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1984 500SL (RHD) 160k miles
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188 Posts
Discussion Starter #1
I'd quite like to get my AC working as it's hot where I'm going.
The system hasn't been used for 14+ years, I think because it was never recharged. When I operate the knob on the console (mine has the sliders L + R, a knob for the fan (I-II-III-DEF) and a knob for the AC with a blue ring of increasing thickness, so it's none of the units in the EG).
I will get an AC guy to check for leaks and recharge it, but before I do that are there any checks/things I can do to see if there's any liklihood of it working? I'm particularly thinking the electromagnetic clutch on the compressor.

The system is completely empty of gas, although I did notice that pressing the needle valve after a long journey in hot weather produced a hissing sound, so some pressurised air/vapour/something was being held (i.e. maybe no major leaks).

I have operated the control knob and I can hear the actuators working.
I suppose if the clutch is working then the engine RPM should drop when engaged, even with no gas in there? Haven't checked this yet.
Couldn't see the AC drains from inside the heater plenum, but I'll jack the car up anyway and make sure the drains are clear from underneath.

Any other suggestions?

And when I get the guy to refill it, which of the needle valves should be used? The one near the radiator (LP Line?) or the one further away (HP line?).

Cheers.
 

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1981 380SL
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256 Posts
I just did the same thing, and am happy to report that as I was charging the compressor magically came to life and the system works great!!! Your AC compressor is one of the few electromechanical components not made by Bosch (it's a GM A-6) . . . and that's a very good thing. They're durable.

If you get a chance, you can reach down and give the portion of your AC clutch that is disengaged from the belt a turn. It should turn smoothly with light resistance. That will tell you that your compressor is not seized.

Manual controls!! You're lucky. After I got the refrigerant portion working, I then had to move on to replacing several vacuum pods on the infernal ACC-II.

These cars have an idle speed computer, so there actually isn't much RPM drop when the compressor engages. But if you listen, you can definitely tell that there is additional load.

If the AC place knows what they are doing, they'll be connecting an HVAC gauge set to both valves, the high side and the low side. Vacuuming and filling are done with the gauges in place.

You should do an R-134A conversion while you're at it.
 

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1984 500SL (RHD) 160k miles
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188 Posts
Discussion Starter #3
Thanks! Will try the clutch. Any idea is what the procedure is for refilling the system? I may be taking it to a non-merc A/C guy (basically whoever I can find here in middle-of-nowhere central france) and I'd like to know what he SHOULD be doing in case he doesn't.

Thanks again, that's super helpful advice.

Cheers.
 

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1981 380SL
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256 Posts
Procedure is standard. Any qualified automotive HVAC person will be able to service the refrigerant charge.

1. Attach gauge set to high and low sides.
2. Evacuate system to 29"Hg of vacuum (or whatever measurement you use in the UK). If they are obeying the law, they'll evacuate with a refrigerant recovery machine as it is illegal to vent R-12 to the atmosphere. I like to let the car sit with a vacuum overnight. This proves there are no leaks and also gives the dessicant in the dryer a chance to dry out. However, someone doing this for a living won't have time to do that. Still, the longer it's under vacuum the better. You open both valves on the gauge set while vacuuming to vacuum both the high and low sides.
3. Shut off the high side valve on the gauge set, start the car and turn on the AC with the blower on high and windows down. Note that the compressor should NOT run. That tells you that your refrigerant low pressure switch is working. Begin filling the system with refrigerant through the low side.
4. Eventually, the pressure will get high enough that the compressor will engage (intermittently at first and then continuously). At this point, monitor the gauges and run the engine at 1500-2000RPM. Fill the system with refregerant until a specific low side pressure is reached. The pressure target depends upon the refrigerant and the outside air temperature. Also, monitor the high pressure side to make sure that the compressor is operating properly and there are no blockages in the system.

That's it. Enjoy cold air. Sounds like a lot, but it is actually simple and well within the DIY realm.
 

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1981 380SL
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256 Posts
BTW, we've actually been having an exceptionally mild Summer here in North Texas. We've even had rain!! My theory is that charging the 380SL AC system caused the mild weather.
 

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1984 500SL (RHD) 160k miles
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188 Posts
Discussion Starter #6
Many thanks. I checked the clutch and it's not seized, so that's something. I would be happy to do the refill myself, problem is I don't have any of the gear, and I think it'll probably cost me more to get the right gear than it will for a guy to do it. I've done everything on the car so far, with the exception of installing a new windshield and balancing the wheels.
Is there a cheap DIY kit you can remommend?

thanks again for the info. The UK is continuing to have the shittest summer on record, with barely a dry day above 20C since april, which is why I am in France and heading to Germany!

Cheers.
 

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1981 380SL
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256 Posts
I have a quality gauge set that I use often on my various rides. Cost about $100. It can do both R-12 and R-134A. Got it from Mechanics Tool Supply (Welcome to Mechanics Tool Supply - Discount Mechanics Automotive Tools).

I also have an air-operated vacuum pump because I have an air compressor. I like it. Less expensive, reliable (no moving parts) and draws a great vacuum. About $35 from same place.
 
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