Blower
Control Voltage - Blower Motor
DIY ACC Blower Control Voltage Typical Results
Repair - Blower Motor
The Issue
Several BenzWorld Members have had trouble with the ACC Blower Motor regulator multiple times on different M-B models. In some cases, they found that the motor rotor was hard to turn. This caused a high current draw and if not caught in time fries the regulator components. After freeing up the motor, the unit will work again. But the root cause of the problem seems to be corrosion of the motor assembly due to excessive moisture due to blocked drains and debris in the plenum. This is another example of poor M-B design. Frequent checks and maintenance in this area must be conducted.
Replacement - Blower Motor
DIY Blower Motor Removing Repairing Replacing
Regulator Repair - Blower Motor
Reference: http://www.benzworld.org/forums/w140-s-class/1470248-blower-regulator-electronics.html#post3706621
Summary: The power transistor driving the motor is actually an N-channel power MOSFET (BTS282Z) which incorporates two temperature sensors mounted on the chip (making it a TEMPFET), sensing the gate and source temperatures. I think the MOSFET operates pretty much as dmac described, allowing for the difference in transistor type. The regulator also contains a pcb with one IC on it - a 14-pin DIP - which carries the IDs: 2901 and 90H815. From the 2901, I guess this is a quad comparator. The rest of the circuitry was not decodable. I assume the four comparators are used with the temperature sensors to stabilise the motor speed as the chip's temperature changes - also to protect the chip, although they don't seem too good at that. The 14-pin DIP would definitely needs its own 12 V line and I think that this is provided by the red connection to pin 4 of the regulator.
Regulator Replacement - Blower Motor
It's a 10-15 minute job if you take your time, and this repair worked wonders for my car's A/C!
I got my blower regulator off eBay for $75 with a supposedly German OEM supplier: Hamburg Technic. I called stealership and they quoted $292 and $399 for two brands. I didn't ask but assume it's Bosch or equivalent. Autohaus has German name brands for ~$170-230 so I took a chance with eBay.
Tools needed: Philips screwdriver #2 and Torx-20 driver.
Symptoms: Very weak air flow especially though out the day in Dallas, TX heat. I noticed air flow getting weaker in a period of a month. So I broke out my Fluke DVM and only measured 2-3VDC across the blower terminals. If your press REST on the climate controls for 10sec. you get into the diagnostic mode. I pressed the left AUTO button to cycle up to step 10 then adjusted (+-) fan buttons. On MAX, I only measured 4VDC. First start up in the morning yielded me 7VDC but still didn't flow a lot. When I commanded 1 or 2 bars (MIN), I got 1VDC.