I am at wits end. Last summer (2010), we bought a 1984 300SD because the 240D's trunk wasn't large enough to accommodate my mother-in-law's walker and my son's baby stroller together, so any family outing required two cars or that one member of the family be excluded. When we picked up the beautifully maintained SD in August 2010, the underside was cleaner than many late model cars I've seen and without a spot of rust save for the exhaust system. The paint had been touched up with a poorly matched silver on the hood and behind the two front wheels, but other than that the car was in top shape. Everything was fine until late September when the climate control blower motor stopped working. I replaced the fuse and it worked for a few days and stopped again. We took the car to a local mechanic who had done basic maintenance on the 240D (such as the brake disks and a few oil changes I didn't have time to do myself) and seemed to have a handle on MB. They replaced the blower motor, and the blower motor control regulator, and ran a separate line to the fuse box with a 25 amp fuse. They also told us the A/C was finished, and would cost several grand to repair.
Living in Canada, and the car being equipped with a sunroof, that's not such a big deal, but heat is, and so is defrost. A couple of days later, the blower stopped working, and upon closer inspection I found that the fuse had not only blown, it had melted the fuse carrier. A quick search on the internet revealed that they had probably bypassed the resistor, which led me to question their work. A closer inspection showed some poor electrical practices, wire junctions loosely wrapped in electrical tape that were just a recipe for a fire. Also, the electric antenna stopped working, but they claimed it wasn't working when we brought it in.
After that, we brought the car to another mechanic (over an hour drive away) with more experience in MB, and they undid everything the first place did, then replaced the in dash control unit with a used unit. The blower worked for a few days, but then melted plastic fuse #10. Replaced it with a ceramic fuse after cleaning the contacts with a brush and worked for a few days, then the blower failed entirely without blowing the fuse. Brought it back to the mechanic, and they replaced the speed resistor pack. It worked fine for a few days then failed again. Brought it back and they held on to it for a week, and when we picked it up it was fixed, or so we thought. They had repaired it, but it worked intermittently for a period of a few months (worked when temps were moderate but not in extremes of hot/cold) but then there was nothing. Also, around this time, the dash was randomly losing power, and the running lights were all failing at once, but cleaning the fuse box with a brass brush and some contact cleaner seemed to resolve these issues. The fuse box looks spotless by the way, it is so clean it looks better than the one in my 2009 Jetta.
Reading the forums, I discovered that the ignition switch could be the trouble, so brought the car back and they replaced it. The blower worked for less than a day and stopped. Also, the old switch doesn't seem bad, but since I asked them to replace it I can't complain.
In the summer months, I can do some work myself, so I have tested the motor, and even bought a second as a spare, both work when connected directly for power. I have tested the sensor at the top of the windscreen by removing it and blowing on it while measuring the resistance (I also have a used spare and they give similar values). I have disassembled the in dash control unit, and discovering that one of the pushbutton switches was defective I replaced it with one from the other, testing all operations and refreshing all doubtful solder joints (all with a temperature controlled soldering station @ 700 degrees). I have loosened, cleaned and re-tightened the several grounding points behind the dash. We have not replaced the monovalve, but the MB mechanic didn't think that was the issue. Can the monovalve prevent the blower from working? I am tempted to have them replace it since it seems to be the only thing that we haven't replaced!

Here are the symptoms. A/C/heater blower is inoperable, no matter what setting I select on the fan speed control or on the system selector. We get heat flowing through the vents when at speed and the air pressure is pushing it through so the basic system seems to be working, just no blower. If I twist fuse #11, then press a button, there is a “puff” from the blower, as if it is spinning a few revolutions and stopping. Sometimes it will do this at startup (that's why we thought the ignition switch was bad). There is currently no voltage at the 3 wire plug at the fan motor in any settings. There is no external fuse, the one that on later cars would be on the drivers side just in front of the master cylinder it's in its own black rectangular box (I've seen pictures, and looked for it.) I have cleaned the entire fusebox with a brass brush and tested each fuse, as well as having switched the relays around. The car is incredibly clean, even after a winter of driving in Canada there is still no rust in the engine bay aside from exhaust, springs and pulleys. We had the car rust proofed with Krown when we first got it.

Items replaced include the blower motor, blower motor control regulator, the dashboard control unit, the temperature sensor, the ignition switch and the speed resistor, (the one that sits on the drivers side just under the master brake cylinder). At this point we have spent more on the climate control that what we originally paid for the car, and my wife wants to get rid of it.

Any thoughts or advice would be greatly welcomed.

If it works when directly powered, why don't you run power on a fused and switched line directly from the battery and be done with the problem? With all that troubleshooting and no solution, I wouldn't bother trying to get the OE system operating.

__________________
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

I've considered that, and it might turn out to be our final option, but the car is so perfect otherwise, it'd be sad to cut it up, not to mention that most of the climate control has been replaced.
Also, few others have had the motors blow as a result, so despite having a used spare, I'd like to avoid destroying an almost brand new OEM motor (I'd probably start with the used one, but what if it blows in the first week?)

I'm sorry that you've struggled with this issue for such a lengthy period of time. We should easily get to the bottom of it as the circuit that feeds the blower motor is quite simple.

