We have had unusually warm weather here in the SF Bay area the last 3 days - 100F days, albeit without the great humidity typical of other parts of the US.

The past weekend I had to replace the condenser in my 1990 560SEL as I had twisted the high fitting off the old one in attempting to replace the R134a valve - the old one was leaking, and I did not do a proper job of getting a counterhold on the base when removing it. In the event, even when twisted off (the condenser tubing is soft copper) I could not separate the R12 Schrader base from the R134a top, so maybe it was meant to be.

Mike Tangas kindly pointed out that the high pressure fitting is on the condenser (unlike on the 1983 300SD where it is on the pipe to the condenser). Thus I had to replace the whole condenser, reckoning repair would cost almost as much as a new one. The replacement is a standard German AGM, without fancy parallel flow technology as far as I know. In contrast to the diesel's condenser, this one is extensively shrouded around its edges with rubber strips, maybe enhancing cooling air flow and aiding condensation.

In the process I learned there are two kinds of R134a fittings used to adapt the R12 bases. The type used in the 'death kits' which screw over the old R12 Schrader and have a 'pass through' plunger that activates the original Schrader valve core. Mine had this and the pass-through part was getting stuck, leaking refrigerant. The other kind, sold by ackits.com, has its own Schrader valve with valve core built in, necessitating the removal of the original valve core (you use a special cylindrical slotted screwdriver to unscrew these - I got mine at a Napa store); the replacement will then screw over the R12 male thread. It appears to have Red Loctite on it so once on it will be very hard to remove.

After replacement (as my web site suggests, the twin fan-fronted condenser is an SOB to replace compared to the single fan model in the diesel), appropriate oil injection, etc. and a 30 minute vacuuming, I recharged it with the factory specified 2.1lbs of R134a (almost 3 12 oz cans at all of $5 each at Target).

Driving in the 100 degree weather (I have no tint on the windows to improve thermal efficiency) I am being frozen out of my car on the freeway if I crank the a/c up. After a cool down period the fan can be set to low and I get adequate cooling. My on-board thermometer's reading of 100F was confirmed with an outside independent reading. Plus even my border terrier's nose was warm and his tongue out some 1"-1.2" at rest (he wouldn't let me get the dial caliper on it so I had to estimate), a sure sign of trouble.

So, R134a can work well even in (some) older cars. The system is certainly subjectively more efficient than that in the diesel (also with a new condenser, this time also with a new expansion valve and receiver/dryer), and as the engine coolant never rises above 87C in either car, the auxiliary fans never come on and are, hence, not a variable in the comparison. (One in the diesel, two in the V8).

I'm wondering whether the A6 compressor in the 560SEL is the cause of the greater efficiency; the diesel uses an R4.

Anyway, though this might be of interest.