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Old 03-08-2019, 10:48 AM
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Join Date: Mar 2019
Posts: 1
C250 Blower Motor


I have a 2012 C250 1.8L. The blower motor was making intermittent noise for some time until I started to see the blower speed jumping around when set to 'high' and eventually, I no longer have any function of the blower motor for HVAC.

I assumed it was the blower motor so I replaced it and still no success. I then used my multi meter to check voltages. I record 0V coming out of the blower motor resistor and into the blower, regardless of what the fan speed was set to one the dash. There is a 3 prong plug which plugs into the resistor (red, white, brown). I measured a voltage of 14V across the red and brown wires at all times. I measured 14V across the white and brown wires when the fan speed on the dash is set to high and as a lower the fan speed, the voltage reduces. If I turn the fan off, I record 0V across white and brown wires. I performed the same test using the frame of the car as a ground rather than the brown wire. Identical results. This all makes sense.

This led me to believe the blower motor resistor had failed. I purchased a new one and still no luck. At this point, I bypassed the resisitor and connected the blower motor directly to the red/brown wires that feed into the resitor. Both the old blower and the new one I bought ran perfectly. Once again, confirming to me that it is the blower motor resistor.

I find it unlikely that my resistor failed and I was shipped a faulty one as well. Do you think the issue lies elsewhere or do you think the new resistor I purchased was faulty?

Thanks for your help! This has been a cold winter in the car!

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Old 03-09-2019, 06:13 PM
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Join Date: Jan 2012
Posts: 7,515
I'd need to see a schematic however if this follows past MB and other brands conventions, the following will apply. This isn't a resistor, it creates a variable voltage electronically.

On many systems, the thin wire receives a variable voltage from the heater / AC controls. Low voltage is slow fan and higher is fast. Monitor the thin wires voltage to ground, it is probably dropping out causing the fan to slow.

To go any farther we need a schematic / test procedure for this car.

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