First and foremost: Turn the ignition key to the #2 position (run). Check for voltage on both sides of fuse #11. It should be a 25 amp (blue) fuse.

If you do not have power at this fuse, the problem is the ignition switch or the wiring from the ignition switch to the fuse box.

If you do have power on fuse #11, find the electronic unit for the blower speed control. There is an input to this unit on socket #12. The wire is red with a yellow stripe and it is a heavy 2.5 mm wire. See if you have power here.

If you have power at this point, plug it back into the electronic unit and check the output of the electronic unit for voltage. That's socket #10 and it is a much lighter .75mm brown wire. This wire goes to the blower.

If you have power at this point (not likely), then check this same brown wire at the blower motor itself for 12V.

That's the complete circuit. Either you have a loose wire or a bad fuse contact at fuse #11 (replace the fuse).


Report back with results................

Responses in-line:
I'm sorry that you've struggled with this issue for such a lengthy period of time. We should easily get to the bottom of it as the circuit that feeds the blower motor is quite simple.
Thanks for helping!

First and foremost: Turn the ignition key to the #2 position (run). Check for voltage on both sides of fuse #11. It should be a 25 amp (blue) fuse.
I have done this, and there is around 12.8v at idle on both sides of the fuse, no drop.

If you do not have power at this fuse, the problem is the ignition switch or the wiring from the ignition switch to the fuse box.

If you do have power on fuse #11, find the electronic unit for the blower speed control. There is an input to this unit on socket #12. The wire is red with a yellow stripe and it is a heavy 2.5 mm wire. See if you have power here.
Question: is this the control unit under the passenger side kickplate? It was replaced but I was not the one doing the work.

If you have power at this point, plug it back into the electronic unit and check the output of the electronic unit for voltage. That's socket #10 and it is a much lighter .75mm brown wire. This wire goes to the blower.
It may take me 24 hours or so to get to this, will advise.

If you have power at this point (not likely), then check this same brown wire at the blower motor itself for 12V.

That's the complete circuit. Either you have a loose wire or a bad fuse contact at fuse #11 (replace the fuse).


Report back with results................
Thanks, I will

From memory, yes, it is behind the passenger side kick panel. If they replaced it with another faulty unit, that would be your problem

This device is the only thing between the fuse and the blower.

The blower regulator is a black box behind the passenger kick panel. It's mounted on a metal frame flat against the large duct that goes from the blower motor housing to the HVAC box.

__________________
1984 300 SD 384,000 Miles

Brian you are a gentleman and a scholar. I have been struggling with this problem for a year, and had two different garages look at it, and this is the first time I feel confident that we have solved the problem. As far as I can tell, the first garage took the rebuilt control unit out of it's electrostatic packaging but never bothered to replace it in the car. I am assuming this because when I plugged the "failed" core, to be returned (we never did because the core charge was less than the cost of shipping to the US), everything worked. I have run the car for an hour now and the fan works as it is supposed to, slowing down in automatic mode when the car interior reaches temperature as advertised. I haven't tried the A/C because it's 40°f outside, but I will take a look at it next summer. Thank you so much for your help, it was so simple but it never occurred to me that the "new" control unit could be bad. I am in your debt!

Excellent.

Glad it worked out for you.

Help is what we do here!!

I took the electronic unit apart and touched the solenoids with a screwdriver and the blower motor goes thru the speeds when touching different ones so now im lost any help would be appreciated

thank you

Well, you have confirmed that the blower motor has power and it is not the culprit.

The first thing to do is to ground the 50C. switch under the hood to be sure the CCU is grounded. Socket #6 on the left side plug should show a ground if the engine is warm or if you ground the wire under the hood.

If you have the ground and the blower still does not work, the problem is either the CCU or the blower controller. The blower controller very rarely fails as it's a solid state unit and the CCU is the most likely culprit. Unfortunately, I have no good way to test these two units without the OE test apparatus. It's a bit of a crapshoot.

I have my own issues with the SD at the moment. After replacing the blower motor, the system works but it won't modulate temperature. It gets full heat...........all the time. The blower does work and the speeds for the blower do function properly.

I bought another CCU on e-bay for troubleshooting purposes. Maybe it will shed some light on my issue.

I have heard that the CCUs soldered connections are weak and often have to be resoldered

The switch under the hood is on the right side of the head close to the thermostat housing. There is a single wire that runs to it. From memory, this wire passes in front of the head and it originates on the left side of the engine.

Yes, the CCU's have this as a common failure. However, there is no assurance that soldering the CCU will fully repair it.

I have a 204 D with a 3 speed radial dial blower motor adjustment knob. The blower motor works great in first two positions. But when I turn it to the third position the 8 amp fuse blows. I replaced the blower motor once and it worked fine for a while now it too blows the fuse on the third position. Does anyone have an idea what is going on and how to repair it.

If you are following the instructions on this thread, you will never have power at pin number 10 on the blower speed control. Number 10 is the ground and it is a brown wire and brown is always the ground. Power output is pin number 6 and the wire is green and red. I was following these instructions and wasted a bunch of time so I'm posting this to correct the information. Also, here is the link to the FSM wiring diagram.

https://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACC83-85.pdf