Ok, now I'm frustrated. Here is the issue that started about 45 days ago. I mostly get heat while idling, but usually after few minutes it gets cold.. When I'm driving I usually loose heat. Its not the loss of air...the fan blows nice and steady, its the temperature of the air that is the problem...it gets cold!

I have replaced the monovalve TWICE. I have replaced the climate push button dash unit. I have replaced the vacuum unit with all the vacuum lines. I have replaced the radiator. I have replaced the thermostat....she runs just over 80*C..... It still acts the same.

What controls the temperature? Is it the monovalve? Is it closing for some reason and turning off the hot water? What would cause this?

If I put 12volts to monovalve leads I can hear/feel it clicking open/shut

I am at a loss and getting very frustrated and COLD......I hope someone can throw me something!!

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'89 300SE
'02 E320 Wagon

Try it in defrost mode. That puts it in full hot and all air to windshield.

Paul

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84 500 SEL (307,xxx miles)

Tried evry mode, no heat at all when it goes cool....no matter what button is pushed, or where wheel is turned.

What controls the temp? Is it the monovalve? I need to see what is telling what to turn cold, or stop the water flow...

How's the littlevaux coolantnpump working?

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Prost!

Unplug the monovalve. No voltage is open. Did you replace the complete monovalve or the insert that goes in the top of the valve? The aux pump should not cause a problem. It only helps during idle. The aux pump in my 300D is disconnected but the heat works fine.

Paul

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84 500 SEL (307,xxx miles)

Plugged in or unplugged, still makes no difference. I have just replaced the pencil thing, not the whole valve. When I put 12 volts directly to leads on monovalve I hear it open/shut. I checked the fuse that goes to this operation and its good, with good contact. What tells monovalve to open/shut? Is it the push button/wheel unit? - I have replaced that also....

If you still have no heat with the monovalve unplugged (no power) then there is something mechanically wrong with the monovalve or something is plugged up. It doesn't matter in this case but the monovalve is controlled by the control panel. I'm not sure where the heater hoses hook on the 300SE (mine is a V8) but both should be hot. If one is cooler than the other, the system is plugged up, either the monovalve is stuck off, or the heater core or one of the heater hoses is plugged. The only thing I can think of now is to unhook both heater hoses from the engine, rig up an adapter so you can connect a water hose to one of the heater hoses, turn on the water and see if it comes out the other heater hose. In other words, flush out the heater core.

Paul

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84 500 SEL (307,xxx miles)

Thanks for the info Paul! Both lines to the heater core are hot. Its not that it never gets warm....it does, then after a few mins gets cold and stays cold until I turn off heat and restart the process, or turn car off.

Im wondering if I have air in the system, bubble maybe? If I squeeze on rad hoses, I get heat. If I stop squeezing, air turns cool again. How do I 'burp' the system?

This sounds so flow related I wonder if your main water pump works sufficiently. Does it overheat in the summer?
I think there is a block vent plug on your M103 passenger side, if you wanted to try.

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Prost!

I was leaning to flow problem also...thats why I replaced the thermostat and radiator. I just got the car in early November so dont know about hot weather yet. I know she runs at just over 80* running or sitting.

Block vent plug? What do I do??

I only had your motor's brother, the M104, and the vent plug was up under the exhaust manifold. You just loosened it and filled till coolant came out.

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Prost!

My car is a generation 1 126 and it has a diverter flap that makes some or most of the air bypass the heater core and directs the air to the upper center vent. It is used when it is in A/C mode. I don't know about the generation 2 126 like yours. If it has this diverter flap, it could cause a problem like you are having. Anyone else know if the generation 2 has this diverter flap? In defrost or heat mode it should not be diverting past the heater core.

Paul

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84 500 SEL (307,xxx miles)

Great info Paul! I will start investigating this!!

Ahhhhh ok I see, I will check to see if I have that! THANKS!!

Do you have it set on auto when this happens? Maybe something is wrong with the cabin temperature sensor and its telling the control unit to turn off the heat because it thinks it's too hot in the car.

When the heater turns on and off in my car (300E), it makes a clicking sound that's audible inside the car, but it doesnt come from the middle, it comes from the passenger side where the valve is located. I dont think the vacuum pods make clicking noises, rather, they make a kind of muffled "pop", but anythings possible.

Was there any air coming out of the middle vents before/after you heard this click? If there was no air coming out before or after the click, then the center vent pod is not the issue. I seriously think something is making the monovalve shut off your coolant supply to the heater core. This is just my unofficial $.02 Good luck!

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C