AC Help !! convert back to R12
 

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.

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.

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.

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.

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

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).

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.

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 ?

If there is a tear in the rubber diaphragm within the monovalve assembly, then you will get hot coolant circulating in the heater core when you want a/c.

Parallel flow condenser will work **substantiallly** better with r134a than the original tube condenser which was designed for r12. The original r12 condensers the w123 and w126 chassis Mercedes came with will work with r134a but will have lackluster performance in hot weather. You can get universal parallel flow condensers, but you will also need to know how to make custom a/c hoses, which a good a/c shop can do. Cabin temps will be in the 40's with a parallel flow condenser and r134a.

https://www.youtube.com/watch?v=M5lvHxCVxWg

I ordered one from here, he raised the price today !!! he said he is getting more calls about them, his name is Matthew, one kit with brackets and hoses

https://klimakit.com/product/parallel-flow-condenser-upgrade-kit-w126/

I'm not so certain you could find a better price on a universal p flow condenser. Measure clearance and then find one that will fit with the fittings in the correct place.

I don't know what the total oil capacity is on this application.

Sure seems like a lot of money to throw after a solution that isn't the problem. These Gen II W126's blow mid 40s on a 100+ day running 134a. I don't know what you think the parallel flow condenser is going to achieve since the pushbutton unit switches off the compressor when the evaporator coil reaches 38-41˚ (42-44˚ vent temp). The Gen 1 (Pre-'86) cars are a totally different story.

I'm just waiting for the OP to come back complaining that his $300 coil and $100+ worth of R12 didn't fix his problem.

I have 3 spares climate controls, I plugged them before to test the system, no change at all, my compressor is running all the time I can tell from the clutch and the gas flowing on the glass sight, never seen my AC less that 53F from center vents since I bought the cars 4 years ago. its eaither full of oil or something else, even when I checked the service order from the dealership that the previous owner gave me, when he paid $4K to convert the AC to 134A at there, the note says (no leak detected, the system is holding pressure, the owner claimed the AC was blowing hot air from the center vent) after 4K and 3 months at the dealer ship as they ordered all the hoses from Germany, the note says (AC works fine, 50F is blowing from the center vents)

When the system is operating do you see any bubbles in the sight glass located on the receiver/drier?

I see one solid yellowish fluid flowing by. Its not clear, I remember my old 1988 VW Passat LS that has the same as Mercedes Benz AC system, the sight was clear on R12, buy I heard you can't get the sight clear in A134A, is that correct ? that was back in the 1991till 2003 when I owned the car

+1

Absolutely incorrect. If the charge is right in the weather we're having (I'm in Texas too), you should have a clear sight glass. ZERO bubbles in our current weather. Cloudy/foamy/bubbling/opaque sight glass indicates low charge. Yellow suggests they have dye in the system, that should not be present if the system didn't have a leak previously.


You're obsessed with throwing a LARGE amount of money into converting back to R12 when the refrigerant being used isn't the problem.

R134A will, if low on freon, show bubbles in the sight glass. It ias amazing that the PO spent $4K to have the system converted to R134 A. Diseasel 300 is correct. There is still dye present in your system.

The last charge I used a scale, I charged exactly 2.03 lbs (70%) or 2.9 lbs of R12 (I hope my information is correct), Yes the glass sight was glowing as hell when I used UV light on it, I hope the dealer didn't flood the system with oil, do you think so ? that is why I decided to go the long way and flush the heck out the system, because this is my 4th time dealing with AC system. There was a leak before according to the dealer notes in the suction hose and they ordered one for him from Germany, the whole AC job took 3 month to get the parts, the car was sitting there.

are the sealant and dye ... etc are bad for the AC system ? I am old school and I don't believe in that crap, in my old days it was pure refrigerant from 35LBS tanks, if you have a leak go and fix it. and done with it.

They charged him for new Denso compressor, a couple of hoses, dryer, expansion valve, and retrofit charging ports and labor for sure + freon , all of these parts wort $700, people don't call them stealership from nothing. I started having that feeling that the system is flooded with oil, as I charged the system before to get the sight clear, it was effecting the system in a bad way, lower temp from the vents, height/low pressure side going up. but the sight still cloudy.

I am scared to flush, I heard that flush stuck inside and you can't get it out you are done. I am right in the middle, scared to flush, scared to convert to R12 scared to drain. I don't know what to do

70% you're way undercharged. Having recharged many Gen II's, the ideal weight for an empty system in our climate is 42 ounces. Factory charge is ~46 ounces. Roughly 90% of the R12 charge by weight. Lots of armchair experts like to quote hard numbers when converting "because that's what the Internet said". Most of them are also the same ones that ***** about their converted A/C being weak. Charging a converted system is more of an art than a science, you're not dealing with the factory weight of refrigerant specified by the engineers who designed it.

If you're uncomfortable going straight to 42 ounces, add in 36 ounces, park a reliable temperature probe in the center vent, fan on Max, temp wheel on min cold, all 4 doors wide open. Continue very slowly adding gas in short bursts and watching the temperature gauge in the center vent. Keep adding gas and waiting (1-2 minutes) in between additions until the center vent temperature is 30˚F below the ambient temperature (if it's 85 outside, keep adding SLOWLY until you're at 55). If you want to get the charge perfect, continue adding very short gas bursts and giving the system time to equalize between additions until you see no further reduction of temperature at the center vent. You'll find you added 41-42 ounces of 134a.

