[W126 Coupe] A/C conversion

[Dick Spellman]

Hello Hein:

This is what a pro shop recommends:
Evacuate the system of the former R-12 refrigerant
Replace the Receiver Dryer (this comes with new O-rings)
Confirm that both Receiver Dryer Sensors are working properly and replace if defective
Replace the ETR valve
Disconnect the AC condenser and flush with AC flush and clear with nitrogen
Add and set to correct thread depth the new R-134 Schrader valves on both the high and the low side fittings
Reconnect all system joints using new O-rings and Nylog sealant
Using a vacuum pump, vac the system down and let sit for an hour or two to see if there are any leaks and to remove any lingering moisture from the system
Set-up your gauges and add 80% of the former system capacity of R-134A.  Which in my 300SE systems (I've done 3 now) was 2.2lbs (35.2ounces); 80% equals 28 ounces of R-134.  Check your gauges while filling to obtain and stay within the pressure thresholds for operation

Now, in reality, here is what I do:
Evacuate the R-12
Replace the receiver/dryer and both sensors (they are cheap enough, so I just do them)
Inspect the condenser line to see if there is any gray colored oil - if none connect it up to the receiver/dryer with no flush, if there is some gray oil use the aerosol flush with hose attached ( I can give you the part # and source to order) and flush the condenser out, followed by a tire valve pressed in place and a shot of compressed air to clear the flush agent. (I do not have access to nitrogen).
If the compressor had no obvious leaks at the manifold hoses, I proceed with the vacuum pump, let it sit for a few hours (not necessary, 15 minutes is probably good enough, but I figure if there is any free moisture or a leak in a seal it will show up given a little more time)
Then hook-up your gauges add the first 16 ounce can of R-134A plus UV Dye followed by most (watch your gauges, I don't use a scale) of a second can this one with R-134A plus Sealant.
I use Nylog refrigerant system sealant on every connection and O-Ring I replace.

So, if you had a tight system it's replace the receiver/dryer, add your new R-134 Schrader valves (2), vac it down, add a little under 2 cans of R-134A and you are good to go.  Other subtle issues will be your cooling systems ability to cool, the viscous fan clutch age and ACC vacuum elements condition so all that nice new cold air is not just flooding your windscreen through the defrost vent instead of the dash vents or being diluted because the fresh air flap diaphragms (2) are letting way too much outside air in to the cabin on a hot day.  I've added a relay on one of the 300SE's so that whenever AC is on the aux fan is on.  On the other's this was not needed.  I've replaced the rubber diaphragms on all of them where they seem to leak after 10 years or so of operation making the air delivery and ratio of outside to inside air correct once again.

One note on screwing down the new Schraders, place a little nylog sealant in and around the valve so that as you tighten it down you know you are seated with no leaks and that you have not gone too far causing the system to release the R-134.  In other words, it's a fitting that requires you get it just the right depth.

I've done compressor R&R on the 380SEC with a new GM A-6 and rebuilt just the head seals on a Nippondenso using Mike Ramay's advise with a $10 kit that provides new seals and O-rings.  I'm not a pro just a learning DIY'r and who knows if I've got it all perfect but, so far I am happy and on year 3 with 1 of the 4 MB's I've done and year 2 on 1 other.  I've converted 2 this year both 300SE's.

If you've got questions, drop me a note or just call me at 508-641-8477.  I am in Boston where today will be a scorcher but, I am cool running in the MB's.

Dick Spellman
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