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

By the way, what do you mean by you replaced the vacuum unit and all the vacuum lines? there are lots of vacuum units and lines in '80s Benzes!

Oh, and a little more on the cabin temperature sensor, air gets pulled through it by a little fan motor mounted behind the dash. The sensor (at least in a 300E) is located behind the little grille next to the front dome light. The sensor can get tons of dust stuck in it from years of air being pulled through it.

Even if that's not the problem, it might just help your climate control to work better and its super easy and cheap to replace.

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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

I replaced the vacuum junction strip thing. It has about 7 or 8 vacuum tubes going to it. It is on the passenger side, under the side carpet/trim piece. I have inspected the vacuum lines and connections, and they look great. Could I have missed one, sure, its not easy to see or get at in there!

I think you are right about the monovalve.

Here is what is weird. I start out with heat, lasts 10 mins or so. When it started getting cold I unplugged the monovalve and got a reading of 13.7 at the leads. So something is telling it to close( power on close-no heat, power off - open heat ....is the way I understand it). What sends the signal to open or close? Is it just the temp wheel? I could have got a defective one....

Another weird thing. If no power to monovalve should mean full heat...I still get no heat when totally unplugged, that also makes no sense.

I have ordered a new temp wheel/push button unit that is bench tested and 100% good, so I will try that when it arrives.

Is there another working part in the monovalve that could go bad??

Ok here is my latest plan.

I have a spare monovalve. I am going to sacrifice it and pull out the plunger that blocks the flow of water. If that is totally gone there is no way to stop the flow. If my heat still goes away, I'll know to chase down a vacuum door/pod issue

The only thing I know of that opens/closes the monovalve is the pushbutton control unit, because that's the brain of the climate control operation. Why don't you try taking OUT the mono valve and putting in small length of pipe or something, so that you know you have full flow of coolant all the time? If you DON'T lose the heat, then you know its not a vacuum problem, and something was shutting off the valve. If you STILL lose the heat, then you know the heater core is getting by-passed somehow.

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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

Same principle using the old monovalave.. The monovalve seals itself with the larger rubber disk. What I did was remove the plunger so there is nothing to block the flow. WELL IT WORKED!! I have heat....I have had heat for longer than 10 minutes....I drove for about 15 mins with heat!!!

This is not a solution, but until the other(bench tested and guaranteed to work :-) ) CCU arrives I will have heat. It must have been the CCU the whole time. I have swaped it out with 3 different ones with no luck, but this proves that it HAS to be the CCU!!

Oh wait, you just said you were going to put in a gutted valve, so forget what I said about the pipe. What tells the valve to open or close is whatever you have the temperature selector wheel set to. When you have a certain temperature set, and it's on automatic, the system will do whatever it needs to maintain the set temperature. This is why I said to check the cabin temp sensor, It's what tells the control unit how hot/cold the cabin is.

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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

Great! this simplifies things. So try the unit, and the temp sensor, and see what happens.

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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

Yes! and sorry I forgot to reply to the temp sensor. I did the tissue test and it is sucking just fine. I put my eyeballs on the little fan also while I had all the trim pieces and glove box out....fan is working fine....infact when I loose heat, the fan blows stronger because the temp sensor is telling it its too cold.

Everything now is pointing to the temp wheel...I hope CCU #4 is the charm!!!

Thanks again for your help, I have been cold and frustrated since around Christmas with this issue!

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ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

Good luck. If this ccu doesn't fix it, it's time to start looking for other possibilities such as an electrical gremlin. Keep us posted!

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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

WOW!! Those illustrations are AWESOME!! I am printing them and have saved this thread to my desktop!! THANK YOU!!

I assumed if my temp sensor fan is getting air, then temp sensor was good, BUT the sensor itself could be bad. HMMMMMMMM

If this CCU doesnt solve it, thats the VERY next thing I am going to try!

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'89 300SE
'02 E320 Wagon