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1971 250, W114 series
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10 Posts
Discussion Starter #1
Guys, as Midwest winter approaches, I need help with getting power to a fine and functional 3-speed blower which I restored on my 71 250 27 years ago. The car was in storage for 14 years and I am just getting it fired up again. I have the drive belt off the A/C compressor and will never use it again, as the weenies have banned our refrigerant. But I need heat. Here's the problem.

With the A/C rotary temperature control knob turned anywhere clockwise above the "Off" position, my blower fan works fine on all three speeds, but there is no warm air, as incoming air is only being drawn over the (unused) refrigerant cores and directed out the cooling vents.

If I turn the A/C knob all the way counterclockwise to the "Off" position, it cuts all power to the blower fan, and I can hear a vacuum bellows smartly changing over a door for heating, to draw air over the hot water cores. I know they work because, at road speeds, a small amount of convective warm air is coming out the vents. And I have functional levers to adjust the hot water flow to both cores. But no blower.

Since the blower works fine through the A/C switching circuitry, I know it is able to draw power from Fuse #6. Because it appears that is the only way the blower gets power. Correct?

So with the A/C turned off, and the heat door switched over, where is the blower now supposed to get power? Which switch or plug needs cleaning or repair?

I have a pile of MB service manuals accumulated over 50 years and six Mercedae, but none of them have an electrical schematic showing how or why the A/C temp control would interrupt power from Fuse 6 to the blower. It makes no engineering sense to me. The blower should always have power.

Can anybody tell me what is going on and how to fix it? Thanks in advance.

I love this old 250 best of all I have owned, including newer and fancier MBs. And I've put many hundreds of hours into restoring it over the last few decades. But without a schematic, I'm stumped.

Oh yes, and for you fellow Mercedes blower hassle victims, the motors can be restored...with no help from MB. In 1992, after two days of work getting the infernal thing out, I found both OEM carbon brushes fused into their sliding holders, making no contact with the armature. I had to chip them out. After much research, I found the right size brushes, with braided copper connecting wires even. Sadly, I don't remember the source or part number, but they are out there, generic. Then and now, the blower worked like new...if only I can get power to it.
 

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'71 250C, '99 SL500
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126 Posts
When you turn the AC switch full CCW, the Changeover switch should convert everything to heat mode via vacuum. The Changeover switch is located above the Gas pedal by the console. It is an electromechanical switch controlled by linkage from the vacuum actuator that is controlled the AC Temp control knob. The power for the Heater and AC motors are switched at the Changeover Switch. Clear as mud, right? Hope this helps.
 

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1971 250, W114 series
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10 Posts
Discussion Starter #4
Snebcb, that was extremely helpful. I think there's a blown secondary fuse or open circuit I didn't know about. I printed out much of the ref you directed me to. The unnecessary complexity of this system is utterly preposterous. It reminds me of being taken through the Smithsonian Air and Space Museum's Paul Garber restoration facility decades ago, where they had just finished restoring a multi-row radial engine for a German FW 190 fighter plane of WW-II. It was a beautiful, compact, hopeless, high performance engineering mess. Looked like the designers were just trying to show how smart they were. No mechanic could get his hand into the mechanical tangle to fix or replace anything, without tearing the entire engine apart. Any rifle bullet hit, during a low-level strafing run, would have knocked the engine out. Ah, that German engineering. Now I understand why this website has so many puzzled posts on what should be a simple and straight forward automobile function, the Mercedes-Benz heater blower. Ah well, keeps us off the streets.
 

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'72 250C, '74 280C, '85 500SEL, '81 300CD.
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984 Posts
To add to the complexity of the system, the fan speeds are controlled by a ballast resistor arrangement on the ground/earthing side of the circuit. Electrical circuitry is NOT a Teutonic strong point.

No wonder Tesla left Austria for the United States.
 

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1971 250, W114 series
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10 Posts
Discussion Starter #6
Yes Mike, when I first looked at the relatively simple blower circuitry in my service manual, it showed the motor/switch and resistor circuit alone, unrelated to any A/C complexity. Fuse 6 fed the three speeds through the main key switch, and it would always have power if the vehicle was powered. What could go wrong? Then Snebcb kindly steered me to this 17-page discourse on how the A'C system is tangled into this single blower motor system. Yoiks! What a nightmare. What were they smoking? My similar experience in trying to keep a 1977 300D and its heating and A/C system running for more than a brief period caused me to lose all interest in any MB manufactured later. I drove that car for 23 years and value this 250 much more highly. Simplicity usually means reliability.

FYI, in the early '70s Daimler-Benz adopted their Climate Control system entirely after Chrysler had abandoned it as hopelessly unreliable. That was about the same time that Daimler began welcoming former Detroit talent into top management. I've never understood any of this, unless they thought it might lead to a better understanding of the U.S. market and higher sales.
 

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1971 250, W114 series
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10 Posts
Discussion Starter #8
Thanks Patryk. I'm slowly getting there. What's embarrassing is that, checking my detailed records, I went through, and resolved this entire same heater and A/C problem in April 1992. I just forgot. I spent days tearing the whole system out from under the dash, analyzing and trouble shooting it. Nightmare task. And I found that one of two rubber diaphragms was split in the vacuum module which operates the changeover switch via a metal rod. After correcting this and servicing the A/C system, I had the whole thing working right. Then 27 years slipped by. There is still a possible A/C fuse receptacle oxidation problem, but I have not had time to check a few things with my VOM meter.

With this car, and with a later '77 300D I had, I was shocked to discover Mercedes' extensive design use of a maze of plastic and rubber vacuum lines, vacuum switches, check valves, and dual rubber diaphragm servos for the sole purpose of operating simple electrical switches, to do simple jobs. This is utter engineering insanity, without purpose. Unless the purpose actually is to make the system unmaintainable for very long, or outside a dealership. I began having to spend a lot of time repairing that system when the 300D was just over one year old. And I could write a book on its Chrysler Corporation reject Climate Control system, adopted by MB after the guys at Chrysler threw up their hands and gave up on it. My Annapolis dealer spent thousands of dollars in parts and manpower trying to fix it, until I asked that they loan me their pile of service manuals to study over a weekend. I theorized where the problem might be, and we fixed it in about 20 minutes on the following Monday. It was a thin spider web in an under-dash venturi which draws air to be sampled by sensors controlling temperature. It very slightly changed the direction of air flow, so the system work in reverse, making the car colder when you wanted warmer, and warmer when you commanded colder. The service manager couldn't wait to tell all the guys about it at the next training get together in New Jersey. And some dopey owner figured it out. :cool:

These cars are lots of fun to drive, but you have to be stubborn and knowledgeable to keep them going. I've never been in a Mercedes traffic jam.

Any of you other guys still trying to keep something of this vintage on the road, just for occasional local enjoyment?
 

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1971 250, W114 series
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10 Posts
Discussion Starter #10
Good for you, Mike. You have two of the nicest ones, better than anything they are cranking out today. It's all about taste.
 

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'72 250C, '74 280C, '85 500SEL, '81 300CD.
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984 Posts
Yup, the good thing about the W114's is they are relatively simple. No accursed ACC for climate control, relatively uncomplicated vacuum controlled door locking system.

Got rid of the Mickey Mouse transistorized ignition and did some modifications on the fueling. Made a world of difference.
 
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