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Mercedes 1986 560SEC
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399 Posts
It is my understanding that the default position of the valve in the monovale is open, so that the surge of water does nothing in the full heat position. The valve is closed to reduce heat by the coil surrounding the insert, int the housing of the monovalve. Turn the thermometer control to lower heat increase current through that coil, and gradually closes the valve, against the flow of the current.
 

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1987 420 SEL
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7 Posts
That is my understanding, as well. Before, I assumed it was one of three things: vacuum, electrical or cooling system. Based on my results after unplugging the auxiliary pump, and everything now functioning relatively normal, I am convinced it's something with the cooling system.

My car operates at about 85 degrees C, but I noticed yesterday that it will run closer to 80 C much of the time. So, it seems prudent to go for a good coolant flush to remove any possible blockage, followed by careful refill to avoid any possibility of air pockets.

If that does the trick, wonderful. If not, then I move onto the temp sending unit. I am also intrigued by the theory that perhaps the monovalve replacements are not built quite to spec and are weaker against strong coolant flow. I may pull the replacement and compare with the original. Stranger things have happened!
 

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94 S350, 06 E320CDI
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It is my understanding that the default position of the valve in the monovale is open, so that the surge of water does nothing in the full heat position. The valve is closed to reduce heat by the coil surrounding the insert, int the housing of the monovalve. Turn the thermometer control to lower heat increase current through that coil, and gradually closes the valve, against the flow of the current.
That is also my understanding, but in actuality, what was happening with the replacement inserts (both of them in different cars) was that in operation even with the monovalve unplugged, when coolant flowed the valve was closing and I was getting only cold air. That tells me that the flow can in fact overpower the insert and that no power to the valve needs to be there for that phenomenon to happen.

The valve theoretically should not close unless power is applied to the magnet to close the valve, but I can tell you that is not the way it worked for me in two different cars with the MTC inserts. I still think it has something to do with the weight of the spring in those inserts but that is only a guess.

I'm done with those thigs and both cars are now working but what a PITA.
 

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Mercedes 1986 560SEC
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399 Posts
I see your point...it really is a PITA, and an inscrutable one at that. It does seem to me that with the degree of expertise we have in this forum it must be possible to resolve it....here's hoping!
 

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75' 280C & 92' 300TE -was now 91' 420SEL
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I am having the same issue. My troubleshooting steps included to disconnect the washer heater circuit from the system. This got me heat again simply by inserting 2 screws into the hoses on the washer unit. My washer fluid was very hot, which lead me to believe this wiper/washer valve was open all the time. It is only supposed to be open below 4 Celcius. I think if you have a little air in the Monovalve and the washer circuit is going, the little floater in the bottom of the Monovalve is closing of the heater flow off, hence no heat inside the car.
To get heat and isolate the problem I removed the electric insert of the Monovale completely and covered up den opening with a self cut washer and a 21mm craftsman ratchet nut and a little sheet metal cover. This way I could be sure the coolant flow is not pushing the plug down. This got me always full heat on 3 drives in row. Reinstalled the Valve today and hoping that heat works tomorrow controllable☺.
 

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1987 420 SEL
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7 Posts
Interesting. I'll be looking at the washer fluid system this weekend, but I'm still leaning towards a vacuum issue.

After disconnecting the auxiliary water pump, I had full climate control, but heat only up to about 1600 rpms. So I dropped into neutral at high speed, and voila... Heat! Put her back into gear, and no heat.

So... It's apparent to me it's not a speed based issue, but engine rpms. And to me, that indicates a vacuum related issue. I'll keep experimenting.
 

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94 S350, 06 E320CDI
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Interesting. I'll be looking at the washer fluid system this weekend, but I'm still leaning towards a vacuum issue.

After disconnecting the auxiliary water pump, I had full climate control, but heat only up to about 1600 rpms. So I dropped into neutral at high speed, and voila... Heat! Put her back into gear, and no heat.

So... It's apparent to me it's not a speed based issue, but engine rpms. And to me, that indicates a vacuum related issue. I'll keep experimenting.
That is interesting. There can be so many things on these systems. Mine was completely the opposite. Any time the engine revved above 2000, I lost heat.

If you think it is vacuum based, you should be able to tell by how much vacuum you have going into the hose to your inside valve. Another quick way is that if you lose the inside center vents and it reverts to airflow to the top defrost vents, then you have a vacuum problem. Much easier to fix, IMO
 

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Mercedes 1986 560SEC
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399 Posts
MIne does the same thing...not heat above 60 mph, but it is rpm based. I too disconnected the auxiliary pump. In my case the heat flap isn't working either, need to get on that one!
 

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Mercedes Benz 300SE - 1989 - W126
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Ended up being a bad mono valve.

Being in Canada, the winters are pretty cold.

I had the exact same symptoms and I had a brand new MTC branded mono block valve.

I finally removed the new valve and compared it to the original mercedes 000-835-06-44-MBZ valve.

Turns out the spring in the cheap 13$ valve was not strong enough. The coolant rushing from the engine or trough the secondary pump would close the valve at higher rpm.

Ended up buying a 141$ CAN valve at the dealership. Problem is now resolved. The heat is so strong I could cook eggs on the vents!
 

