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  #1  
Old 02-05-2015, 12:22 PM
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1983 300D//Fuse Box

Good day everyone-

Been reading through the the threads and I think I have my course of action planned out, but I'd like some confirmation that I'm understanding what I'm reading. I bought this car a couple months ago as my next project. All fuses were changed out to MB ceramic.

Problem:
Fuse #8 had melted away the contact points in the fuse box.

Solution:
Thinking I'll install the MB external fuse mod for the blower. It seems like I can unfasten the fuse box and tilt it fwd to access the back and do the connections. Is this actually how it will happen?

Also, the A/C push-button controls do not stay in when pushed. With #8 fuse out, is this why the defrost button will not catch and stay down. My first thought was I needed to find a new push button assembly, but after fixing the fuse box will this fix that problem?

Any comments are greatly appreciated.

Thanks,

Lance-
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  #2  
Old 02-05-2015, 09:44 PM
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Location: Alexandria, Virginia
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Quote:
Originally Posted by eelsfoot View Post
Good day everyone-

Been reading through the the threads and I think I have my course of action planned out, but I'd like some confirmation that I'm understanding what I'm reading. I bought this car a couple months ago as my next project. All fuses were changed out to MB ceramic.

Problem:
Fuse #8 had melted away the contact points in the fuse box.

Solution:
Thinking I'll install the MB external fuse mod for the blower. It seems like I can unfasten the fuse box and tilt it fwd to access the back and do the connections. Is this actually how it will happen?

Also, the A/C push-button controls do not stay in when pushed. With #8 fuse out, is this why the defrost button will not catch and stay down. My first thought was I needed to find a new push button assembly, but after fixing the fuse box will this fix that problem?

Any comments are greatly appreciated.

Thanks,

Lance-
I had one of these melted 300D fuseboxes a few years ago where I was able to pull & flip the box far enough to transfer the blower wiring from the melted section to one of the pairs of unused fuse contacts. Can't say about the pushbutton control.

Happy Motoring, Mark
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  #3  
Old 02-05-2015, 09:57 PM
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With ANY electrical issue ... the fuses and the box are the 1st places to go.
Id suggest repairing the box and making sure ALL your fuses are good.
A full set with extras is bout $10 on fleabay.
Make sure the right value fuse is used too. Should have a paper roadmap on the
inside of your fuse box cover.

Once that is done, make the aux fan mod and continue
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  #4  
Old 02-06-2015, 06:56 AM
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I'm surprised the contact points melted away,seems that would take much heat.

Anyway, the fuse blew for a reason. The blower motor may have very short brushes, which will draw too much power and blow the fuse. Try and isolate the issue, otherwise you'll keep blowing fuses.
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  #5  
Old 02-06-2015, 08:01 AM
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Quote:
Originally Posted by toomany MBZ View Post
I'm surprised the contact points melted away,seems that would take much heat.

Anyway, the fuse blew for a reason. The blower motor may have very short brushes, which will draw too much power and blow the fuse. Try and isolate the issue, otherwise you'll keep blowing fuses.
The metal contact points might take the heat. It's the plastic fusebox they're attached to that melts.
It would be a lot of work to switch all the wires to another fusebox AND not get any of them mixed up. Switching the fan wiring to a vacant spot on the existing fusebox is the easiest fix and allows for simple fuse replacement. Wiring in an inline fuse-holder to the back of the box means you'll probably need to go under the dash to change the fuse.
Otherwise, any issues with the fan itself still need to be addressed.
Note: Some aftermarket fuses have plastic bodies that melt, or have aluminum strips which corrode at the copper fusebox contact points, causing or contributing to this problem.

Happy Motoring, Mark
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Last edited by Mark DiSilvestro; 02-06-2015 at 08:28 AM.
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  #6  
Old 02-06-2015, 09:59 AM
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Short brushes does not draw more power and blow fuses. Bad bearings, dry bearings (extra drag), blower cage rubbing (extra drag) will.

If there is poor contact between the fuse and fuse holder, a hot spot is created from the blower trying to draw 18 amps through it and that can get hot enough to melt where it attaches to the fuse box.
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  #7  
Old 02-06-2015, 06:53 PM
toomany MBZ's Avatar
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I see.
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  #8  
Old 02-08-2015, 10:54 AM
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short brushes

Short brushes do not draw more current than longer brushes but a motor with short brushes that has not been cleaned or blown out will draw more current due to carbon brush dust accumulation. Sometimes excessive current draw is simply due to carbon accumulation in commutator slots. The dust is easily cleaned out once the commutator is accessed.
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  #9  
Old 02-08-2015, 11:51 AM
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The AC Push buttons are unrelated to Fuse #8.
With the Ignition off the Buttons should stay pushed in.

You have two separate problems. Your PBU (push button unit) is faulty, along
with the fuse problem.

Charlie
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