My CCU for my '83 300SD had some mechanical issues (off button would not disengage any of the other buttons). Turns out that there's a slide rail that shifts as you press a button to release all the other ones. I ended up desoldering the entire unit, took off the daughterboard, then took off the switches from the mainboard.

I THOUGHT I soldered it back on properly, and when I plug it in, I hear the vents opening/closing, but when I put it on defrost, the fan does NOT turn on. Nothing I can do (not even turning the fan speed to HI) will turn the blower on.

I suspect I failed to solder a very specific spot, but I've been trying to look over the electrical diagram to see how the blower is turned on and I can't see how the CCU does it. I've looked at the bottom and don't see any cracks in the side connectors' solder.

How can I test the CCU with a multimeter to verify that it tells the blower to turn on? I have the CCU plastic housing off so I can always follow the circuits to see what joints I may have done poorly.

I take the blower control module's plug and wire #12 (power) to #6 (blower motor), and the blower turns on like a charm.

And if by some chance it's not the CCU (the blower did work this morning before I did any of this work), where do I check next?

I've already checked the fuse box and #10 fuse (powering the CCU) is OK. I'm gonna resolder the whole thing again and see if that helps.

I believe the blower speed is controlled by varying the resistance on the ground side. If you look at your diagram there is a ground (C1 pin 3) (about middle of the pushbutton control assembly). When switched to DEF, a direct ground is applied(through the PBU) to pin 8 of the Blower Speed Controller.

This forces the Blower Speed Control to HI speed.

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Sam

84 300SD 350K+ miles ( Blue Belle )

With DEF on, there is a tiny resistance of 0.3 ohm between pin #8 and #3 of the CCU. Yet the blower doesn't turn on.

I've tried hot wiring the CCU plug wires (#8 to #3), but the fan didn't turn on. Should it? If so, maybe the speed controller is (suddenly) faulty?

I've tried using the low speed (Speed 1), but don't hear a peep (I know it's quiet, but still). I'm pretty sure the CCU has the low speed setting working.

I'm wondering if maybe there is a different connection to tell it to turn the fan on (regardless of speed)? I've hotwired the blower speed controller plug so POWER goes directly to SPEED 6 OUTPUT. And it turns on. But I'm suspicious my CCU was re-soldered wrong.

Any clue what the "Automatic Blower Speed Input" does (#1 on the speed controller, from C2/#5 on the CCU)?

If the problem is only in DEF, I don't see how the Blower Speed Controller can be the failure. Have you checked grounding of C1 pin 3?

The "automatic blower speed control" gives control of the speed to the Temperature Controller.

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Sam

84 300SD 350K+ miles ( Blue Belle )

No matter what speed I put the CCU on, whether HI or AUTO or LOW (and trying all button combinations), the blower will NOT turn on. I hear some gurgling noise (something to do with a circulation pump?), but that's all. Absolutely NOTHING works to trigger the blower motor.

The C1/pin 3 GROUND is properly grounded because it and BSC pin 10 (ground) are connected (no resistance).

Is there a way to turn the blower fan on from the climate control plugs, by shorting a particular circuit/path (one pin to another)? I've tried connecting C1/3 to C2/8, to no avail. It sounds like that's all that's needed to turn on the blower to HI (even if the CCU is totally removed) but maybe someone can correct me.

I measure the resistance from C1/3 (Ground) to C2/8 (SPEED 6 CONTROL) on the CCU, when DEF is on, and it measures 0.4 ohm (I think my multimeter is just miscalibrated so it's probably really 0 ohm). There's no reference whether it should be a particular resistance or not. Maybe that is just normal?

If I measure the resistance from SPEED 6 CONTROL (#9 on the Blower Speed Controller) to GROUND (#10), I get no less than 30 ohms with the DEF on. Is that's what's expected? I really am at a loss here. I wouldn't even know where to begin to test the blower speed controller.

I've pretty much gone through all the connections on the circuit board and tested for resistance. Nothing showed an open circuit, but there definitely was some resistance, I don't know if that's the solder or if it's normal or what.

