Replaced Mono Valve Internals – Still no heat at speed.
 

The symptoms were, no heat above ~45mph. The original mono valve diaphragm was torn in the center. Replaced it with an MTC kit.

Still about the same, no heat at higher speeds. If anything, the heat at lower speeds seems warmer but the transition to cold seems sharper as speed increases.

What does your coolant temp gauge say?

Coolant temp is normal. ~80-85 deg. C.

There have been a number of reports on this forum of aftermarket monovalve inserts not operating properly. Evidently, the spring is too weak to hold the plunger open when coolant flow increases as a function of engine rpm.

That would make sense. I guess I will bite the bullet and get the Mercedes monovalve kit and see what happens.

Swap in a piece of 1/2 inch copper pipe for the monovalve and see what you get.

Test only, to rule out blockage elsewhere in the system.

Quite often the electromagnet in the mono valve has failed. You might want to try a mono valve from a wrecked car (Pick a Part).

According to the FSM the mono valve has a dc resistance of 11-19 ohms.

Also you can try momentarily applying 12 V dc and verifying it clicks on and off. If you can't hear it click, put a compass next to it and see if it swings the needle (old control guy trick).

I like the compass trick. It should work with the electronic compass on cell phones too.

I had same problem after I rebuilt monovalve with kit. I later added some coolant, making sure that I got the air out of the system and that seemed to solve the problem.

I have thought about that too.

How does the mono valve work, does it allow flow when it is activated or does it block flow under activation?

Spring-loaded open, electromagnetically closed.

If you select max cool and don't have any heat, the electrical part of the monovalve has to be working properly. If you disconnect the monovalve electrical connector and don't have more heat than you can stand, the problem is mechanical, not electric.

Interesting, then some force pulls the plunger out with the coil off to stop the flow of coolant.

I can confirm if I turn the climate control to max cold, I have no heat at any speed.

Not quite. With no power, a spring inside (under the diaphragm) pulls the plunger upwards to allow flow.

There is also a one way valve in the "bowl" right above the lower hose which prevents back flow. It is a free floating plastic ball and moves up/down about 1/2". If you take the monovalve off and shake it, you should hear it rattle. If you don't hear a rattle or if the rattle is very faint, that ball may be stuck or that bowl area is clogged and can impede coolant flow (even if the electromagnet plunger is working). If that's the case, unclogging it and cleaning it with a CLR solution may restore its functionality.

Yes. And in your case, that "force" is most likely the higher coolant flow that occurs at higher engine speeds. The spring, which should hold the valve open, is loosing the battle.

You might experiment with partially obstructing coolant flow to the valve. Like by partially clamping a heater hose, for example.

That would reasonably confirm the proper operation of the valve solenoid.