The heater on our 560 isn't working, replaced the control head and the mono valve, still no heat. The air conditioner compressor seems to be working fine and the mono valve is opening and closing as it should. The heater core is fine, no leaks or blockagr. Is there any control that would cause either cold air only to go through the system or not allow heated air?

so its just blowing cold air, or no blower at all?

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

It blows cold air, the fan works fine.

man the heater system is pretty simple. the only things i can think of is:
the climate control unit still does not work(used one)
the monovalve is bad( used one)
not enough coolant
or perhaps a vacuum leak
when you have it on heat, does air come out the center vents like the a/c would?

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

I hadn't really noticed the air blowing from the very center vents, but I believe so. Both parts are brand new. I just aquired the car and had those parts replaced thinking that would be it. The vacum leak may add up since the check engine light is on now. I'm wondering how they are connected?

Disconnect the wires from the monovalve and it should go into a "Default" mode, always hot with open valve and hot coolant flowing through the heater core, if this works than the CCU is faulty. Or the vents are faulty. Or the core is clogged. What does the temp gauge read ?

Ben

The check engine light is only for o2 sensor. When my heat didn't work, it was my thermostat. It didn't allow the engine to warm up. Of course it got a little warm, but not the 140F I get get out of it now at the vents.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

The temp gage goes up to about 80 and tends to stay right there, that's driving in 35-45 deg F. here. I'll give the disconnect a try. I re-checked the fluid, it's fine, I noticed one hose warm-hot while the other was cool so my assumption is that water is not flowing through. The heater radiator was checked for flow when they replaced the control head.

Is it possible that if someone had crossed the hosed from the heater core to the mono valve body that water would not flow because of the ball valve in the body?

Also, when you mention the CCU, is that a component other than the control head?

The CCU is the brains that the control panel connects to.
I believe it is located under the passenger side of the dash, but I may be mistaken on that.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

What code is it throwing?

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)

OK, so I discovered the hoses were backwards and changed them, it now gets some luke warm air. I'll try disconnecting the mono valve again. It is showing a code "4". We may get to the bottom of this yet.