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Pelican Technical Article:

Blower Motor Testing

Nick Czerula

Time:

2 hours2 hrs

Tab:

$200

Talent:

****

Tools:

Phillips head screwdriver, T20 Torx driver

Applicable Models:

Mercedes-Benz W211 (2003-09)

Parts Required:

Blower motor final stage unit

Hot Tip:

Use a T20 Torx socket for the rear fasteners

Performance Gain:

Proper working blower motor

Complementary Modification:

Replace cabin air filter

The blower motor controls air flow into the passenger cabin in conjunction with the blower motor resistor (final stage). It is mounted in the heater housing behind the right side of the dashboard. The blower motor speed is adjusted via the HVAC control panel through the blower motor final stage. The HVAC control panel receives a request from the driver or passenger, then sends a serial data signal to the blower motor final stage unit to the blower motor.

The blower motor final stage unit (resistor) is utilized to control blower motor speed. It receives the driver demand signal from the HVAC control panel and sends a high current signal to control blower motor speed. The blower final stage unit can fail resulting in a malfunctioning or inoperative blower motor. If you suspect your final stage is failing, look for the following symptoms: Blower motor speed fluctuates from high to low. Blower motor does not turn ON. Blower motor only runs at one speed.

Over time, a blower motor may become noisy as the motor electrical contacts wear. I have also seen debris from a rodent get trapped in the blower cage creating a vibration or noise. If your blower is noisy, pull it down and see if there is something stuck in it. If you find debris, be sure to clean out the HAVC housing before installing the new blower motor. If you have a vibration, see our tech article on blower motor replacing.

If you push the button to change the blower motor speed and nothing happens, you may have a blown fuse, a bad blower motor relay or a bad final stage. There have been cases where the final stage has kept the blower motor running while the ignition key is off, therefore draining the battery and leading to a "no-start" problem the next morning. In this tech article we are going to review how to test each component in the system

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

I suggest using a current wiring diagram when testing your vehicle, as the wiring or circuit connectors may vary from my subject vehicle.

Working at the left corner of the dashboard, open the fuse panel door.
Figure 1

Working at the left corner of the dashboard, open the fuse panel door. Check fuse #25 30 amp, fuse #41 15 amp and fuse #30 30 amp for continuity. Use the fuse sheet supplied with your vehicle (yellow arrow) to confirm fuse location and assignment. The paper should be stuffed into a compartment in the side of your fuse panel. Also, confirm with a current wiring diagram, as I find year to year there are changes.

Working below the glove box, remove two T20 Torx lower trim panel fasteners (red arrows).
Figure 2

Working below the glove box, remove two T20 Torx lower trim panel fasteners (red arrows). The fasteners shown are Phillips head screws, however they should be T20 Torx. Someone has replaced them with the incorrect fastener.

Detach the lower corner of lower trim piece (red arrow) by pulling it down until the retaining lug disengages.
Figure 3

Detach the lower corner of lower trim piece (red arrow) by pulling it down until the retaining lug disengages. Pull the lower trim panel down from the dashboard far enough to access the footwell light electrical connector (yellow arrows). Disconnect the footwell light electrical connector by squeezing the release tab and pulling it straight out of the light. Then remove the trim panel from the vehicle. Be sure to store it in a safe place. 

Next, the lower blower motor cover (red arrow) has to be removed.
Figure 4

Next, the lower blower motor cover (red arrow) has to be removed.

Start by pulling the wiring harness out of the retaining tabs (red arrow), then disconnect the electrical connect by squeezing the release tab and pulling it straight out of the blower motor (inset).
Figure 5

Start by pulling the wiring harness out of the retaining tabs (red arrow), then disconnect the electrical connect by squeezing the release tab and pulling it straight out of the blower motor (inset).

Working at the blower motor cover, remove the four T20 Torx fasteners (red arrows).
Figure 6

Working at the blower motor cover, remove the four T20 Torx fasteners (red arrows).

Remove the blower motor cover from the blower and store it in a safe place.
Figure 7

Remove the blower motor cover from the blower and store it in a safe place.

Working at the blower motor resistor (red arrow), disconnect the electrical connector by squeezing the release tabs (yellow arrows) and pulling the connector straight out of the blower motor.
Figure 8

Working at the blower motor resistor (red arrow), disconnect the electrical connector by squeezing the release tabs (yellow arrows) and pulling the connector straight out of the blower motor.

