I had this exact problem on my 80 300SD and let it go until the blower finally stopped. I removed the motor and ground out the compression clamps to remove the back bearing plate and discovered the brushes were completely worn down. You can go to a parts supply store for new brushes - I used some out of a Hoover vacuum that I had laying around and they worked fine with a small amount of modification. When you put the bearing plate back on make sure it is properly aligned and tack weld it back on. Before you go ripping into the motor check the power supply circuits and all contacts to make sure that isn’t the problem!

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1983 Mercedes 300SD
1987 Mercedes 300SDL
2001 VW Passat 2.8 AWD
2007 OM642 Jeep WK 4x4