Hello, its my first post in here.

I hope some one can help me with my AC issue.
I have a w126 300SEL 1988 M103 engine, 3 years ago the previous owner converted the car to 134A at the dealership, new denso compressor, expansion valve, suction hose. in Texas heat i am getting 55F at night from the center vents, 70F during the day. the AC is just not there and not doing anything. I found a good deal on R12 freon and want to convert back, I also asked my bud to make aluminum P-condenser for me with brackets, also I picked S70 volvo 3100CFM fan to delete the mechanical fan and convert to electric.
Can someone help me what do I need to do to convert back to R12, already bought new drier and expansion valve. how to flush the system, drying time after flushing, compressor oil, are the green seals compatible with R12 ... etc.

Appreciate any comments

You have other problems. The Gen II 126's are easily capable of delivering vent temps in the mid 40s in our heat with the stock system running R134a. You have a poor charge, a leaking monovalve, or a recirculation flap that isn't closing.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

The mono valve is brand new from low millage donor car, I took out the dash a month ago, replaced the felt on all doors and vacuum pods, rubber elbows, the suction return hose is just cool not cold at all, I saw online some members reported great results using propane mix from enviro-safe, bought the kit and the same result. I am planning to install a valve on the heater to prevent hot coolant from getting inside the car in the heater core, I don't know where yet to put it, on the manifold gauges i am getting 40PSI low - 200PSI height

I agree with Diseasel300. Converting back to R12 will not achieve your goal. Limiting hot water to the heater core is a good step. I think you have a weak charge of freon.

The car had been charged 4 time in the past 3 years:

Dealer ship result:55F from center vents at night , mid 70 during the day
AC shop result: 52 from center vents at night , 70 in during the day
Me result: 52 from center vents at night , 70 in during the day
Me Using enviro safe 134A replacement, same result

engine temp above +80, when driving 80, sitting in traffic its better to roll down the windows and 100c engine temp

As I said before, you have something else wrong. The A/C system in these Gen II cars is quite strong. Even with R134a, it should cool just fine and pull down the cabin quickly. You have an enormous evaporator, a large dual-circuit condenser, a large pusher fan (dual fans on 1988+ cars), a modern Denso compressor, and a good control system. Even at 100+ outside, you should be in the upper 40s, lower 50s running on 134a.

You likely have a bad monovalve or a bad pushbutton unit that is failing to turn off the monovalve. The entire point of the monovalve is to stop coolant flow through the heater core, in cooling mode it should be energized. If not, your heater core and evaporator are duking it out (and the evaporator will lose every time).

Converting to R12 may get you some slight performance improvement, but the difference isn't profound compared to a properly converted and charged 134a system. Even without window tinting, my 350SD has as good if not better A/C as most modern cars.

Also skip the "alternative" refrigerants. They *SUCK* at idle. Having been through them all (Propane, butane, Enviro-safe, R152a, Freeze-12), I can assure you that R134a has the best performance of any gas other than R12 you can put in the system.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Also consider tint your windshield with clear 3M ceramic tint, and also all the windows. The ceramic tint will substantially reduce interior cabin temperatures. "Normal" black window tint will not reduce the heat the same way that the more expensive ceramic tint does.

Go on youtube and search for "ceramic tint vs regular tint" you will see massive differences in temperatures with heat lamps between ceramic tint and normal (cheap) tint. And a clear ceramic tint will not change the aesthetics of your car if you don't want a totally limo look.

Disconnect everything, flush it all out, add proper amount of mineral oil, vacuum and charge. Make sure you remove the compressor, turn it over and empty any existing oil in it, put mineral oil in it and then flush/empty again then add oil before bolting it back up.

__________________
Jim

My 1985 300D was converted to R134 several years ago. The original compressor was replaced at the time with a genuine Mercedes (Delco?) compressor. I live in Southern California. I consistently measure 45-50 degree air from all 4 vents. I also replaced the monovalve with a complete new unit at the time of the conversion to R134.

Does your system have UV dye in it? If not, it should be put in next time it's recharged. This way you'll know where it's leaking.

Ceramic tint makes a HUGE difference on interior temps. It was on the agenda for my 350SD before the plague happened, now put off for a while. My point to the OP is that even a completely stock car with no tint should be able to blow cold when converted to 134a.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Yes, No leaks

Will a parallel flow aluminum condenser will boost the AC ? I will take everything a part flush and put back with fresh oil

Thanks !! shall I pour the whole 7.75 Oz to the compressor itself, or pour some in dryer or condenser ... etc after flushing

Thanks for the notes !!! appreciate it, to narrow it down I bought a manual valve switch, I will plug it before the metal black hose in M103 engine gloing to mono valve (correct my information if am wrong) I replaced the monoo valve twice, once after market that was a piece of junk, them from apart car,I tested the coil its working and the seals are all good