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Old 08-10-2007, 08:07 PM
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Join Date: Aug 2004
Posts: 38
brainstorming mercedes AC

While i was replacing the evaporator in my 87 560 SL, and finally getting the thing to cool, I had to remove the heater coil which is on the inside of the AC coil in the air box. while working on the expansion valve and tightening the fittings where i found a leak, I inadvertently touched the heater piping coming into the heater coil from the engine compartment. The thing was hotter than hades.

So i spin the rotor control to heat and the things pours the heat out, telling me that the monovalve works ( since i replaced it last fall.)

So if i am thinking correctly, and since everyone seems to agree that mercedes ACs generally are not considered the primo system, wouldnt this be a problem with the AC trying to cool against two 1/2 inch tubes of very hot water located down blower from the AC evaporator?

my AC is cooling despite the 112 heat index here the past few days, but i was thinking it should be colder without having to battle that hot water piping in front of the evaporator.

Does anyone see any problem with putting a valve in the heating lines coming from the engine to the heater coil, and keeping all that hot water out of the interior? An old trucker told me that he used to do that on his truck, and it made the AC much cooler. Just open the valves when the weather cools and you are back in business. I dont want to restrict any flow of cooling water to the engine, if that would be a concern.

Just looking for ideas of whether my idea would be beneficial. I dont suppose in germany in 87 they had heat like we have here, and the AC was plenty cool enough.


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Old 08-11-2007, 01:10 AM
KCM KCM is offline
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Join Date: Jan 2004
Posts: 633
Stopping the flow of the coolant to the heater core is the function of the monovalve on the 560SL. It actually regulates the flow by switching on and off during normal operation. When off, full flow is achieved. When on, flow is shut off. It grounds out to change states. Sounds like you still have an issue with the valve. According to my CD manual (section 83-603), if you move the dial to min and press the total ventilation button (second from the left), there should be no hot air from the ducts and the heater core (exchanger) should be cold. The manual basically says if it is hot, either the monovalve is bad or the circuit controlling it is bad (I'd bet on the valve). It does say to check to see if "positive and negative are available on double coupler", but also warns of a possible short circuit, and I assume damage to the electronics if the wires are touched or shorted out. It also says to check for proper connection and bending of the connector at the valve which may not be making contact.

So yes, your idea is valid and there is a provision for this in the system, the monovalve, that apparently isn't working. Either the one you replaced is faulty, you have a bad connection somwhere, a bad fuse, or a problem with the pushbutton unit or climate control computer. You can always add the manual valve, but it will affect the climate temp control from what I read.

I recently retrofitted my 560SL to R-134a. I was amazed that with only half the charge the A/C was cold. So I know the A/C system in the 560SL works very well.
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Old 08-11-2007, 11:10 PM
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Join Date: Aug 2004
Posts: 38
thanks KCM. i have also changed gasses and it is cold without a full charge, according to the new gas instructions for replacement amount. i have to tweak my system according to the charge pressure/temp chart for the replacement gas. i am going to get some help in doing this--my goal for now in messing withthe system is to get gas in there and not leak out---i think i have accomplished that, and think i need about half a can more to tweak it. soi know it will get colder.

I agree with the monovalve function on the car. You have this one valve that restrcits flow through the coil--what i am wondering is, if the valve is closed and operating properly with no flow, will the water inthe heater coil lines still get hot? the line coming into the driver;s side of the car is still connected to the hot water in the engine block, and it seems to me that heat will radiate into that eventually without any water flow. this probably happens on every car and i just dont know it, and the AC system just overcomes this small source of heat. i do know that when i do crack that heater and the monovalve opens, there is definitely mega heat there!!!

i was just wondering if you kept all the hot water out of the passenger compartment, if this wouldnt make the AC system even colder. from what i see about this car, i would have figured some german engineer would have installed 2 monovalves, one on either side of the heater core, to gain maximum thermodynamic efficency of the AC unit by not allowing any hot water to come past the firewall until heat is called for. they seem to over engineer everything else to my way of thinking.

thanks again for the response. I will check my controller again with the button you suggest, and get the gas charge tweaked. i guess i am now thinking too much like an engineer--i have gone deep into this car and AC, and i want every cold atom i can get out of this thing!!!

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Old 08-13-2007, 04:19 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
The way the monovalve works has a giant design flaw IMO. When you shut off the ignition, you lose power to the valve, and it goes to open, allowing hot water into the core on the inlet side.

And on the outlet side, I don't think there is anything to stop hot water from getting into the core by convection.

So I think your idea of valves to allow for summer and winter mode makes a lot of sense.

Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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