So I have reduced my HVAC system on my car to merely the defrost buttton, and that works just fine for me most of the time, but now I am going through fuses and somewhere there must be a short. It takes a 16A fuse, and the last one got so hot that it melted the plastic!

I figure I either have a short in a line somewhere, or the fan motor is having problems, though it sounds fine.

Has anyone out there had this problem and is able to offer some advice on where to start looking? Being winter in the NW, the car is not very usable without heat and defrost! Thanks in advance.

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Where there's a will there's a way
1978 300D

it's a standard problem with MB blower circuit. the holder cannot handle the sustained load. ONE you need to pull your blower, and clean and lubricate it. likely the bearings are dirty and grinding, which is drawing extra juice.
TWO you need to get yourself a heavy duty fuse bypass socket, and install a quality 16 amp fuse in it.
likely you are using aftermarket plastic body fuses, and you need to order ceramic body fuses with copper elements. (fastlane has them) you will likely have to dig melted plastic out of the hole in the fuse body and clean the two copper sides.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks vstech, that is what I am looking for. Checking the blower motor makes sense as it is 32 years old! And yes, I am using cheap plastic NAPA fuses. Funny thing is I have had the car for years and never blown a fuse, and now it is eating them. So something has changed. I think the blower motor is a great place to start, as it probably needs love anyway.

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Where there's a will there's a way
1978 300D

yeah, read my thread about 123 blower motors being discontinued by MB... over 4bills to get one now!!! be sure you take care of it!

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

there's a 30 amp upgrade fuse holder for the blower on the w126 made by MB; not sure if it applies to your w123 though. usually if just the defrost works that means there's problem with climate control unit (again on w126; i really don't know much on the w123)

So I removed my heater blower and found that it was a sealed unit. Original, I'm sure, as it was stamped West Germany and had plenty of crud around it, but you can't access bearings in any deep way. So I cleaned it off and blew out gunk from the inside. Then I shot some WD40 into the bearing spaces, wiped that off, and then dripped some oil into those spaces. It did seem to spin a little easier after that.

It still melts the plastic fuses, however.

So now I will upgrade the fuse to ceramic, as recommended. It makes sense that if the 16amp fuse metal is not breaking, then it is not a problem with overcurrent, but with a sustained current that heats the metal but is below the level of blowing the fuse.

Bummer MB is not making the blower anymore. Is that really true?

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Where there's a will there's a way
1978 300D

Apparently the Volvo 850 blower motor fits at least some cars.

My update is that I ordered and tried a good quality non-plastic fuse and after running a few moments it still burned through the fuse element. This fuse is glass with metal conical ends and so there was no plastic to melt.

I had removed the blower motor and it seemed to spin fine.

So now it seems I need to isolate a short somewhere or something. That the fuse doesn't blow immediately, but instead lets the blower work for a few seconds before blowing, makes me think it isn't a short to ground.

Any ideas on where I can start to look for this problem? Can't use my 300D in the meantime because it is wet/cold and without heat/defrost it isn't really safe.

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Where there's a will there's a way
1978 300D

You could run a temporary switched and fused wire directly from the battery to the blower motor to make the car useable until you track down the problem.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yeah, I'm reading other posts and starting to consider my options. I've done lots of DC wiring on my sailboat, and so am very familiar with doing it in a more severe environment than a car. Hate to get mickey mouse about it though. Where to mount the switch...where to run the wires...This damn HVAC system!

Car is about perfect in every other way, but this climate system problem just about has me willing to part ways. I'm already settling for only defrost, and now even that is a pain. Everything else about my beautiful 78 300D is just great.

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Where there's a will there's a way
1978 300D

You've got the evil servo. That is never good.
It's not mickey mouse or redneck if it's just a temporary solution while you chase it down

The solid state retrofit for the servo has gotten good reviews.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Hi there
Have you thought to replace the brushes in the motor? If they are worn down to the wire that is embedded in the brush it will cause a higher current. I had a fault like yours and a brush change fixed the problem. Car was 1983 300d.

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1983 300DT 401,200 + Miles

I like the thought of checking and replacing the brushes, but when I pulled my blower motor I found the case is crimped and not apparently or easily serviceable. It would be great if I could access it, and find that the brushes were the problem and fixable.

Anyone with any advice on gaining access to a 78 W123 blower motor with a crimped housing?

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Where there's a will there's a way
1978 300D

In the DIY Section there is some info on the Brush Changing.

In the below thread I believe there is some links with pics.
http://www.peachparts.com/shopforum/diesel-discussion/308273-need-advice-removing-84-123-blower-motor-fan.html
In the Thread above in my case I had a Seimans made Blower and the Brushes.
Mine had stopped working completely but the Fuse was good. As it turned out one of the Blower Brush connections must have gotten corroded inside and was no longer making Contact (no conduction when tested with the Ohm meter).
There was also signs that there was some localized overheating due to the bad connection.

The Bearings on mine were the self aligning Bronze Spherical Ball type. All I could do was drip some Oil on the Bearings and let is soak in and drip some more and wait for it to soak in; until I felt it was not soaking in any more.
Not a Mercedes but on my Van Blower Motor that had similar bearings a lot of crap had built up between the Motor Shaft and the Bearing making the Blower hard to turn.

I do not know how it worked but on the Passenger Side Fender Well up behind the Head light is a square Box with holes around it. That is some sort of resistor for the Blower; that I believe has to do with the Blower Speed. But, I am not sure.
Since I do not know how it works I cannot say if it could cause the Blower Fuse to blow.

Some pics of the crimp on your Motor would be helpful.

In some of the threads I looked at it appears that all of the Motors are not aliked. Some had the Brushes at the Rear of the Motor; mine at the front.

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84 300D, 82 Volvo 244Gl Diesel

yes, the motor is crimped together, but you should be able to get to the brushes regardless. Simply take the black plastic bottom off of the motor. So you only have the motor and the circular fan "blade". you will have to take off the Mercedes electrical connector piece to get to the brushes. simply fold over the brass brush holder, and don't let the spring inside fly away.