Questions and Answers

Mercedes 300D Blower Motor Q. Vince Hello, how are you? I just discovered your writings on About.com, and eagerly checked it out for some good tips on my 1984 Mercedes Benz 300D, legit American version, automatic transmission, 150,000 miles. I'm a 1950's through 1970's American muscle car gear head historically and now as a hobby; age notwithstanding now (50) I've discovered these 300D's are economical, dependable "drivers!"

But in some respects they present real challenges!

Problem: Out of the blue, the blower motor has taken on a "mind of its own." Without rhyme or reason, it comes on and while it's working, the speed controls (HI, LO, and AUTO) work fine, as do the pushbutton HVAC functions, etc. But, intermittently, and now almost constantly, the motor sits completely dead, at any "speed" setting or pushbutton position, all of which have "zero effect" when it goes comatose.

Outside climate/temperature, cabin temperature, engine temperature, all appear to matter not, nor does banging on the dash help (although once it seemed a bump in the road jolted it to life, for a few minutes). I've double checked the fuse in the fuse box for the circuit, as well as the blower motor itself, both are quite fine and sound. What's left?

A parts house suggested replacing the whole in dash HVAC control unit, you agree? Can that be checked? Anything to check before that? Read about the mid-80's 190D (?) on the site, does mine have a relay that could be sticky? How do I find this stuff, get to them, test them, and get them out if necessary?

FYI: Appears the fan motor relay is "double duty," controlling the A/C compressor power feed when the "Defrost" pushbutton is selected, and the relay functions properly to feed power to the compressor in such Defrost pushbutton "mode."

Interestingly, in true hot rod fashion, I've hot-wired the blower through the cigar lighter, but needless to say, I need a prudent, permanent fix.

I've got lots more about these cars to chat about, but in respect of your time, later!

Keith
West Virginia

A. That climate Control Computer is about $1100.00. I don't think you want to replace that unless you were 105% sure it was bad, which I don't believe it is. I believe it is more likely the blower motor itself. I went through this myself with my 1883 MB 300D. Same exact symptoms and conditions. What led me to the blower motor was having seen several other 300D's all do the same thing.

The brushes inside the blower motor wear down and the springs that keep them in contact with the armature don't have enough push to keep them seated. They will lose contact and then a bump or bang on the end of the blower motor will help the springs push them out and make contact again for a while.

A new blower motor for that car is, are you sitting down? $362.00. You can try getting a used one, but I checked every bone yard in the country and no one had one. In fact, used parts for these cars are hard to find because they are all still on the road. They just don't die.

What I did with mine was open the blower motor up and took out the old brushes. I went to an Auto Electric shop and matched up, as close as I could, a pair of brushes. I used a file to file them down to the same shape as the old ones and put it back together. That was four years ago and it is still going strong. You'll need a soldering iron to take them out and put them in. And I had to change the end of the brush wire but it works great. And in fine German tradition, I did not replace the part, I repaired the broken part.

You sound like you have the skill for this so it should make a good weekend project. It only takes about ten minutes to pull it out and another ten minutes to put it back in, so it's worth a look.