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  #1  
Old 02-22-2014, 02:22 PM
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No heat @ hi rpm - new mono valve - still happening

1987 300sdl - 95,000 miles. I've read many of the no heat threads having to do with hitting certain speed / rpm and haven't found quite this problem. Mine has heat until I reach 2000-2200 rpm and then it goes cold. Unlike many I've read, just dropping below that rpm range will not start the heat back up. I must turn the key off and on - staying below that rpm range. As soon as I go above that range, heat goes off again.

I ran like this for a couple of months. This week I pulled the mono valve cartridge and replaced it with a new one. I was happy to see a tear in the diaphragm of the old cartridge and was expecting success.

Nope.

Still does the exact same thing. I pulled the new valve to make sure I hadn't torn it when installing - it was fine.

I pulled the plug on top of the mono valve and drove the car, this doesn't make any difference - still goes cold above that rpm range. Still must turn the key off and then on.

Used a test light on the plug and it lights up when the key is on, so I replaced the plug, drove again and...

Nope.

Any suggestions?

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  #2  
Old 02-22-2014, 02:44 PM
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I'm thinking that the cooling system is not brim full? BTW, tell me about the monovalve cartridge - I didn't think there was one available. I dunno if the cartridge would be the same for a 124....?

Rgds,
Chris W.
'95 E300D 413K
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  #3  
Old 02-22-2014, 03:05 PM
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Quote:
Originally Posted by Chris W. View Post
I'm thinking that the cooling system is not brim full? BTW, tell me about the monovalve cartridge - I didn't think there was one available. I dunno if the cartridge would be the same for a 124....?

Rgds,
Chris W.
'95 E300D 413K
The 126 has replaceable cart, the 124 does not IIRC.

I
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  #4  
Old 02-22-2014, 03:28 PM
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Yes, its a replaceable cartridge. And yes, the antifreeze is topped off.
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  #5  
Old 02-22-2014, 03:52 PM
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On my W123 the mono valve electro magnet had failed. I too replaced the rubber diaphragm that did not fix the problem. I got a used complete mono valve assembly from a wrecked car (lucky find). Problem fixed. Prior to fix had to turn the key off and on to both turn on and off the heat.
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  #6  
Old 02-22-2014, 04:40 PM
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But, if the magnet was bad, wouldn't that mean I would have heat all the time? Same as if pulling the plug?

I did that, disconnected the plug, had heat up to 2000-2200 again and then bang, no heat unless I turn the key off an on.
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  #7  
Old 02-22-2014, 04:50 PM
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Just the fact that you need to turn the key off and on points to an electrical problem. The magnet in the mono valve is an electro magnet and not a permanent magnet. It might be worth the effort to find a used mono valve assembly and give it a try. I was able to get one for $10 at a Pick a Part.
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  #8  
Old 02-22-2014, 08:08 PM
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i had the exact same issue with my w123. if i left the temp wheel on high, the valve would only open at low rpm. if i went over about 3500rpm it would quickly fade away until i came back to an almost idle rpm. very annoying and i would have to drive slower than everyone else on the highway with extremely hot air because i couldn't mess with the wheel or i would have to come back to an idle. turns out the newer replacement "elements" don't have a strong enough internal spring to hold it open when your water pump is moving a lot of water through the monovalve. i wound up using a bike tire tube to mend my original for now. and btw, the one that didn't work was an mtc brand.
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  #9  
Old 02-23-2014, 03:48 AM
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Well, the one that was in there was a replacement from a couple of years ago. The problem is that this is what the car was doing before I replaced the cartridge. Even though the old cartridge had a tear in the diaphragm, its still doing exactly what it was doing before I replaced it.

I think there has to be two things going wrong here. I fixed the cartridge, but now, what is the second problem?

Something is telling the system to quit calling for heat. But the fact that I have to turn the key off then on, means I'm resetting something. Now all I have to do is find out what exactly I'm resetting.

The magnetic coil that moves the mono valve is charged with the key on. So something has to shut off the charge to the valve, allowing the cartridge to open. At 2200 rpm something is charging the magnetic coil again...but what?

Of course, all this is assuming that I even have a clue....
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  #10  
Old 02-23-2014, 05:04 AM
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you can unplug the monovalve and go for a test drive. the heat should be VERY hot no matter what. that would narrow it down to a ccu issue. if it still acts the same, you have an issue with the monovalve or something with the cooling system.
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  #11  
Old 12-05-2014, 09:25 PM
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Bad/Weak Spring in new(ish) MonoValve

http://www.peachparts.com/shopforum/3414667-post43.html

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