1979 300SD HVAC fuse melting - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 02-18-2019, 08:16 AM

I can not figure out why, but when I turn on the heat on any setting including defrost, it will run fine for a few minutes and then stop. When I check the fuse, its melted. Literally. Which to me means that there is a short somewhere???

Sugar Bear · #2 02-18-2019, 08:57 AM

Is it certain the correct amperage fuse is installed?

Look closely at the ends of the metal strip of the fuse where it failed, if the ends are glob like versus sharp/blunt then there was a slow blow from heat due to a moderate current overload such as a blower motor wearing out. If the ends are sharp then there was a short to ground with a sudden large current overload.

Good luck and post a picture of the blown fuse if you wish.

Diesel911 · #3 02-18-2019, 11:34 AM

Is it the metal strip that melted or the body of the Fuse? Fuses with plastic bodies have had the plastic melt before the metal strip.

There used to be an upgrade to a higher amp fuse I think it was only for older SD's but you need to look it up. It is not for W123's

AUX Fuse For Blower Motor (not sure what is in this thread.)
Write up with pictures, aux fuse for blower motor - PeachParts Mercedes-Benz Forum

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Not that as time passes the lube in the bushings dries up and at least on mine the blower motor was full of gunk and I even had one of the wires going to a blower motor brush due to crossing making poor contact and you could see that it had been over heating. Anyway resistance to turning or electrical resistance cause a higher amperage draw.

Personally even if you do the fuse mod I would remove the Blower Motor and see what is going on there.
Some people also have found worn brushes.

Not all of the motors are Bosch. Mine was made by Siemens.

sixto · #4 02-18-2019, 11:39 AM

If there’s an aux pump in the circuit, unplug it then see if the fuse blows.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Diesel911 · #5 02-18-2019, 11:52 AM

Quote:

Originally Posted by sixto

If there’s an aux pump in the circuit, unplug it then see if the fuse blows.

Sixto
98 E320s sedan and wagon
02 C320 wagon

I don't know on SD's but on W123's there is no fuse in the Aux Water Pump Circuit and when something shorts it burns up part of the Climate Control Circuit Board and even has caused one fire.

There used to be a few threads on adding a fuse to the Aux Water Pump circuit.

sixto · #6 02-18-2019, 12:02 PM

I mean maybe the aux pump rather than the blower motor is drawing the amps to blow the fuse. Unplugging it is a simple test.

Does the 116 have a ventilated resistor box for blower speed control? Maybe there’s a short in one of the resistors. Blower starts on high then fuse blows at lower speed.

Sixto
98 E320s sedan and wagon
02 C320 wagon

turbozeke418 · #7 02-23-2019, 11:57 PM

it's a slow burn (melt). I played around today with it and found that it works fine on auto hi and lo. the fuse is not hot and and doesn't melt. It get's hot and melts when you put the defroster on.

Clemson88 · #8 02-24-2019, 05:37 AM

Quote:

Originally Posted by Sugar Bear

Is it certain the correct amperage fuse is installed?

Look closely at the ends of the metal strip of the fuse where it failed, if the ends are glob like versus sharp/blunt then there was a slow blow from heat due to a moderate current overload such as a blower motor wearing out. If the ends are sharp then there was a short to ground with a sudden large current overload.

Good luck and post a picture of the blown fuse if you wish.

There's gold in them thar hills.

turbozeke418 · #9 02-24-2019, 07:22 AM

I put the largest fuse possible in there and it still melts. I guess I just wont use the defroster button

Rooster300SD · #10 02-27-2019, 07:53 PM

Not sure if the 116 is the same as 126 but my car had a 30 amp fuse
soldered into the # 11 AC/motor blower fuse position when I bought it.
Seems like I read once that this was a known problem and the larger fuse
retrofit was a MB recommendation? I haven't had a any problems running the larger fuse.

Jeff

turbozeke418 · #11 03-31-2019, 08:21 PM

I did a little trouble shooting and the fan works great. But, I suppose something could be wrong and also not know? When I turn on the defroster the fuse for automatic air conditioning #3 gets beyond 160* and melts. I also have no bi level hi or lo function. I checked the servo and powered from an external source and it works. When I plug it in though it does now. I have several electrical diagrams etc.. but no clue how to read the blueprint looking stuff. Can anyone tell me (in simpleton context ) what places I should check in the system that would be suspect?

dude99 · #12 03-31-2019, 10:00 PM

In my 85 300SD, my fuse kept blowing when the blower motor bearings began to seize up. Would run for a few minutes over drawing and then blow the fuse. Pull your blower motor and see how tight the bearings feel.

turbozeke418 · #13 03-31-2019, 10:11 PM

I did....spins freely. Feels like a regular fan would.

dude99 · #14 04-01-2019, 12:55 AM

Just for giggles, does the fuse blow when the blower is disconnected?

turbozeke418 · #15 04-01-2019, 01:37 AM

nope not at all...doesn't even get hot.