please help

I have a 1983 380SL at work and the HVAC blower won't come on. The blower motor has been replaced and the blower speed control module behind the glove box has been replaced. The ducts switch when I push
the buttons and if I take the cover off of the old module and close a relay with my fingers the blower comes on. Four relays, four speeds...so I guess the motor, resistor, and associated wiring is ok. Any help, suggestions, or wiring diagrams would be wonderful. Any part failures that are common? thanks in advance,
doug
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Isn't the blower switched ON / OFF by a vacuum switch?
Older, "servo" cursed 116s and 123s switched it that way, your more modern version may differ.
Since you asked - my 2 cents.
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doug wrote:

I wonder if you have the same problem my wife's 1983 300SD had. Two different dealers worked on the problem over several years and all were just expensive temporary fixes. The fan was replaced because the motor was "bad". It fixed the problem for about a year. I kept the fan and years later hooked to 12v and it ran just fine. Another dealer replaced the resistor box because it was bad. I didn't keep it, but this fix lasted only 3 weeks.
So, the only thing left was the push button control box. It was replaced and it did cure the fan problem. Total cost for fan problem is probably over $3,000. I have been afraid to add it up!
Being the curious type, I disassembled the old control box. We have all heard and experienced German engineering, but down at the circuit board level, there is engineering that should bring shame to all German engineers. The main circuit board has two daughter boards attached to edges of the mother board with only solder fillets. Over time and vibration, some of the solder fillets developed cracks and eventually opened up the circuit.
When the fan and resistor box were replaced, the connecting cables had to be disconnected and reconnected to the controller. When this was done, the flexing of the plastic housing and circuit boards forced the solder breaks back into contact and the whole thing worked for awhile.
The control box was a real bugger to replace as all the "wood grained" material on the center console had to be removed. So, naturally the service manual says to replace all the other components first. Dumb! The controller was designed to fail!!!!!
So, I don't know if this helps or not, but may give insite into the potential source of the problem.
Good luck!
Paul
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It's nearly ALWAYS the pushbutton assembly that fails. The fan gets weak long before it fails as the brushes wear out then the brish holders act like lathe cutting tools on the shaft. The poor fangets slower and slower.
When the pushbutton unit fails it fails usually by having only one or two button work then one day - nothing. The time between one button failing and the whole thing failing was in my case about a week. I suspect, but do not now for sure it was related to having a high power stereo installed below it and running both on a hot sumemrs day.
As you say, onc eyou look inside those things you get a bit quesy. But, they are state of the art 1970s technology because that's exactly what they are much in the same way the shuttle still uses core memory and 35 year old computers.
The pushbutton assemblies can usually be found for about $50 used on ebay.
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for all those who offered help, it is appreciated. it turned out to be the control head in the dash. I borrowed one from a friend with the same car, 10 minutes out and in and it worked. Ordered up a reman and all is well. thanks again, doug
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