W123 Monovalve - Building a replacement with new parts - PeachParts Mercedes-Benz Forum

Demothen · #1 04-20-2017, 02:18 PM

So I've still been fighting monovalve issues in my 85 300d. Primarily the issue is that at high coolant temp/pressure, the monovalve is not able to stop the flow of coolant to the heater core, meaning the heater overpowers the AC system.

Last night I swapped in a new MTC core, and so far it's working well but I don't really trust it long term, and i just want this problem to go away.

My plan is to pull the entire monovalve assembly and replace it with modern parts. I've found a 3/4 12V normally open solenoid valve made from brass that's seems like it's overbuilt for this situation: 3/4'' 12V DC Normally Open Electric Solenoid Valve 12-Volts - this is rated to 90PSI, 265F, and runs on 1.5a. I still need to check my existing monovalve to see how much current it draws, but I imagine worse-case I can just add a relay to the circuit to protect the logic boards. I'll also need to build an adapter to add a 1.5a fuse and convert from the Mercedes style plug to the standard spade terminals that the valve seems to use/

Next up is the check valve. The monovalve housing contains a ball type check valve that seems to be intended to prevent back-flow from the auxiliary water pump side of the block to the heater core. My plan is to find a brass check valve to replace that. Something like this:

Next is the adapters. I'm thinking I'll order a few 90 degree 3/4" NPT to 3/4" barbed elbows, as well as some 3/4" NPT to 3/4" barbed straight adapters, so I can work out the plumbing. I'll need to use teflon tape for the NPT connections, right?

Finally, the mounting bracket. I'll have to figure this out as I go, but I want to mount this using the same mounting location as the original monovalve used. I've got a welder and reasonable fabrication skills, so this shouldn't be too much trouble. I might need to get some bulk heater hose to deal with differences in port locations.

Does anyone see any flaws in my plan? I would think brass would hold up well to the coolant - I believe the stock monovalve has a brass filter element and other brass bits. I can't imagine the cooling system ever reaches anywhere near 90PSI, and it certainly shouldn't exceed 265F.

Of course the other alternative is to add a manual shutoff valve so I can swap between heat/cold as needed, but I'd really like to maintain the automatic climate control functionality, as I've spent a lot of effort rebuilding the rest of the system.

Dmitry at Pelican Parts · #2 04-20-2017, 03:27 PM

If you're planning on doing a write up of this procedure, take a look at the link below. If we decide to publish your tech article, we'll give you a credit and 10% off your next order!

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-Dmitry

bsmuwk · #3 04-20-2017, 08:00 PM

Dump that MTC and get it direct from Mercedes. They're grossly mispriced on Pelican and other websites - cheap direct from Mercedes

Lifting Magnet | Genuine Mercedes-Benz | 000-835-06-44 : Mercedes-Benz Parts - Genuine OEM | MBOEMParts.com

BillGrissom · #4 04-20-2017, 10:30 PM

The only possible flaw I see is that this solenoid valve may be designed for on/off instead of proportional action. Some devices are designed to "pop open" and "snap shut", like the difference between a "pressure-relief valve" and a "back-pressure regulator". It would still work OK in your climate control system, but would always be cycling instead of reaching a steady stem position. If you add a relay, it will definitely operate in on/off mode.

But, it would seem much easier to install a solenoid valve from a later W124 car. I think they are much cheaper and more available, and might last longer. One guy here installed one in place of the aux water pump (a useless device), by extending the wires.

SD Blue · #5 04-21-2017, 08:08 AM

The original monovalve is on/off with +12v on one side and a pulsed ground being applied by the pushbutton controller.

Diseasel300 · #6 04-21-2017, 10:32 AM

Having watched the behavior of the monovalve in my SDL I can second what SD Blue says. The valve is pulsed on and off to cycle it. It is not a "variable flow" type valve. The variable amount of heat comes from how long the valve is left open to heat the heater core.

Demothen · #7 04-21-2017, 10:50 AM

Thanks guys,

BSMUWK- Thanks for the tip, I wasn't aware that the mono valve plunger was actually available from the dealer anymore. However, I'm going to stick with my plan of re-engineering the system, and here's why: I've been running a MB plunger with an MTC seal for quite some time, with inconsistent results.

Here's what I've actually been fighting:

At low engine temperatures, the mono valve functions correctly. I have heat or cool air on demand.
Once the engine comes to temperature on a warm day (when AC is most appreciated, obviously), the behavior becomes erratic. Most likely situation is that I'll make a quick stop, then get back in the car and start it. At this point, I'll have uncontrollable hot air from the CC unit. This is more than adequate to overpower my (recently rebuilt & improved) AC system. I'll have to drive with the windows down from this point on. My theory is the mono valve plunger is unable to push against the higher pressure coolant with enough force to seal to cut off hot coolant from the heater core.

I've also disassembled a plunger unit, and found that there are no substantial wear items inside of it besides a spring (which when weakened could cause problems with the valve being sucked closed - not the situation I'm running into)

I think the reason that this is happening is that, as we know, pressure in a sealed constant volume system increases as temperature increases. The interesting thing about the stock mono valve design is that it not only has to act against the pressure caused by the flowing coolant, but it also has to act against the pressure differential between ambient air and the cooling system. The plunger seal (the one that likes to tear) has coolant pressure on one side, and ambient/near ambient air pressure on the other. That means that at higher pressures, the solenoid has to work harder. So that means that either A) I've got higher than normal coolant pressures, even though the car runs at 80-85C. Maybe I have a high flow water pump? Was this ever available? Or maybe my radiator cap (recently replaced) is poorly calibrated? My aux water pump is functional, but disconnected currently. Or B) My monovalve coil(s) are weaker than they should be. These shouldn't be a wear item.

As SD Blue said, the mono valve is not varying action, it's actually a switched on/off cycle (in EE terms, I believe this is called a bang bang controller). I've tested mine to be sure it behaves properly by adding a 12v bulb in parallel with the solenoid. I should have grabbed a video, and maybe I will soon, but here's what I remember.

Climate Control set to max cool: power supplied to the coil constantly, meaning valve is always closed
CC set somewhere in the middle with AC off: Power pulses, duty cycle increases at cooler temperatures, decreases at hotter temperatures.
CC set to max heat: power always off

The reasons I'm not using a w124 mono valve are: They're more expensive (new) - they run from 140$ up on PelicanParts. They also seem more difficult to mount and package due to the port locations. Finally, I'll still need to add at least one elbow with a short section of heater hose as a splice in order to mount them in the original location. That's one more point of failure that I have to add to the system.

I've ordered the new solenoid valve, once it arrives I'll start piecing together the rest of the system, locally if possible.

dieselbenz1 · #8 04-21-2017, 11:21 AM

The new solenoid maybe noisier than the original at least thats what I've observed using those in domestic hot water heating systems.