyone have a wiring diagram for a 300SL, or maybe able to point me in the right direction?

Is there a relay for the air con? If so where is the blessed thing?

I have checked the compressor fuse, that's fine.

[r129sl]

Bad pressure switch?

[Jubag]

It's odd because I've checked for power to the pressure switch and there is nothing there, all so the switch has continuity so I'm guessing that the circuit is closed because there is gas in the system.

Are there any wiring diagrams anywhere?

[jimdollar]

Try this site http://www.sf.nexedge.com.my/129/etm/webetm/129/st... or join the benzworld.org forum, acres of info on there, hth, Jim.

[Jubag]

jimdollar said:

Try this site http://www.sf.nexedge.com.my/129/etm/webetm/129/st... or join the benzworld.org forum, acres of info on there, hth, Jim.

Absolutely spot on Sir! Thanks for that info. 👍🏻

[alabbasi]

Check the pressure, it overcharged, the system will prevent the compressor running. Also check the pressure switch and relays are functioning properly.

[Jubag]

Hello! Thanks for the advice, in the end I just ended up rewiring the main circuit with a standard relay and a hidden switch. Works a treat!

[alabbasi]

Best of luck. The pressure switches are fail safe devices that protect the compressor from damage in the event that the pressure gets too high or too low. The compressor also needs to cycle in order to prevent the evaporator from freezing up. You might run into issues down the road with this bypass.

[Zonergem]

A failed evaporator is a BIG deal.

https://youtu.be/A90TwLDsguU

The gag on Benzworld is that Mercedes hung an evaporator on a piece of string, then built the car around it. Getting to it is one thing but putting everything back together to factory-spec without breaking much or starting a million new rattles is a big challenge.

I believe Mercedes used some O-rings and other parts in their cars which don't get along well with R134a, so failure is quite probable. Probably not the end of the world in the UK for a sunny-day convertible, mind.

[alabbasi]

On W123's at least, it seemed to be the brazing on the older evaporators on early cars that did not like R134a.