I'm honestly tempted to just find a parts car and get the CCU and Blower Speed Controller off of it. Yesterday morning I had two perfectly good CCU's and in the process of swapping parts (the bulb sockets), the buttons and slide rail broke due to age. Now I have two junk CCU's that I can't get either working. Definitely not paying $200+ for a refurbished one.

Have you tried unplugging the connector from the blower and apply power to it directly? That will tell you if the problem is the blower or elsewhere.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I've unplugged the Blower Speed Controller and shorted POWER (#12) to BLOWER MOTOR (#6). Fan turns right on. Not the fan. Power is going to the BSC. Worked yesterday morning. I rebuilt my CCU, and it at least partially works (the aux pump seems to kick on), but not sure whether it's the BSC or the CCU at this point. There isn't much in terms of testing the BSC out there. I found one transcript of a diagnosis and am looking at that now.

http://www.justanswer.com/mercedes/0z76p-ok-guys-i-m-back-still-working-82-mercedes.html

OK! I've made progress diagnosing things.

The CCU properly grounds when DEF is on. I read full voltage from the BSC #12 (POWER) to #9 (SPEED 6 CONTROL).

The Blower Speed Controller DOES have power. #12 (power) to #10 (ground) reads voltage.

Full voltage for SPEED 1 (#2 to GROUND) when I have it on LOW.

Blower resistors all measure resistance to the blower motor (#6).

The blower turns on when I short POWER (#12) to the blower motor (#6).

There's no voltage between POWER (#12) and the blower motor pin (#6), or any of the resisters between, even though there IS voltage from POWER (#12) to SPEED 6 CONTROL (#3) - which is grounded.

If I wire power to the SPEED 1 OUTPUT resistor (or any other resistors), the fan turns on to that respective speed. The blower resistors seem good.

Is it safe to conclude that the blower speed controller is bad (something between the speed input and speed output)? It seems to me that all the power is there, all the correct input is present, yet it won't turn the blower on.

I just don't understand since it was working yesterday morning. I DID perhaps foolishly plug in the CCU while the car was running, and the blower turned on with only the left CCU plug on. Not sure if that shorted something.

Should I take the Blower Speed Controller out and see if there are any hairline cracks in the solder, something blown, etc?

One more thing is needed. There is a cold engine lockout switch and relay at the other end of the diagram you provided. It puts 12 volts on pin c1-12 to allow the fan to come on. It is usually energized by the engine temp but also energized when in defrost mode. If this relay doesn't send 12 volts out c1-12, the blower will stay off.

Paul

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84 500 SEL (307,xxx miles)

I must have a 2nd generation W126 unit ('86+). This CCU has 6 relays. The diagram I see only has four. I have another one that's 4 only.

Only two relays turn on (and in all modes), but they must be the wrong ones. I've wired my multimeter from C1/#12 (SPEED ENABLE OUTPUT) to C2/#12 (POWER), and then fiddled to trigger each relay by hand, and I found the right relay (which just isn't tripping when put on DEF) by checking for resistance (0 ohm once the proper relay is tripped). If I plug it all back in and then use a screwdriver to push the relay switch shut, the blower fan turns on. So it looks like some wiring issue with the relay. I think I safely have the problem pinpointed.

Some information here.

Middle-aged Mercedes Electrical Repairs

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Tomas, the generation II 126 control unit is completely different (looks similar but electriclly very different). The plug in's have more pins (14 each I believe) while the Gen 1s have 12 pins). Most people think a 126 is a 126 but electrically they are very different. They change just about every year with the gen 1 and gen2 being verrry different.
That cold engine lockout relay can either be intergized by the lockout temp switch or by the mode switch being in defrost mode. Once the relay is intergized, it will stay intergized until the system is turned off. If the system is on (heat, A/C, Defrost) it should stay intergized as long as there is power to the system (Ign on) and the system is on.
I am looking at diagrams for my 84 500SEL which should be close to your 83 300. Don't take everything I say as being 100% correct, but should be close. I'll look some more and see if I can give you some more things to look at to get this thing working.

Paul

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84 500 SEL (307,xxx miles)