W124 Manual Blower Control DIY

[Jeremy5848]

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