Many people have complained about the automatic blower speed control in the W124 automatic climate control system. It's almost as if Mercedes is saying, "We know what's best so keep your darn fingers off of the controls!"

For some reason, folks seem to think that the W124 CCU is digital and not modifiable. Well, some of it is digital, but the blower control is analog and adding a manual control is easy. Here's how.





This first picture shows the PBU (N22) at the top center, N29 the "Electronic Blower Control Unit" at the left center, and M2 the blower motor below. N29 (buried under the blower motor) is simply an analog amplifier. It takes a small signal from N22 and with it controls the big current to the blower motor.

Today I measured the current and voltage from N22 to N29. With the High Speed button selected, the current flowing from N22 to N29 is 2.8 milliamperes (0.0028 Amps) at 10 Volts. This is a very small current, barely enough to light one of the little lamps in the instrument cluster.

Since the blower draws 28 Amps at High Speed (we who have fried a lot of fuses know all about this, right?), N29 has a current gain of 28/0.0028 = 10,000. That means that N29 is more than a single transistor but it's still just a big analog amplifier (hence the big heat sink).





The second picture shows the same diagram modified to break the wire from N22 to N29 and add a variable resistor. A value of 20,000 Ohms is enough to take the blower from its maximum speed down to a little more than Low Speed. That's all it takes! To use the manual blower control, all you do is push the High Speed pushbutton and then control the blower manually with the potentiometer.

Practical matters: The PBU has two 14-pin plugs. The one you want is on the left side; it's even labeled "L." Pin 12 has a white/red wire (mine is faded and looks white/pink). Cut that wire, extend it as necessary, add the potentiometer. (I can't find quite the right value in the Radio Shack on-line catalog but it will be available somewhere. You can use either 20,000 Ohms or 25,000 Ohms.)

Where to put the potentiometer? I have a spare panel (the one that covers the upper console switches and PBU) that has a hole drilled in it. Since it is already "ruined," I plan to mount the potentiometer in that hole. Add a nice black knob and it's done (pictures when I finish).

Caveats: AFAIK, all W124s have exactly the same kind of blower control from 1986 through 1995, based on what I can find in the FSM (CD-ROM version). Remember that if you select pushbutton "Auto" or "Low," the blower speed is still affected by the new control so turn it all the way up or those blower speeds will be proportionally lowered.

This mod won't work with the W123s and their fixed resistors. It might work with W201 and W126 cars -- I don't know what their climate control systems are like. As usual, YMMV.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Great idea. I'm going to mod mine tommorow. I hate the noise of the fan on high.

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85 300CD Gramps 390K
89 190D Baby 322K
93 300D Grumpy 176K
99 SLK230

Even easier than I thought!

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
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'87 300TD 225k mi.
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Interesting, and cool, idea.

Gen II W126 cars use the W124 climate control system, so this mod should work for them too.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

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Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
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2013 Fiat 500E.

Excellent post - thanks for sharing! I have wanted to do a similar mod for a long time. What I'd like is for the "auto" button to be a fixed medium speed, instead of automatic control. This would require internal modifications on the PBU / CCU though - not even sure it's possible.

Better yet, it would be neat to replace the three fan speed buttons with a dial identical to the temp wheel, where the dial would offer infinite adjustment from high to low speed. That's probably pushing the envelope though, lol. The wood trim would allow a dial on the right side, but it would take serious surgery on the PBU. I have a spare / broken one that I should probably play with for grins...

Sixto, what sayest thou?

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Dave
Boise, ID
Check out my website photos, documents, and movies!

Would the stock 124 audio fader work as a pot for adjusting the blower speed? The traces inside the fader seem to have three different sections: 0-100 ohms, 100-1K ohms, and 1K-55K ohms (the board is printed with "100-1k-20k"). It's not a linear gradient, but it would be great to use a mercedes knob.

