1979 300SD Def blows fuses

I've got 278K miles on this great old car. The engine is in great shape, but the rest of the systems aren't quite so resilient. The latest problem is that my defroster for the front windows is blowing
two of the fuses whenever I try to use it. It has consistently blown the #3 & #9 fuses, which correspond to the HVAC system and the air conditioner blower. The rest of the systems are fine, it's only the defroster.
Does anyone have any experiences with something like this? I would like to troubleshoot it myself before I have to take it to the garage. They don't seem to be great sleuths.
Any advice would be terrific!
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I've owned a '80 300SD since new and sympathize with you; this climate control is the most difficult "system" in the world to fix.
I've looked at the wiring schematic and have a suggestion or two.
#3 and #9 fuses are connected via the double pole relay, so there's reason for both to blow.
I first suggest you temporarily disconnect the electric auxiliary water pump as a test; it may be shorted and blowing #9 fuse. If that's excluded and the fuses still blow go to step two.
Then I suggest you temporarily disconnect the electric auxiliary fan as a test; it may be shorted and blowing #9 fuse. If that's excluded and the fuses still blow go to step three.
DEF causes the blower to run at maximum speed and heat, if the fuses also blow when AUTO HIGH is pressed and the TEMP wheel is set to say, 60 degrees on a hot day then I'd check the blower motor. I suggest this because the blower motor runs at maximum only at these times. The motor is located behind the glove box, which can be removed by removing its plastic rivets. If the blower motor PROVES to be OK, despite all its use, then I'm afraid the servo is the most likely suspect, especially if it's more than 7 to 10 years old.
Unfortunately one just doesn't know; it's too complicated to comprehend and the repair becomes a guessing game. That's the problem that everyone, not just your shop, has with this old system. Unfortunately the fellows who understood it have mostly retired from wrenching.
I personally don't touch it, I just leave it on Auto and hope it runs another year.
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