Here is my "conversion" experience from over the past few days. Friend brought his 300SD over that I had installed a parallel flow condenser in last year and converted the car to r134. The compressor clutch was inoperative. I replaced the compressor, vacuumed and attempted to recharge with 134. At around 200 psi hi side I was getting 0 on the low side. Added a little more 134 and the pressure came up to around 300 psi with less than 10 on the low side. Shut the car off and when I started it up again the hi side headed for the sky. At around 400 psi the load on the compressor overcame the friction coefficient of the compressor clutch and a little bit of it vaporized. Removed the refrigerant and started disassembling the system to determine where the it was plugged. Pulled the expansion valve out and there was oil sitting on top of one side of it. Seems the expansion valve is plugged up. Plans are to change it out, flush the system real good, install a new dryer and re-install the compressor (after I attempt to rinse the 134 oil out of it) and go back to r12. Nothing like it in my opinion. It is going to be interesting to see how the r12 works with the updated p flow condenser:).

If you mean the center vent has very little air coming out then maybe the vacuum module that controls it is leaking. If when air is supposed to come out the center vent it comes out the defrost vent then center vent vacuum element is probably leaking.

No, all the vents up front, including center, sides, and defrost, work properly. I'm referring to the vent at the back (rear) of the center console. It's under the armrest and points at the rear seat passengers. Sort of like a rear A/C vent. I can't get any air to blow out of there.

UPDATE: Over the weekend I removed the condensor with the thought of replacing it with a P-flow condensor from a W210 I had sitting around. Preliminary measurement told me it would fit, but it did not. I seriously considered flushing everything while it was apart and going back to R12, but due to lack of time and motivation I just buttoned it back up.

I pulled a vacuum for about an hour and this time I charged by weight (easy since I was going from an empty system). R12 capacity is 46 oz. so I calculated 80% (36.8) and charged up with 3 cans of R134a (12 oz. each). To my surprise, I now get consistent vent temps of 48 degrees moving, 62 idling. Pressures were approx. 30/340 @2000 RPM with ambient air temp of 86 degrees and approx. 30% humidity. I drove around all the next day running errands and verified the vent temps - I will hook the gauges back up and double-check pressures when it heats up to make sure everything is still in check.

The A/C is now what I would call pretty good. Not great, but noticeably colder. Rear passengers (aka "the kids") no longer complain. I'm really shocked. Of course, the real test will be when it is 100 degrees out, as 86 is fairly cool, but either way the temps are about 10 degrees colder, and within 10 degrees of the R12 system in my 300E.

I have no idea why vacuuming and recharging would have this type of positive effect on the system. Can someone explain this? Could there have been air or moisture trapped in there before? Is the relatively lower ambient temp of 86 the factor? When getting the car from the PO I just topped up and "guesstimated" the charge based on gauge pressures. These results tell me that is not always the best way to do things. Maybe I should have pulled a vacuum and charged by weight to begin with.

The high side pressure of 340 @ 86 degrees still bugs me. Who knows what it will shoot up to at 100 degrees. I'm guessing somewhere near 400. Is it wise to rely on the cutoff switch for high side pressure control? I know that these compressors were only designed to run mid to high 200s under R12 and perhaps that is why compressors just don't last as long after a R134a conversion, even if the conversion was done properly. Somehow this compressor is still the original one and has survived 9 years of running R134a.

Would installing a P-flow condensor help things?

The problem with adding refrigerant opposed to vac/recharge is that the system can have air in it from previous repairs , resulting in good gauge readings , but inefficient cooling.....
Plus , who knows what else is in there .. I have seen all kinds of mixes...even propane !

One last post to close the loop on this thread. On my 45-min drive home, got the following vent temp measurements:

First 30 min: 57 moving 65 idling.
After 30 min: 48 moving (after 2 min) 60 idling (after 1 min) - actually hit 46/47 after doing 60mph for 3 or 4 mins near the end of the drive (hard to tell exactly on that tiny thermometer while doing 60).
weather conditions: 92 degrees ambient, 20% humidity.
Hooked gauges up when I got home:
45/235@650 - 28/290@2500

Summary:

1. Pressures are well in line with my original readings, leading me to believe there may have been air or something else in the system preventing optimal refrigeration. This was evidently remedied by evac, pulling a vacuum for 1 hour (dual stage pump, 2.5 cfm), and charging by correct weight.

2. Idling is where this system suffers. When it's 100 degrees out and your vents are only putting out 65 degrees at a light, it's going to take a LONG time to cool anything. May never happen if you idle frequently and/or passengers get in/out frequently.

3. When the vehicle is moving, this system is quite cold. Not as cold as an R12 system, but definitely within 8 or 10 degrees which I consider very good. Of course, it seems to take about 1/2 hour of continuous operation to reach this level of performance.

4. I believe the condensor and/or condensor fan is/are the weak link. Apparently it is incapable of providing good cooling without tons of air moving through the coils (witness the cold temps when vehicle is moving). This must simply be a limitation of the R134a conversion, as R12 must be more efficent at transferring heat to the atmosphere and condensing to a liquid. I believe a parallel flow condensor and maybe a higher CFM fan would improve idle cooling tremendously. I may try a custom P-flow at some point.

5. I am predicting at 100 degrees my pressures would be in the neighborhood of 32/360 or so, pretty much back to where I started. Probably about 100 more than the compressor was originally designed for. Oh well, didn't gain much as far as bringing down head pressures, but that's just the nature of the converted R134a beast, something I'll have to live with. At least things are 10 degrees colder now.

6. This system would be just fine up north, where temperatures don't often venture much over the low 90s. In Chicago, where I used to live, I thought my converted systems were adequate. Here in TX, when temps hover around 100, you tend to notice the limitations more readily.

In summary, I think I'm happy with it the way it is.

Woohoo! With the vehicle moving, today I got 53 degrees from the vent with ambient of 101* and 27% humidity - that's nearly a 50* differential! While idling at a light, needle creeps up to 62. Not bad for a converted system if you ask me.