AC systems designed for R12 - PeachParts Mercedes-Benz Forum

lsmalley · #1 05-04-2017, 11:57 AM

This topic has probably been discussed over and over again, but I am still having some issues (very minor now) with the ac in my 201. The problem is this, if I am sitting at idle for an extended period of time say 10-15 minutes, then I will get maybe between 55°- 60°F at the vent. In a car that was made post R12 could be any make it model or year after R12, sitting in the car idling, the ac feels extremely cold. All my components for ac are new, including a brand new Denso compressor. So considering that the only major difference between pre R12 and post R12 systems aside from the refrigerant is the condenser (tube and fin vs pf) does that make that much of a difference? I am not running r134 in my car I have put in r152 and I will tell you that it is cheaper and at least 60% better. Does anyone have any recommendations on a near drop in with minimal modification for a pf condenser for a 201?

Diseasel300 · #2 05-04-2017, 10:14 PM

If you're sitting idle for 10-15 mins, that's going to happen in just about any car, especially if your ambient temperature is high enough. About the only thing you can do is increase the RPM's of the engine a bit to move more refrigerant and make the engine-driven fan blow a bit more air across the condenser. Mass flow of the refrigerant matters a LOT. When you're idling, you have very little mass flow, so the system can't move a huge amount of heat.

Additionally, if your car is black (I bring this up because of your signature photo), you can amplify the effect since the car's skin both absorbs and radiates additional heat into the vehicle and onto you. In a hot climate, dark colors aren't your friend in the summer. I drive a black Honda and a Midnight-blue Mercedes and it gets in the triple digits here in the summer. A/C struggles at stop lights...

lsmalley · #3 05-04-2017, 11:59 PM

Yes, the black on black in the desert during summer months has been miserable. I don't know, I do get what you are saying about mass flow, but I still feel that the system could be improved. Actually today was about 106° and this was my first time really putting the r152 to the test since I put it into the system about a week ago and I must say that it did an amazing job. Actually, I'll see how it goes in the next few weeks, but this has probably been the best the system has been since owning the car. I will keep everyone updated. I'm running the r152 with PAG150 oil it's compatible with r134 also.

rocky raccoon · #4 05-05-2017, 07:11 AM

I converted my 1983 W123 to an R134 system. It was done as recommended by changing all components except the condenser. I now wish I hadn't done so. It has been about 5 years. The system cools fine while underway but just does not put out well at idle as you describe. I live about 25 miles from the closest town and if I park at work on a hot day even with my windshield covered the interior just starts to feel cool by the time I get home.

I am told that a good parallel flow condenser would cure my ills. I have been looking (not very hard) for one since. If I can find one at a reasonable cost I may install. I will also talk with my local ac expert irt R152 based on your experience.

Thanks for that. This is what this forum is for.

Mike Murrell · #5 05-05-2017, 08:13 AM

I have seen discussion about the use of a cross-flow condenser. I have not done it yet. Just thought I'd mention it. You might want to do some research on it.

Diseasel300 · #6 05-05-2017, 08:59 AM

If you are idling, the condenser will not help you beyond the limitations of the system design on a hot day. You can blow all the air you want across that coil, but if enough refrigerant isn't moving through it, the system capacity is greatly reduced. Remember that the compressors in these cars are FIXED CAPACITY. They're designed to produce the correct amount of refrigerant flow when the car is cruising. If the RPM is too high and the pressure in the system climbs to a point that is outside the design limits of the system (such as when accelerating or passing), the pressure switch will cut the compressor off.

The converse is true when sitting at low engine RPMs. The expansion valve will attempt to keep the pressures elevated to maintain cooling, but it has to throttle down to do this. Your system capacity is reduced, and as a result your car heats up in hot weather.

If the A/C blows frosty when you're moving, cools the car off quickly (5 mins or so in the dead of summer) when it's been sitting out and sun-baking, and the temp rapidly drops when you get the RPM up around 1000 RPM or higher, the system is probably just at its capacity.