Having been charging Fords, Hondas, Mercedes, BMW's, Mazdas, Mitsubishis, Chryslers, GM's, and too many others I've forgotten about over the years, I can promise if you charge the system carefully and deliberately, you'll have ice-cold air with 134a.

Your head pressure will be higher than you expect it to be on a modern car. Ignore that. You're dealing with a converted system, the head pressure will be higher at a proper charge. The Denso compressor will handle it no sweat. The head pressure will actually DROP once you're moving and have air blowing through the grille and the system catches up with the heat load in the cabin.

42 Oz ???? someone was wrong from another thread !!!! OMG I was struggling for 3 years !! I want to take the junk out inside the system (the dye, the sealant) and whatever in there, do you recommend the flushing ? drain the compressor ? I don't know how much oil inside there.

Depending on the system design and BTU capacity, retrofitting to R134a from R12 will be anywhere from 70-95% of the charge weight of R12. The vast majority fall in the 80-90% range.

You can flush the system if you want. While you have it apart, replace every O-ring in the system and use Nylog (blue bottle) smeared on the O-rings. Remove the expansion valve while you have it apart, it needs to come out to flush the system anyway. After you flush, blow everything out with compressed air until nothing else comes out.

When you put the system back together, pump it down on the vacuum pump for a minimum of 1 hour to make sure you have a nice dry system. Keep in mind that the dye will stain the oil residues in the compressor, so when you recharge, you'll still have some yellow dye in there.

I am taking down to compressor to drain, first to see how much oil inside and put POA68, any recommendations on flush brand, also can you confirm the oil capacity in 7.75 Oz. Thanks for the great information

It might already have been mentioned and by itself it shouldn't impact cooling as negatively as you're seeing but it can't hurt to check whether the outside air flap is closed when AC is running.

If you have the same setup as the Gen I (I assume you do since the early Gen II didn't have the passenger side airbag yet) then the way to check this is by either pulling the glove compartment (easier to see)or popping out the little grey plastic oval in the back of it.

Shine a flashlight in the opening. See pictures which show the open flap (lets outside air in) and closed (recycles cabin air -hence colder). The flap is the part with the deteriorating foam on it.

Thanks, I took the dash out a month ago, replaced all pods, rubber elbows, and checked all flaps functionality before putting back the dash, also I replaced the old thin layer of foam on all of them with black thin felt.

Good work! I hope you're soon to be rewarded with ice cold air.

Can the Gen II W126 condenser be adapted to a GEN I system, if yes do you think it worthwhile?

No real preference to flush brands. Buy the bulk liquid and a flush gun that connects to the air compressor, it'll do the most thorough job compared to an aerosol can. You'll need at least 2 quarts to do a thorough flush.

Most of the Denso compressor car take between 7.5-8.5oz of oil. 7.75 oz should be plenty. My preference is PAO68 since it has superior lubrication properties to PAG46 and is not hydroscopic, meaning that if any moisture ever enters the system it will not turn the oil acidic.

That I do not know. I'd suspect if so you'd need custom hoses at a minimum. By the time you're in that far, you'd be better off to build a parallel flow setup to compensate for the weaker performance of the Gen I A/C setup (and it goes without saying, convert to a Sanden compressor to get rid of the unpolished poo that the R4 is). The Gen II's have a vastly improved system all around.

Lol. To be fair my '83SD did see 34°F (even below) at the vent at minimum of 45mph with outside temps of high 80's low 90's and typical FL humidity running Duracool (hydrocarbon) refrigerant. Of course in traffic/idling, as you note, it would go up to 50°F and more.

Have a can of grinding rocks R4 in my new '85SD giving at best 55°F at highway speeds so it's due for a refurb. Hence my previous question.

before and after pictures

https://ibb.co/6NpC8Cv


https://ibb.co/gMz9WmH

Thank you for the great information !!!!

Quite the pristine specimen!

The Gen I cars have a mechanical evaporator thermostat to trip the compressor to keep it from freezing the evaporator. They are supposed to trip right around freezing, but there's some tolerance and it's not uncommon to see vent temps in the low-mid 30s on them. The Gen II cars have a thermistor sensor that communicates with the pushbutton unit to trip the compressor at 3-5˚C (38-41˚F) so the lowest realistic vent temps you will ever see on a Gen II car is 42-43˚F under ideal circumstances regardless of what refrigerant is being run.

I've been thoroughly disappointed with hydrocarbon refrigerants. When the engine RPM is up and the car is moving, they cool like crazy! Painfully cold! Then you come to a stop light...... 65-70F vent temps are simply not an option for me, sorry. R134a seems to keep the Gen II cars at <55F at idle with the fan on high. Once the cabin has pulled down, it'll significantly longer to creep up into the 50s. Soon as you start moving, down it goes again.

Yes, I imagine the Texas environment is even more demanding of the AC.

Have contemplated doing a hack grafting the innards of cooled seats off a Saab/Lexus/Caddy into the MB seats and tapping a duct into the rear AC conduit...

The heat-gain in this area during the summer is crazy. We routinely get into the triple digits with variable humidity levels and lots of sunshine. The hardest the A/C has to work is when it's over 100 and the humidity is low, vent temps matter! Ceramic window tint helps a LOT, but I haven't yet done it to my 350SD. Despite that, it still has one of the best A/Cs of any car I have access to.

I would be more than happy if you recommend a flushing agent, I spent the last 2 hours searching for one and checking the review, everything says "the solvent is super oily" "did not evaporate" "leaving a film behind" "leaving residue" :-( , Jesus !! I din't know its that hard to find a good product