AC Help !! convert back to R12 - PeachParts Mercedes-Benz Forum

Fred Jan · #1 05-11-2020, 09:11 PM

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

Diseasel300 · #2 05-11-2020, 09:15 PM

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.

Fred Jan · #3 05-11-2020, 09:28 PM

Quote:

Originally Posted by Diseasel300

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.

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

Roncallo · #4 05-19-2020, 09:47 PM

Quote:

Originally Posted by Fred Jan

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

Aftermarket monovalves are known to be bad in these cars. Get a nerw one from Mercedes.

Usaguy · #5 05-13-2020, 08:24 PM

Quote:

Originally Posted by Diseasel300

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.

+1

BWhitmore · #6 05-11-2020, 10:43 PM

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.

Fred Jan · #7 05-11-2020, 11:59 PM

Quote:

Originally Posted by BWhitmore

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

JHZR2 · #8 05-15-2020, 12:59 PM

Quote:

Originally Posted by Fred Jan

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

So now youre using ES-12a?

Regular or industrial? Did you pull a vacuum?

Max pressures can go haywire with these blends depending upon how they are charged. That can affect efficacy.

Fred Jan · #9 05-15-2020, 11:29 PM

Quote:

Originally Posted by JHZR2

So now youre using ES-12a?

Regular or industrial? Did you pull a vacuum?

Max pressures can go haywire with these blends depending upon how they are charged. That can affect efficacy.

ES 134 industrial 134A replacement
Yes pulled vacuum for 1 hour

Diseasel300 · #10 05-12-2020, 12:23 AM

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.

Fred Jan · #11 05-12-2020, 12:05 PM

Quote:

Originally Posted by Diseasel300

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.

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

Diseasel300 · #12 05-12-2020, 01:03 PM

Quote:

Originally Posted by Fred Jan

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

Not as much as it will a Gen 1 W126 or a W123. The small-tube dual-circuit condenser of the Gen II W126 is quite efficient. It also has the huge benefit that it can be flushed, a parallel flow condenser cannot and must be replaced if clogged or compressor fails.

Quote:

Originally Posted by Fred Jan

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

Use PAO68 oil. It is usable with R12 or R134a. All of it goes in the compressor, the Densos have a sump and do not require the oil distributed around the system.

Quote:

Originally Posted by Fred Jan

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

Replacement monovalve core should ONLY come from the dealer. They're expensive ($130 or so) but the only ones that work and last. An old hardened core will not work, the MTC aftermarket ones typically don't work properly either. Make sure your coolant hoses are routed correctly.

You have not confirmed whether or not the coil is energized with the A/C running, you can only test it properly with everything hooked up due to how the transistor pack in the pushbutton unit does its switching. Adding a manual valve is totally unnecessary, the monovalve should stop all water flow to the heater core when the temperature wheel is set to the blue dot (max cold).

engatwork · #13 05-12-2020, 02:09 PM

Personally I'll add probably 75 to 80% of oil to compressor then remainder in other parts, dryer, hose to condenser, etc.

Fred Jan · #14 05-12-2020, 10:29 PM

Quote:

Originally Posted by engatwork

Personally I'll add probably 75 to 80% of oil to compressor then remainder in other parts, dryer, hose to condenser, etc.

Correct my information please is it 7.75 Oz ? the sticker under the hood does not say anything about compressor oil.

Fred Jan · #15 05-12-2020, 10:38 PM

Quote:

Originally Posted by Diseasel300

Not as much as it will a Gen 1 W126 or a W123. The small-tube dual-circuit condenser of the Gen II W126 is quite efficient. It also has the huge benefit that it can be flushed, a parallel flow condenser cannot and must be replaced if clogged or compressor fails.
Use PAO68 oil. It is usable with R12 or R134a. All of it goes in the compressor, the Densos have a sump and do not require the oil distributed around the system.

Replacement monovalve core should ONLY come from the dealer. They're expensive ($130 or so) but the only ones that work and last. An old hardened core will not work, the MTC aftermarket ones typically don't work properly either. Make sure your coolant hoses are routed correctly.

You have not confirmed whether or not the coil is energized with the A/C running, you can only test it properly with everything hooked up due to how the transistor pack in the pushbutton unit does its switching. Adding a manual valve is totally unnecessary, the monovalve should stop all water flow to the heater core when the temperature wheel is set to the blue dot (max cold).

Thanks !!! great information out there !!! can you tell me more about the mono valve ? so the is coolant coming from the engine through metal hose them from the bottom of the valve, if the core pushed up the coolant will run into the heater core ? the core should be pushed in all way all the time unless you turn on the heater ?