W126 fuse melted but not blown?
 

Well gentlemen this last weekend before taking the Mrs. out for a cruise to the local breakfast place, we decided to plug the GPS in. In doing so we realized there was a small piece of a metal tip from a different 12volt device in the plug. It blew the fuse for the AC blower motor. It was a nice day for keeping the windows down so I told her I was hungry and that I would replace it after eating. Next stop was to the parts store for the W111. I threw a new fuse in and watched it discolor in seconds, but did not "blow". I went to pull it as the tab inside the fuse box was also discoloring. Man was that little %&$# HOT! Burnt the crap out of my fingers. The fan should have started right up when the fuse was replaced. But it did not. Only ruined the fuse and discolored the tab in the fusebox. Any suggestions on troubleshooting?

The melted one was from when the small metal tip was found. The discolored one was the replacement.
http://vintagebenz.org/photos/W126_fuse.jpg


How is it that it did not "blow"?
http://vintagebenz.org/photos/W126_fuse2.jpg

That means that the load through the fuse was right at the rating of the fuse -- blue is 25 Amps? The blower draws about that much; if the blower was hung up on something that had fallen into the squirrel cage it would draw current but not move. Once the fan gets to spinning, the motor will develop a "back EMF" and the current load comes down a bit.

Thanks for the quick reply Jeremy. I will check and make sure everything is clear. The only thing I find odd is the timing or the other fuse blowing and hours later something getting stuck in the blower motor.

OK, maybe I didn't understand completely.

I'm not very familiar with the 126 -- can you unplug the blower by going up under the passenger-side under-dash panel like I can in my 123? If so, try a new fuse after unplugging, see if it is OK. Then check the blower to see if you can spin it by hand. If it won't turn or is hard to turn, it could have bad bushings or some other problem.

It is also possible that bad brushes or broken wires or some weird thing that I cannot imagine is causing your problem. You could also run some wires and try operating the blower directly from the battery, just to see if it will work.

Jeremy

The blower on the W126 doesn't use 25A...holy cow....that'd be 300+ watts at 13+ volts! :eek: I think more like 8-9A is what it uses. Blue fuse is a very high rating (25A)....are you sure that is what was supposed to be in that slot? There are very few of those used in a W126.....most are white (8A) or Red (16A)....it melted because its plastic....use the real (and correct size physically) ceramic fuses...they are much better and super cheap (online for about .10 apiece)

I am nearly 99 percent positive it asks for a 25A fuse. I drove the W210 today so I will have to double check when I get home. The vehicle came with all plastic fuses when I bought it. Wouldn't mind starting to replace them with ceramic. Know of a good source? If the fuse actually calls for a 25A, what should my next course of action be?

My 82 126 uses a blue fuse in that slot, and my fuses did the same thing... until I replaced the blower motor.
no problems since.
as a matter of fact, I think there is a service bullitin replacing the fuse for the blower with a 30!!! amp fuse block! from MB!

Fuse upgrade for blower
 

Mercedes did a mid-year mod on the early 124 cars (the '87 300D) and perhaps on others, as you mention. It involved adding a special 1-fuse holder outside of the fuse compartment and wiring the blower fuse wires to it instead of the regular spot in the fuse box. The special holder takes a 30 amp "strip fuse" (like the glow plug relay uses). A thread on this is here.

Jeremy

Correct. I just replaced the 30amp strip fuse for the blower on my car. In 1991 it was standard design from the factory. I would guess the blower motor is over drawing the circuit which is why the 25amp fuse is melting. It could be the brushes have worn completely down and are binding/dragging against the armature.

That's a good guess. I don't know how big a job it is to pull the blower in a 126 -- in my 123 it was easy. The brushes had not only worn down, in the process they wore the commutator as if it had been in a lathe (with a dull cutter!). I ended up replacing the motor -- about US$85 in 2006. In my case, there was no overload but the blower was intermittent due to poor contact from the excessive wear.

it's just as easy in the 126

Thank you for the help gentlemen. Does anyone else find it odd that within one hour of finding a piece of metal in the cigarette lighter socket (the same circuit as the blower motor) causing the fuse to blow, the blower motor has all of these problems? It also managed to blow the fuse right next to it, the 25A for the radio. One would think the two would have to be somewhat related. I might take a peek tonight as I will be in the garage knee deep in the M180's fuel system. I will let you know what I come up with. Thanks again for the help fellas.

New Brushes for the "Blower" motor
 

And a little ATF gently applied to the motor shaft (For a lubricant)

This has already been seconded, I'll "third" it. My 300SDL (w126) also has this modification. When my plastic 30 amp fuse went, it melted the plastic fuse casing (nearly catching fire, i'm sure) rather than burning the strip fuse in two. When this happened, we simply went to Advance and bought a sturdily wired "fuse holder" (built for this purpose, introducing a fuse to a circuit) and a 30 amp standard stick-in American-car type fuse that fit it, plus a spare fuse to keep in the glove box. Replacement original parts are of course available but I don't like the idea of a potentially hot fuse in a plastic casing :eek:

Yes, I know the entire fuse box is plastic... but those seem, as a rule, to pose a bit less risk than strip fuses.

Well again the MercedesShop forum members save the day.

So I take a break from working on the W111, which I got running for a second BTW to look under the passenger's side dash at the blower motor. I pry the cover open a bit and feel around inside there to see if anything could have caused the motor to be jammed. Sure enough I felt what I thought to be Kleenex so I pulled it out. Now it was dark underneath there so I took my handful of Kleenex out to take a peak.

http://vintagebenz.org/photos/dead_mouse.jpg

Looks like the blower blade popped him right in the skull. Damn mouse must have been thinking he was living large being shacked up in a Mercedes. Put the fuse back in, air as cold as the arctic. So I also replaced the blown fuse next to it for the radio. Got in and went to turn the radio on, nothing. Next thing I know I see a lot of smoke coming from my fuse box. I left the cover off just in case thank God. Popped the fuse out and sat there and scratched the old noggin and went back to working on the W111.

So I again bow to the MercedesShop experts and ask what this problem is?

Before smoke
http://vintagebenz.org/photos/radio_fuse.jpg

Right after smoke
http://vintagebenz.org/photos/radio_fuse2.jpg