Hello All, I've been chasing down an intermittent blower issue on my '83 300SD for some time now. I have verified the following:

1. There is power on both sides of the #11 Blue fuse in the fuse box.

2. There is power at port #12 going into the blower motor regulator.

3. There is NO power at the blower motor plug itself.

I have tested the blower motor regulator with a reman unit, and still get no power to the blower.

When pushing the different buttons on the CCU, I can hear the system adjust to the different settings. ANY ideas or suggestions would be greatly appreciated! Thank you.

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2015 E250 Bluetec Black/designo Sand
2014 GLK250 Bluetec Iridium Silver/Almond
1999 E320 Sedan Bordeaux Red/ Parchment
1999 E270CDI Estate Black Opal/Black
1983 300SD Champagne/Palomino Leather
1982 300SD Silver Blue/ Pacific Blue Leather

Update:

I have located the blower motor resistor, under the master cylinder. There was some debris around the unit. I have cleaned all the debris, and inspected the coils (as well as you can in this location), and all looks to be in good shape.

Does anyone have ANY theories? Ready to put it in the shop...

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2015 E250 Bluetec Black/designo Sand
2014 GLK250 Bluetec Iridium Silver/Almond
1999 E320 Sedan Bordeaux Red/ Parchment
1999 E270CDI Estate Black Opal/Black
1983 300SD Champagne/Palomino Leather
1982 300SD Silver Blue/ Pacific Blue Leather

What does "blower motor regulator" mean?

The final element is the "blower relay box" and is where you should focus measurements, since you get nothing at the motor. It is a black plastic box that sits above the glovebox liner (in my 300D's, beside the shiny "TemperatureRegular" box). That box has a relay for each of the 6 blower speeds. Each relay switches in a different series resistance from the "resistor box" you played with, with the last relay being direct 12 V (no resistance).

Look for my post where I added a relay to power that box directly from the battery, to avoid the "melted #8 fuse" problem in 300D's. In my 1985 300D, I found excessive resistance came from a crimp below the #8 fuseholder. I recently did the relay addition in my 1984 and dropped down the fuse-box to solder that same crimp before it caused problems and found it doesn't have it, so 1985's may be special. I think your SD has a totally different fuse-box. Good hunting.

I do have an idea on shops. They will burn a lot of hours poking thru wiring diagrams and testing w/ a multimeter which could do yourself, so decide if you want to pay for that. Some shops are amazingly crude w/ electrical, hacking up wires and adding butt crimps everywhere. I spent much time fixing those messes in my 1960's cars.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Do you get power at pin 2 , of the Blower Motor Controller, when HI blower speed is selected?

Do you get a ground at pin 9, of the BMC, when LOW speed is selected?

Automatic mode of blower speed is a function of both the Temperature Controller and the Pushbutton Controller.

Also, check the ground of the BMC at pin 10.

It is highly likely to be a problem in the Pushbutton Controller as they are notorious for developing cracks in the solder joints, where the circuit boards join in an "H" pattern. The Pushbutton Controller has to tell the Blower Speed Controller what mode is selected.

Mercedes-Benz 126 Service Manual Library

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Sam

84 300SD 350K+ miles ( Blue Belle )

Open the blower box, unplug the connector directly attached to the blower motor. Apply 12 volt directly to the motor.

That is how I determined my motor was bad. It had the commutator shredded and the brushes were nubs.

New motor fixed the issue.

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1st MBz: 1982 300SD
2nd MBz: 1987 300SDL
3rd MBz: 1995 S420
4th MBz: 1987 190DT
5th MBz: 1984 300SD w/1983 300DT engine
6th MBz: 1999 C230k I'm 3rd owner, got it w/57,235 miles.
and manages Mom's 2007 R320 CDI