I agree with Jaime that it's likely the valve insert but differ
in that you do need to drain some coolant. Enough to get the
level down below the valve. If you don't then you will start the
life of the new seal with water on top where the magnet part is
and have it working from the first just as if it was bad. Try
taking the hose off the side of the monovalve first and let the
coolant drain out of the system till it stops. Now open the top
and replace the insert. When you put the insert in there should
be no water above the insert. Of course you may want to put a
catch basin under the car to catch the coolant. You can reuse it
if you run it through a filter. I have used a coffee filter for
that. Slow but it cleans it good. OR use the occasion to replace
all the coolant in the system if it hadn't been done in a while.
Manfred
Date: Fri, 23 Dec 2016 19:14:47 -0500
From: Jaime Kopchinski <jaime...@gmail.com>
Sounds like you need a monovalve. The failure results in water getting
into the tube where the magnet while moves up and down to open and close
the valve. When water gets in, the valve moves much more slowly or get
stuck temporarily, resulting in very erratic behavior. Take out the 4
screws, pull out the insert, and have a look at the rubber part that seals
the tube at the bottom, and see if there is any sign of water into the
tube. A tiny tear in the rubber, or any water inside (possible with no
tear when you have a cheap chinese one), then you need a monovalve repair
kit.
Here is a good photo of where to look:
http://www.benzworld.org/forums/attachments/w123-e-ce-d-cd-td/487593d1357169603-heat-barely-works-1983-300d-monovalve.jpg
But keep in mind the tear can be very small.
You don't need to drain any coolant to do this, BTW... just release the
pressure at the cap. Its the highest part of the system so coolant loss
will be nearly nothing.
Its cheap, its easy to replace, and it'll continue to work for many more
years after replacement. Regarding your desire for 2 valves and a switch:
cables seize, o rings fail, and switches go bad (I've had all happen in my
240Ds). So, there is no perfect world.
Jaime
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