From everything I've read, R152A is about as close as you're going to get to R12 performance. Make sure you charge the system correctly if you're using it, you use just a bit over half the weight of R12. If your car has a "recirculate" button, use it judiciously in hot weather, it makes a big difference. My SDL will get so cold with the recirculate button pressed that you wind up adjusting the temp wheel. Of course, I haven't dealt with 106˚ temps yet. I have driven it when it was 95˚ a couple weeks ago and it did fine.

lsmalley · #7 05-11-2017, 10:22 PM

As of now I'm considering the 9 year poor ac performance post r134a conversion, resolved. After putting in r152a my car's ac has been superb. Idling in 105*F weather for 10 minutes and getting high 30s low 40s at the vent.......when driving the temp drops dramatically. I'm using an infrared thermometer and see temps in to the 20s when cruising at 40 mph. On the freeway at extended travel time I have seen the temp drop into the negative double digits. I would definitely recommended anyone that is not satisfied with their r12 to r134 conversion to try this.

ps2cho · #8 05-11-2017, 11:17 PM

I put in r152a last weekend to give it a try, but I am not getting as good as you report. 100F today here and got 49F driving and 58F idle

lsmalley · #9 05-11-2017, 11:19 PM

How much did you put in? I put in nearly 3, 10oz cans

ps2cho · #10 05-12-2017, 08:53 AM

Quote:

Originally Posted by lsmalley

How much did you put in? I put in nearly 3, 10oz cans

I put in about 26oz, at 2000rpm I have 28psi low and 350psi high

lsmalley · #11 05-12-2017, 09:42 AM

28 seems really low. My low side was about 45-50 and my high side was 300±. Although, I did bypass the gauges when I charged the system. I charged the system first and I would get in and out of the car to see what the temp felt like. If it wasn't cold enough, I would add more. Eventually it got to where I felt it was blowing cold enough. Then I would put everything back together and go drive it around. Felt cold while driving, but at a stop is was so-so. Went back and added more and that's when it was actually freezing. When I got it back I to check my pressure it was at 50± low side and 300 high side. One thing that I would recommend is try turning on your high speed fans while you're charging. Also try spraying your condenser with water while charging to get the pressures down. Then charge until it gets cold

ps2cho · #12 05-12-2017, 03:23 PM

At idle it is 55psi, high 20's at 2000rpm

Diseasel300 · #13 05-12-2017, 03:46 PM

My SDL is running R12, which is arguably more effective than the R152A and I'm not getting those kinds of results either. Center vent blows 40-45 once the cabin cools down and stays there. If it gets to 38, the compressor trips off and comes back on when the temp gets to 45 or so. Reading up on how the ACC system works, there's a thermostat to shut the system off at 40˚ to keep the evaporator from freezing.

Take any and all readings with a probe style thermometer, preferably one with some mass to it to even out the readings you're taking. IR guns are notoriously inaccurate with cold temperatures and with the way the A/C systems in these cars work, I'd be impressed to see a vent temp much below 35˚ in recirc mode with the fan on low.

To get a negative temperature at the vent on a 100˚ day would mean that the system has to have a >100˚ temp split which doesn't exist in any refrigeration system.

Hit Man X · #14 05-13-2017, 03:39 PM

I have been using the conversion factor of 0.56 for R12 to R152A conversions. The 300E is on it now as the evap leaks. I only shot in 20oz last I recall as it holds just over 2lbs of R12 (2.1?). It did low to mid 40s last summer. 300psi on the high side is way high on 152A, even if it is 100F outside. It should have pressures very similar to R12.

One of my E23s is also on it as I wanted to see how it performed on a fresh system. Same temps as the 300E, car is black on black.

lsmalley · #15 05-15-2017, 01:00 AM

Interesting, the IR thermometer was not accurate at all in reading the cold temps. Went out today and bought a more accurate probe style thermometer and my readings are still well below 40°f. I helped a friend convert his 93 Honda civic today to r152 and his readings were in the 20s. Ambient outside temp was 93°F. Here are some of my readings taken today. I'm sure if I were on the freeway driving at speed they would be lower.