I replaced the climate control unit last night and the heater was working good in all settings I noticed that when I changed the settings from defrost to different vent settings it took a couple seconds to switch and the fan wood run on low and then switch to high speed and stay there. The next day I went out for a ride and the heat was not working again? When I put the climate control unit in defrost mode it made a buzzing noise and still nothing. I pulled the fan again and bench tested it to the battery it was working real smooth. I did notice when I put a test light to the plug that goes to the fan there is no power. At this point I have changed the ignition switch the the blower control unit and the climate control unit. Thanks guys for all your input. Now Im kind of lost and looking for a little direction. Thanks again

We need to differentiate between "heat not working" and a "blower problem".

If it is heat, then the problem is likely to be a monovalve problem. To test, unplug the monovalve and you should get coolant flow to the heater coil.

For the blower problem, your symptoms seem to be pointing to the Cold Engine Lockout Switch. It is located front RH side of the engine, near the thermostat housing. This switch enables to blower motor (provides a ground) when the coolant is 95-104 degees and disables it below this, when in a heating situation. It may be something as simple as a poor connection at this switch.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I assume your temperature control is the same as my 85 300D. Mine seemed erratic also until I read the User's Manual and found the way it operated was "as designed". Normally, you should only use the two buttons with the "black down arrow". Use the "EC" one in winter to keep the AC compressor from running. The controller will allow hot water to flow (via mono-valve) when it decides, and turn on the blower when it wants to.

To make it work like factory (still quirky), insure the foam tube behind the glove box isn't dis-integrated (usually). I replaced mine with rubber hose. That tube goes to the blower inlet to suck air past the air temp sensor on the dash. And don't block that mini-grill w/ papers on top of the dash.

The problem is the blower not coming on even when I have the climate control unit set on defrost and temperature wheel on high and the fan on high. I have a buzzing noise coming from the climate control unit not sure why that is. I checked the wires going to the cold engine lock out switch they looked ok and were connected. Just to recap on what I have done so far. I have changed the ignition switch the blower control unit and the climate control unit. I also rechecked the blower and it is working when I tested it with the battery. The brushes on the blower looked ok when I took the blower apart. Thanks guys for the replies they are very helpful. Please keep sending your replies.

Do you have a multimeter to check for power and grounds?

First there is F10 and F11. I would verify they are good. Unless they have been changed recently, it would be a good idea to pull them and physically inspect them. The plastic ones especially, are notorious for cracks on the ends and bad connections. The fuses with aluminum contacts are known for corrosion problems.

From your description of buzzing from the ACC controller, it sounds like a relay/s dropping in and out. That is the reason I mentioned the Cold Engine Lockout Switch. It provides a ground that controls 2 internal relays in the ACC controller. It needs to be checked to see if it is indeed providing this ground at temperature.

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Sam

84 300SD 350K+ miles ( Blue Belle )

In my case it was the blower. I had all the symptoms that you described. It will only work with defrost. Bench tested it, it worked fine. Eventually it wouldnt work with defrost anymore.

Pull the connector to the aux water pump when you test. Sometimes when the aux pump is stuck, it creates alot of problems with the CCU. You've done the upgrade to the aux pump yet? Its just putting a line fuse, 1 amp IRCC.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

We had a buzzing from the acc. we pulled it out and took the cover off and plugged it back in. Got the same buzzing noise coming from one of the little contacts in the acc. I went to the thermostat housing and had my son sit in the car, while I moved the wires that plug into the thermostat housing sensor. At first, that did nothing to stop the buzz, then I grabbed the brass sensor part that the wire plugs into and my son started yelling something at me.... so I went over to ask him what he was yelling about. He says," Dad it stopped buzzing." so I went over and wiggled the brass part on the thermostat housing and he said that every time I wiggled it around the buzzing would stop. I pulled out that sensor and put a new one in. The buzzing from the acc stopped, but I still have a problem with getting adequete heat out of this thing. It seems like the blower motor is'nt pushing enough air. We bypassed the monovalve entirely. Still nothing. The system in these cars is a royal pain....every winter one of them is on the fritz. Let us know if you fix it, I am watching your thread too. It's -41C right now. That is cold out...virtually undriveable without a heater. Is it that cold were you are too?

Guys, Is there some sort of big resistor looking thing mounted on the pass. side fender, that has something to do with the climate control...or is that more for a/c. It is a big square looking thing with little spirals of wire inside. Could that have something to do with this guys problems. How do you test if that thing is working?

My tech Mike has been battling Ol' Yeller's heating woes these past two days too. The heat was working up until a couple of weeks ago.

A week ago, it would blow warm for a little while following start-up and then only tepid. Mike installed a new monovalve and now it blows cold. A new thermostat and still no joy. It had a MB thermostat dated 2001, the replacement has a company name that starts with a W.

Ol' Yellow didn't see much use by a previous owner for several years which might explain why there was some oily stuff in the coolent. The person I bought the car from (and his tech) said that they flushed the system late last spring before I bought the car.

With the new thermostat installed, Mike started the car up, backed it out of the shop. We figured we were done and worked up my bill. It blew warm for a short while, then went cold and stayed cold.

He fiddled with something and we could hear the unit behind the firewall click. He ran it stationary on the lot at a high rpm for several minutes, still cold. The upper hose radiator hose still seemingly empty. He installed a new dash (dial and button unit) controller, which made no difference. Brought it back in the shop. Pulled the thermostat to check that the arrow pointed up; it did. Aux. water pump worked OK. We took it for a drive of about 10 miles. Still blew cold but it's the end of the day. I took a cab home.

He couldn't get to it until late yesterday. He flushed the system; he can get water to the heater core. He plans to try another thermostat today.

Just called him with the Lister's thoughts about a bad magnet in the monovalve. He reports that the magnet works. He's battling a heating problem on a 450 SL as well. Since both cars use the same controller, he's ordered a new one to eliminate that possibility and will use it to test both cars.

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- Neil
'83 W123 (Ol' Yeller)
'82 HP 34C

Just picked up Ol' Yeller. Turns out it was the monovalve's electromegnet. It was working off-line but Mike is thinking that the water pressure was defeating it or it was "weak." I'm not sure if I mentioned earlier that we found the diaphragm torn last week and installed a new plunger/diaphragm "monovalve kit" He removed mine and installed a monovalve borrowed one from another car. Now the heat works like a champ. A new monovalve has been ordered will be in next week. Looks like the thermostat was not necessary but then there's no harm in replacing one that was 12 years old.

He flushed a heck out of the system, lots of crap came out. Now Ol' Yeller sports the blue MB coolant.

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- Neil
'83 W123 (Ol' Yeller)
'82 HP 34C

Well finally I figured what the problem is with my heater and blower not working. I did figure it out only because of this site and the people who put there time and expertise to use in helping others. For that I thank you all. The solution to the problem was two fold. One was the controller was not working. So I replaced it and still the heat and blower were not working. What I noticed was that the controller had two connectors that connect on either side of the climate controller (left and right side) and one group of wires is longer then the other. I could not understand why the shorter wire was connected to the furthest side of the CC so I reversed the connections and I have heat and the blower is working on all my settings. What I think is the person that owned the car before myself inadvertently installed the connectors on the wrong sides. The shorter group of wires that comes from the left side of the car connect to the left side of the climate controller and the longer group of wires that also comes from the left side of the car connects to the right side of the climate controller. Temperature here is 25 degrees and Im glad I have heat know. I hope that this helps someone. Thanks again Happy Holidays!!!