Blower
Back to: WIS 83.40 Automatic Climate Control (ACC) or Heating Ventilation and Air Conditioning (HVAC)
Issues - Blower Motor
- ACC Blower Motor Speed Regulator fails due to corrosion.
- ACC Blower Motor hard to turn due to corrosion. This causes a high current draw and if not caught in time permanently damages the motor speed regulator components.
- Root cause of the many blower issues seems to be corrosion of the motor assembly due to excessive moisture due to blocked drains and debris in the plenum.
- A blower motor seized due to corrosion can be freed up but if the cause of the moisture is not addressed the issue will reoccur.
Diagnosis/Testing - Blower Motor
Control Voltage - Blower Motor
ACC Blower Control Voltage Typical Results
DIY Procedures - Blower Motor
Blocked Drains
Frequent checks and maintenance for blocked drains must be conducted. Under Hood ACC Drain Access and Cleaning
Blower Motor Replacement
Blower Motor Removing Repairing Replacing
Regulator Repair
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
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 you 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. For Service Menus see here: Automatic Climate Control (ACC) Service Menus