One obvious issue with using the audio fader is that there are two separate pots built into it: one from center down and one from center up. Since the center position is 0 ohms, that would have no effect on the blower speed and would therefore be the "high" setting of the knob. You would then turn the knob down to achieve slower blower speeds. Turning the knob up (or forwards) would be useless and confusing. To prevent this you'd have to physically stop the knob from going past center - probably with a small screw or something. There are already two built-in stops on the plastic wheel; inserting a screw through the knob with a little nylon spacer would effectively act as stop. It would just have to be placed carefully. Along these lines, another thing to consider is that the max ohms seem to be 55k (even though it's printed with 20k), which is more that what you recommend Jeremy, and if there is a problem operating the blower with too small a current, you could install another bumper at the other end of the knob to prevent it from going past 20k-25k.

Agreed. The fader resistance does not meet the needs of the blower controller. However, it is possible to take the fader apart, remove everything except the wheel, and attach the wheel to a small potentiometer. I did some experimenting (see the attached pictures) and determined that such a mod would work but only those with some shop skills and small fingers would be able to pull it off.

You could move the fader to the switch panel above the PBU but you need a position where the next place is unused, because the fader with an external pot will be too wide for a single switch position. The nice thing is that the only part that is visible is a factory part. [The Zebrano panel is from my junk box; I don't know what the round hole was used for.]

Jeremy

Attached Thumbnails

   

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Dang, you revived a dream I still want to use the the middle position of the fader to pass the ACC's automatic signal to the blower, essentially replicating a W140 blower thumbwheel. Turning below the middle position proportionally slows the blower to the ACC's low setting, turning above the middle position proportionally speeds up the blower to the ACC's max setting (seems the ACC's auto setting never blows as hard as max), both relative to the ACC's instantaneous automatic signal. But Jeremy rained on that dream by suggesting that modifying the signal to the blower might set the ACC into a loop.

Sixto
87 300D

It still might be doable but it would take more work. A notch in the wheel at the rear of the fader could operate a microswitch; the microswitch would operate a relay that would connect the PBU to the blower controller "normally" (meaning, automatic speed control by the PBU). Out of the detent in either direction, the 'automatic' connection would be defeated and the potentiometer would govern the speed of the blower.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

" 'Somebody's discovered Shrimp and Grits and I wanna Know where the Lobster Sauce is ?"

Back in "Days of Yore" ,wasn't "Someone" fiddling around with a "Simpler" less
expensive replacement for the W124's Blower Speed Controller?
('Given the Blower Speed Controller's "Finicky"/Delicate nature.It's the second weakest link in the Forced Air delivery system,)

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'84 300SD sold
124.128

Why, because it goes above 20k? If so, can't that be controlled with a physical stop (like I mentioned)?

That works except let's say ACC has the blower at 30%. If you roll the thumbwheel down to set the blower even lower, you'll crank to 50% before getting back to 30% on the way to 20% or 15% or whatever you desire. I'm looking for a way to have the ACC signal as a floating reference for the high end when reducing blower speed and the low end when increasing blower speed.

Yes, I am looking for a W140 SD

Sixto
87 300D

Compress: Yes, I may be reinventing the wheel. You've been a member here a lot longer than I have. I think I did a thorough search before I started this thread but don't remember.

Zebellis: I don't know why I said that; you could be right.

Sixto: Yes, you could run the PBU's speed control output (which varies between 0 and about 6 Volts) through a unitary op-amp, so the output is exactly the same as the input. That would allow the PBU's 'automatic' speed control to function normally.

The potentiometer (whether Mercedes radio fader or something else) could then be used to adjust the control of the blower speed controller up or down, depending on your wish to have the blower speed higher or lower than the speed PBU has chosen. If PBU changed its output voltage, that would be passed through the circuit and proportionally increase or decrease the blower speed. If you didn't like PBU's choice, you could dial it up or down as you wished. The circuit would need to have an upper limit for blower current of no more than 28 Amps.

As far as circuit design goes, that is pushing against the limit of my abilities and I would have to ask the advice of others. But it is possible.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Come to think of it, I think the biggest obstacle to using the audio fader for any of these mods (including what you're talking about Sixto) is that the trace is non-linear. The range from 1k-20k is in the last few degrees of the arc, which amounts to only a few millimeters of movement on the dial. You'd get the knob almost to the end of it's rotation before you started to notice any change in the blower speed.