I wish I could get my 85 300SD to cool adequately. It had been converted to 134 by the PO at MB dealer. Car now shows about 115,000 miles. The compressor was noisy and only had 60 degees at the center vents so I read here about parallel flow condensers and went to a local ac shop for an update. Replaced the condenser with the largest parallel flow that the shop could make fit, new R4 compessor, dryer, Tx or whatever valve near the evaporator, rings, flush, evacuate, new oil and 134. Result - still 60 degree air out the vents. Shop said the only way they could get it colder was by running water over the new parallel condenser so they said they had done all they knew how to do. 13 months later the compessor blew out near or at the place where the lines attach. This time I went to a different shop for all of the above except the condenser. 60 degree air still with outside at about 80 degrees. I put a new monovalve in in case the heat was on, no temp change. I did find that completely closing the outside air door would get the center vent temp to about 50 degrees after 30 minutes or so on the highway. So, I envy you guys who have cold air using R134 or anything else. My benz cooling is so embarassing that I always ask friends to drive when we go out for dinner so that we don't arrive sweating. My next effort will be to clean the evap and then switch to some other refrigerant. I have read that the thermistor in the evap can be lowered to get colder air so I'll look at that also. I don't know what else to do. Throwing money at ac shops hasn't helped.

That's just not right. With a parallel flow condenser, the early SD should be able to get down to 40° vent temps with 134..........even at 90° ambient.

Can you determine if the compressor is cycling? If you cannot determine whether it's cycling, try providing 12V to the compressor clutch.........all the time.........and see if the situation improves. If the compressor is cycling due to the evap temp switch, you'd have the exact scenario that you're experiencing.

I keep all my R-12 cars R-12 and about 5 years ago,I took an online test to get my EPA card so I can purchase R-12. Its easy to buy on E-bay and is not much more money than 134.
I wont even touch on the politics of R-12 Verses 134 and the whole DuPont soap box that I can jump up on in a minute! Its like the veggie oil thing that some insist is robbing the FEDS from their well spent fuel taxes.....yadda yadda.....in this new culture in this country of screw you bfr. you screw me, I do what's BEST FOR ME!
I did have freeze 12 in the jag for a spell and it seemed colder than 134

My SDL and the 300D I had both have the aux fan that comes on when the high pressure line gets up to temp and also when the engine temp call for more cooling.....on the SL, it comes on when the compressor is activated!

I don't think the compressor is ever turning off but I can try the straight 12v to see. I can tell a diff in the engine load while driving if I switch the ac off, that's why I don't think it's cycling.

You cannot easily determine cycling if you're on the highway. If you happen to shut the compressor down when it's engaged, you make the conclusion that it's not cycling.........but, the data isn't in evidence from one attempt.

See with 12V does for you.

Also, be sure the aux. fan is working properly, otherwise it will never cool anything at low speeds.

OK, I'll wire the clutch direct. The elec fan is on and working. It's been wired so that anytime the ac is on the fan is on.

And so...?
 

and so what happened then? After supplying a constant 12v? Gee, I wanna know! I gotta learn!