Sudden loss of AC performance, Monovalve?
 

A few weeks ago I completed the install of my AC system on my V12 107. When I first charged it it was 70°F ambient and I was getting duct temperatures of 42°F. A week later it was once again 70° out and I'm getting 55° Duct temperature. With the only real change being I got the aux fan now working and installed a electric fan controller that turns on the radiator fan anytime AC is on. Gage pressures have since gone down with the new fan controls from about 275 high side to 225 and 35 to 20 on the low side running at 2000 RPM.

I was suspecting a leaking monovalve. I grounded the brown and white wire on the monovalve to make sure it was closed at all times and no change. But I did notice the output line of the monovalve to the heater core does get warm, not as warm as the radiator hose but it is warm and I wasn't sure if that was normal. Should the monovalve seal of completely tight or does it allow some water to pass.

I hate to try a new monovalve because the only ones now available are MTC aftermarkets and I and many others have had a bad experience with.

This is a bastard system using a universal condenser and running R134. But it seemed to work really well for a moment.

The MTC monovalve cores are so cheap and easy to replace (unless you have buried it during your conversion) I suggest just trying one to see if it cures your problem. If it does, buy a couple of spares and change as needed. That was my approach.

As an alternative, for a reasonable price, KLIMA offers a monovalve elimination kit that promises a different method of heat control. Check it out at:

https://klimakit.com/product/monovalve-and-coolant-circulator-pump-eliminator-upgrade-kit/

I haven't done this; but, read somewhere BMW monovalve internal parts available from the BMW dealer will work.

Anyone here know???

It sounds like your charge is wrong. Return your electric fan setup to how it was before you changed anything and see if your performance returns. 20PSi suction pressure sounds like a low charge or a pinched off expansion valve, even on a cool day you should be seeing 25-30PSI if the cabin is pulled down, fan on low, and coil cold. More than likely the extra airflow over the condenser sucked all the heat out of the system and it now does no work.

never had a working AC in any of my cars since 1970. I lived in 43 different states. From 113 vegas to 95% humidity, and 99 degree tennessee.

It is easy to replace. But my one experience with it was very bad. I like the looks of the kit, have you tried it.

Thanks
John

I have seen that as well and was considering it.

I have not but it is on my "if this doesn't work" list.

IRT the aux water pump, I simply removed mine and put a bypass hose in it's place. No noticeable loss or delay in heating. The rubber mount offers a good place to fold and stow my block heater cord.

Could very well be and I need to figure out the proper way to charge this especially because it has a universal parallel flow condenser and an aftermarket electric fan. I don't know how that will effect the charge. The Denso compressor from the M120 appears to be very similar to the Denso compressor from the M117 except the 1996 SL600 the M120 came from was using R134 and the 560SL used R12. The stock 560SL used 1Kg (~36oz) charge of R12. So that would account for about 32 oz of R134 I put in 36 oz when I got my low duct output. I have installed a new expansion valve which as far as I know if the stock R12 replacement valve. That's why I asked you in the other thread if you were aware of an R134 valve for the R107.

The one thing I found it that pressures can be adjusted either by charge or fans. When I first went to charge the system the only fan I had working was the electric radiator fan that would only click on when the car warmed up to about 85°C. Since the car was cold when I first went to charge it I was getting pressures like 375PSI/70PSI and no cold air at all. As soon as I jumped out the fan those came down significantly to the 250PSI/35PSI range. So I can bring both pressures up just by disconnecting fans but where does that leave me with setting the proper charge. In any case I have already tried disconnecting the Aux fan to simulate what I had previously and still have lost the performance.

There is a possibility that I have a leak or had a leak. I just received a new leak detector so I will be using that.

As far as setting the charge I'm going to have to wait until I can send for my brothers recovery station, so I can experiment properly. In the end I really need to get a known quantity of Freon in the system.

As you can see from the attached service manual page, the R107 really does suck for AC. The red line shows where a stock R107 should perform at on a 70° day. But I did have much better than that for a very short period of time.

You do seem very knowledgeable on this subject so I do appreciate your advice.


Thanks
John

First thing to do is pinch off the hot water supply to figure out if a/c is underperforming or heater is overperforming.

I suspect you may be grossly overcharged. Is the suction line back to the compressor cold or frosty? Which compressor does the M120 use? 10PA15 or 10PA17? Both were used on the W126 era cars with the 17 showing up in the early 90s. The difference is the displacement (15cc vs 17cc). Keep in mind that none of the temperatures or pressures from the FSM will apply on a converted system and even less so on one running a parallel flow condenser. The parallel flow condensers cannot stack liquid in the tubes like a series-flow coil can, and they are very sensitive to charge.

Based on the head pressure and suction pressure you have at 70F, I'd wager that you're grossly overcharged and simply flooding back to the compressor. The PF condenser will hold considerably less gas than the series coil did, so your starting point for charge should be ~50% of the original weight of R12.

My preferred way to charge an unknown system is to wait for a warm day (85˚ or so) and charge by vent temperature. Set up your fans however you want and run the system on the highest fan speed, max cold, and a temperature probe in the center vent with all doors and windows open (leave the top up so you don't have sun bearing down in the cabin area). Very slowly add gas, waiting 1-2 minutes after each addition until you hit 30˚ below the ambient temperature. If it's 85 outside, you're looking for 55 at the center vent. Even more slowly add gas, with even longer waits after each addition until the vent temp doesn't change. You'll be 30-32˚ below the ambient temperature. At that point, the system should be properly charged. It doesn't sound scientific, but it's an old-school way to charge a system with an unknown charge requirement.

Thought about it but I don't like to pinch those off because replacement of them is difficult with the M120 in place. May try to disconnect and plug.

Suction line is cold but not frosty.

Both are 17's 560SL use an 10P17C. 1996 SL600 used a 10PA17C.

The approach you suggested was kind of what I had in mind. Pretty hard to really establish anything at 70°. Its just barely on the scale.

We are in a cold spell for the moment, may have more suitable weather by the time my refrigerant recovery system gets here. Its being shipped in from NY.

Thanks

Peaking & Tweaking the Klima II HVAC
 

Subscribed .

FWIW, back in the mid 1980's when I was doing AC repairs I too tended to use the slowly fill until it gets cold enough method it worked very well .

Tomorrow/today I will test my stock 560SL that had a new Bosch monovalve installed 3 years ago to see if I get the same level of heat bypassing in the hose.