Is there a way to block the mono valve open? - PeachParts Mercedes-Benz Forum

gregp1962 · #1 01-05-2018, 07:23 PM

My heater is blowing cold air at freeway speeds. I want to block the mono valve open to see if that is the problem. Does anyone know if it can be blocked open?

Diseasel300 · #2 01-05-2018, 07:30 PM

Replace the cartridge with an OEM one (skip the aftermarket). Not really an easy way to "block it open" because of how it works.

ROLLGUY · #3 01-05-2018, 08:43 PM

The monovalve should be open (heat is the default) unless the solenoid is energized. You could remove the lower section of the cartridge so coolant is flowing through it all of the time. At least until you get a new cartridge.

Junkman · #4 01-05-2018, 08:48 PM

Doesn't removing the electrical plug gives full heat unless the diaphragm is torn?

ROLLGUY · #5 01-05-2018, 08:55 PM

Quote:

Originally Posted by Junkman

Doesn't removing the electrical plug gives full heat unless the diaphragm is torn?

If the CCU is calling for cold, the valve should be closed. If -12v is present on the connector when heat is selected, you have a CCU problem. Removing the electrical connection is pointless if no -power is going to it. I have heard of the valve sticking, and not allowing coolant to flow.

EDIT: corrected- thanks TF007

tangofox007 · #6 01-05-2018, 09:08 PM

Quote:

Originally Posted by ROLLGUY

If the CCU is calling for cold, the valve should be open.

It's the other way around.

Quote:

Originally Posted by ROLLGUY

If 12v is present on the connector when heat is selected, you have a CCU problem.

There should be 12 volts present at the monovalve positive terminal whenever the key is on. The monovlave is actuated by switching the ground side of the circuit.

tdhawk · #7 01-05-2018, 09:59 PM

Be VERY carefull of the plug and pins on the solenoid if you decide to unplug it. I had trouble finding a used unit that did not have pins broken off in the plug, and finally had a fellow with a parts yard just cut the wire and send me the solenoid and plug. When I first broke a pin off in the plug and lost the connection, I had full heat all the time. With a working connection, and a bad aftermarket monovalve, I had no heat at all except at idle. I think with the bad monovalves, the design allows increased coolant flow to pull the valve closed.

85 300TD 376,000 daily driver

ROLLGUY · #8 01-05-2018, 11:49 PM

Quote:

Originally Posted by tangofox007

It's the other way around.

There should be 12 volts present at the monovalve positive terminal whenever the key is on. The monovlave is actuated by switching the ground side of the circuit.

Thanks, I had it backwards, now corrected. I knew in the back of my mind that it was powered to close the valve, but forgot exactly how......Rich

gregp1962 · #9 01-06-2018, 12:18 AM

ROLLGUY, what do you mean when you say remove the lower section of the cartridge? I'm trying to block it open since it appears that coolant isn't flowing at higher speeds even though it isn't energized. I don't even have the electrical connection hooked up now.

rocky raccoon · #10 01-06-2018, 07:08 AM

+12vdc all the time
- (ground) to energize monovalve = no coolant flow = no heat
if no ground full heat

Counterintuitive but that's the way it works. Sort of British in concept.

Simply unplugging should produce full heat. If you want manual control, run your ground wire into the cockpit and install a toggle switch.

Junkman · #11 01-06-2018, 07:38 AM

Quote:

Originally Posted by rocky raccoon

+12vdc all the time
- (ground) to energize monovalve = no coolant flow = no heat
if no ground full heat

Counterintuitive but that's the way it works. Sort of British in concept.

Simply unplugging should produce full heat. If you want manual control, run your ground wire into the cockpit and install a toggle switch.

That has been my understanding and experience except when the diaphragm is torn. One of the SDs had heat all of the time. Replacing the entire monovalve including the electrical coils let it shut off and work correctly. James Dean had reworked the push button prior to the constant heat issue. Heat is working as it should. No idea about AC. That's a project for this spring.

tangofox007 · #12 01-06-2018, 09:40 AM

Quote:

Originally Posted by gregp1962

ROLLGUY, what do you mean when you say remove the lower section of the cartridge? I'm trying to block it open since it appears that coolant isn't flowing at higher speeds even though it isn't energized. I don't even have the electrical connection hooked up now.

The rubber plunger that makes the seal is retained by an E clip. If you disassemble the valve, you could simply remove the plunger from the shaft.

That said, your symptoms are so classic for a bad monovalve I would not waste a lot time with further troubleshooting, especially if you find a torn diaphragm.

Quote:

Originally Posted by rocky raccoon

Simply unplugging should produce full heat.

With a properly functioning monovalve, that is true. But with a torn diaphragm or a poor quality aftermarket valve, the increased dynamic coolant pressure at higher engine RPM will mechanically overcome the spring pressure of the valve and push the valve to the closed position. Hence, good heat at low RPM and no heat at highway speeds above at 45 mph.

Diseasel300 · #13 01-06-2018, 09:41 AM

Quote:

Originally Posted by gregp1962

ROLLGUY, what do you mean when you say remove the lower section of the cartridge? I'm trying to block it open since it appears that coolant isn't flowing at higher speeds even though it isn't energized. I don't even have the electrical connection hooked up now.

That's a prime example of why you keep being told to replace the cartridge. It's bad. For under $40 you can have a properly functioning heater again. If it's been replaced with the MTC aftermarket part, well, there's your problem...

In the meantime (or if $40 literally breaks your bank - in which case you need a cheaper car to drive) remove the existing cartridge and literally pull the pieces off the end of the plunger in the monovalve. You lose the filter screen, so hope your coolant has been kept serviced. With the valve parts removed, you have full heat, full time. Your new temperature regulator is your windows.

Diseasel300 · #14 01-06-2018, 09:54 AM

Quote:

Originally Posted by tangofox007

With a properly functioning monovalve, that is true. But with a torn diaphragm or a poor quality aftermarket valve, the increased dynamic coolant pressure at higher engine RPM will mechanically overcome the spring pressure of the valve and push the valve to the closed position. Hence, good heat at low RPM and no heat at highway speeds above at 45 mph.

Just to add - a neglected cooling system can cause problems with the Monovalve plunger too. I went through that with the SDL last year. Running water, or letting the coolant stay in too long promotes rusting of the components in the cartridge and will cause it to "stick". Mine would stick open or stick closed randomly, when I removed the cartridge, it had a very "gritty" feel to it when operated, just enough to make it bind up. New cartridge has it working perfect.

gregp1962 · #15 01-06-2018, 02:21 PM

My mono valve is new. I'm going to try running it blocked open to see if the problem resolves itself.