Start by connecting two fused jumper wires to the final stage blower motor output connector (red arrow).
Figure 9

Start by connecting two fused jumper wires to the final stage blower motor output connector (red arrow). I disconnected the HAVC control panel connector (green arrow) for a better view of the rear connector. You do not have to disconnect it. Once connected, plug the blower motor electrical connector into the jumper leads to connect the circuit. Connect your DVOM across the terminals. Connect the negative test lead to the brown wire and the red test lead to the red wire.

Turn the blower speed to high (yellow arrow).
Figure 10

Turn the blower speed to high (yellow arrow). Your DVOM should read battery volts (green arrow). If the blower motor is running, it will be about 12 volts with a vehicle power supply on. If just testing using your battery, expect a two-volt drop from what open circuit battery voltage is at the time. If you get no voltage, try cycling the key off and waiting one minute, then cycling the key on with your DVOM already in place. If the HVAC control panel doesn't see a load, it will cut the signal. So I suggest doing this test a few times to confirm there in no attempt at turning the blower motor on. If you have zero volts or lower than battery volts, the final stage unit may be faulty. In the next steps I will show you how to test it.

It's also a good idea to load the signal to see if the resistor can handle the load of the blower motor.
Figure 11

It's also a good idea to load the signal to see if the resistor can handle the load of the blower motor. I like to use an incandescent test light. In this case, the test light is acting as the blower motor. Turn the blower speed to high (yellow arrow). Your DVOM should read battery volts (green arrow). If the blower motor is running, it will be about 12 volts with a vehicle power supply on. If just testing using your battery, expect a two-volt drop from what open circuit battery voltage is at the time. If you get no voltage, try cycling the key off and waiting one minute, then cycling the key on with your DVOM already in place. If the HVAC control panel doesn't see a load, it will cut the signal. So I suggest doing this test a few times to confirm there in no attempt at turning the blower motor on. If you have zero volts or lower than battery volts, the final stage unit may be faulty. In the next steps I will show you how to test it.

Working at the final stage unit, you will be testing the four wires.
Figure 12

Working at the final stage unit, you will be testing the four wires. Start at the red with blue tracer (red arrow). This wire should have battery volts. Next, the brown wire (yellow arrow) should be battery negative (0.1 volts). Next, the white with red tracer (blue arrow) wire should be battery negative (0.1 volts). Then the green with blue tracer (green arrow) wire will be about 10 volts with blower from low to high speed.

The signal that controls blower speed, sent from the HVAC control head is the green wire with blue tracer.
Figure 13

The signal that controls blower speed, sent from the HVAC control head is the green wire with blue tracer. With a DVOM backprobing (red arrow) that terminal and the negative lead of the DVOM on battery negative, you should have about 10 volts. No matter the speed you put the blower on the signal from the HVAC control head will remain at about 10 volts (blue arrow). If you have this signal, and no blower speed power output, the blower motor final stage unit is faulty. For more detail on the signal the final stage unit receives, see the next step. If you do not have this signal, the HVAC control panel may be faulty. Check for fault codes using a Mercedes-Benz scan tool. You can also inspect the integrity of the wiring from the HVAC control panel to the final stage unit.

The signal that controls blower speed, sent from the HVAC control head is the green wire with blue tracer.
Figure 14

The signal that controls blower speed, sent from the HVAC control head is the green wire with blue tracer. With a labscope backprobing (red arrow) that terminal and the negative lead of the DVOM on battery negative, you should have about a battery volt square wave. No matter the speed you put the blower on the signal from the HVAC control head will remain at about the same. This is a serial data signal sent from the HVAC control module to the blower motor final stage unit to control final output of blower motor speed. If you do not have this signal, the HVAC control panel may be faulty. Check for fault codes using a Mercedes-Benz scan tool. You can also inspect the integrity of the wiring from the HVAC control panel to the final stage unit.


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Comments and Suggestions:
abhay Comments: what will be the serial data from HVAC control to the regulator? how can we decode it?
August 16, 2016
  Followup from the Pelican Staff: You don;t decode it, just confirm it is good and present. - Nick at Pelican Parts  
Daine Comments: Hi. Thanks for the info. Very very helpfull. I have a question though, red arrow on figure 12 has no voltage and I have checked all the fuses. Where is that wirethick red with blue tracer supplied from ? Please help. Thank You.
July 1, 2015
  Followup from the Pelican Staff: On our subject vehicle, Fuse 77 in the main fuse box. - Nick at Pelican Parts  

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