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1987 420 SEL
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7 Posts
Monovalves, etc.

If nothing else works, contact George Murphy at Performance Analysys, Inc. His number is 865-482-9175. He rebuilds original mono valves and other components. I've bought his stuff and it's awesome, as is he. That aftermarket stuff just doesn't work in most cases.

You can also see what he offers at:

Performance Analysis | Climate Control | Diaphragms | Blower Motors | Pushbutton Panels | Vacuum Door Locks | Autotemp | Cruise Control | Engine Oil Analysis | Mercedes-Benz Parts | Mercedes-Benz Club of America | Oak Ridge | Car Care
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,283 Posts
Yes John….

If the posters had bought an MB insert from the dealership, rather than the MTC brand one, the troubles would have been in the distant past first attempt.

I had researched extensively on this site, and did get the factory one several years ago… Worked as advertised the very first time out….. R&R four screws and the insert…. done.

Just when you thought all is well, the el~cheapo MTC insert, if you get one that sorta fits, closes off the hot water flow at anything over .. say 50 - 60 mph.

With the MB branded one, you are warm and have the knowledge the job is done right.

When your girlfriend or wife says; "Thanks honey… Its really warm in here now." That is priceless.

MBL
 

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1987 300SDL, 2011 E350
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15 Posts
It must be the darn mono valve

I am with you folks. This is just maddening. Replaced mono valve insert, aux pump, vented the lines, changed T stat (engine temp stays nice and steady at 80C), flushed engine and coolers, and yet at high engine rpm there is no heat on my 1987 300SDL.
I did notice today that the hose coming from the cylinder head going to the mono valve was pulsing. Is that supposed to do that? I have a sneaky suspicion that the valve insert closes automatically at along with a high coolant flow. Do you guys have any thoughts?
 

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Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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"Replaced mono valve insert" with what?

Does the heat work normally at low engine speeds?
 

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1987 300SDL, 2011 E350
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15 Posts
More mono valve findings

"Replaced mono valve insert" with what?

Does the heat work normally at low engine speeds?
Hi there;
I replaced it with a MTC repair kit. Same result as before. The heat is terrific a idle up to about 1000-1200 rpm. Then the heat fades away until the engine idles.
Today I disassembled the older mono valve insert by drilling a small hole in the back and then driving out the insert. I inspected all the little parts and I am wondering if the little spring has the correct spring rating. It seems so weak and I am wondering if auxiliary pump and high engine rev causes so much water flow that the little valve seat may get sucked on to its little valve seat. Once the low engine speed lowers flow the valve should be pulled into the open position (temp dial is on max heat the whole time) and heat returns. Many previous replies to this thread recommended disconnecting the aux pump. I did and wouldn't you know it the heat stays even at high rpm. Then I tried dialing the heat down with the temp selector and the heat "follows" the temp selection nicely but it does not get back up to heat when I turn the dial to max heat - until I lower the engine speed. Hmm. Could it be the little mono valve spring that is causing this phenomenon? I am beginning to think so.
I went on a treasure hunt in my parts bin and found a spring of almost identical size but with a slightly thicker wire. I reassembled this little troublemaker and soldered the hole shut. This week I will give this rigged valve kit a try. If this works I will be either leaving it in if this fixed the no heat at high rpm issue or be on the lookout for a genuine valve repair kit. I do not think that I will be buying another cheap part like this.
 

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'91 560SEC, '91 300SE (sold), '98 Yota, '02 S-10 Hauler
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Yes John I've read those. Just a week or so ago I pulled and replaced the monovalve with one from my parts car, it had the OEM markings and it's rubber was like brand new to replace the torn boot and no heat above 1500 RPM (it also has OEM markings, ???) I have an MTC unit here (ordered a Bosch and MTC is what they sent :| ) I'm going to swap the good rubber boot onto my OEM plunger, something about the internal springs being wrong on MTC's, and still have a spare valve for later

BTW, the swap of valves here, I now have heat on demand at all speeds :wink
 

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1987 300SDL, 2011 E350
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15 Posts

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1987 300SDL, 2011 E350
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If your mono valve does not have the correct spring there will be no heat at high rpm

Attached is the photo of the disassembled mono valve insert. See the little spring (do not mind the fraction of a spring, that was part of my little experiment) in the center? That is the one that keeps the valve in the default no voltage to coil full heat position. With aux pump running and high engine rev the valve seat (all the way to the right) is literally sucked into the closed position. Only an insert with the correct spring rating will keep the valve seat where it is supposed to be. Disconnecting the aux pump will only help you with continuos heat if you keep the temp dial set to max heat - it will give you all kinds of funky heat / cold trouble the moment you try setting the temp dial lower.
By the way, the back of the insert (all the way to the left) will always see coolant. The screen allows it to enter the back of the diaphragm through a tiny pin hole in the brass part (you may be able to see that in the photo) of the valve seat. So do not throw out the insert just because you got coolant come out of the back part of the insert. A torn diaphragm will definitely reduce the force toward the closed valve position and thus add to the maddening no or erratic heat at high engine rev symptoms of this simple device. :eek
 

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