Mono Valve
 

I have a 90 300 SEL with a mono valve that is not working, I have a 94 300 TE with one that does. Anyone know if they can be swapped out? 94 looks a bit different but otherwise the same.

It can be swapped if the water connection lines up because the inside is the same. Are you sure it is the mono valve? Whats the problem? Maybe the diaphragm is bad on the inside.

Sometimes no heat, If I squeeze the heater lines going to the core sometimes I get heat. If I unplug the Mono Valve I seem to get heat. I am not sure if it may be the CCU or the mono valve. I am not sure if this car has an aux pump. Can't seem to find it unless it is in behind the firewall somewhere. If that seizes I believe it will take out the CCU so I need to make sure of them all. I have a w124 donor car.

W126 mono valve is not compatible with one from a W124 or W140. Most likely the inside diaphragm part is torn and needs replacement. Have done this on ALL of our W126's at one point or another, just did it on my 420 a week ago.