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Discussion Starter · #1 ·
Need some help with my A/C blower. <br>
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No matter where I set the 'volume' wheel (Min, AUTO, Max) it blows a very low volume of air. As the control is rotated from one end (Min) to the other (Max), the air volume changes but it's blowing at what would be considered a MIN setting.<br>
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The A/C is fine; compressor cycles and the evaporator temp ranges from 34F to 40F, so it's blowing very cold air but just not a lot of it!!<br>
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The sensors (internal temp and external temp sensors seem ok based on the readings of the using the DIAG mode (my page; menu #14) <br>
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Poked around and it seems to be this 'critter'. What is it?? Looks like a triac? But that's for AC? I'm confused?<br>
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Thanks in advance for any help.
 

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Discussion Starter · #2 ·
I now understand completely how it operates, as an 'emitter follower' amplifier. And how it can fail! <br> <br> Here's how it can. Since it's controlling the power for the fan motor, it's must withstand a lot of power dissipation, thus the required heat sink to the blower's frame. Roughly here's the scenario: 13.8V (terminal 15) and 15amps (when the transistor is about 'half' on, yields about 90 watts dissipation. The transistor must dissipate the energy not transmitted to the blower. Thus when the fan is on MAX, the dissipation is minimal since the transistor passes all of the power to the blower. <br> <br> Most people operate their cooling controls around halfway on so it's constantly dissipating a large amount of energy. So I can see how it can easily fail. <br> <br> The part number is: 140 821 83 51 about $170. Around $300 at an MB dealer.
 

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Where is this part located?

And if mine is bad, would the diagnostic show the correct...


Where is this part located?

And if mine is bad, would the diagnostic show the correct voltage (8 - 60 reading) delivered to the blower?
 
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