Case Studies

Audi Electrical Diagnosis – HVAC Blower Motor and Control Module Failure

Awww Audi
Mike our shop foreman at the Atlantic Technical Center (ATC) discovered this today when diagnosing a HVAC fault in a client car. Nice call Mike! Now this is bad…whenever we find melted connectors, it’s what we call a “clue”, but also is a very interesting find, instructive even. Especially if you understand Ohm’s Law or E = IR, and also the relationship between heat and resistance, also known as P=I2R.

Great question!
Normally electric power is useful, making a lamp light or a motor turn for example. However, electrical energy is converted to heat whenever a current flows through a resistance and this can be a problem if it makes a device or wire overheat. In electronics the effect is usually negligible, but if the resistance is low but not close to zero (a wire or low value resistor for example) the current can be sufficiently large to cause a problem. You can see from the equation P = I² × R that for a given resistance the power depends on the current squared, so doubling the current will give 4 times the power.

What Happened
In this case, a worn blower motor was drawing much more current than normal, to overcome the friction of the worn bearings. Yet, not enough current to cause the fuse to blow, which would have alerted to the problem. As the worn bearings created excessive drag, which in turn causes the current to rise, more current equates to more heat at any resistance point. More current means any resistance in the system is going to get warm. Electrical connectors are the weak point, heat is generated there due to resistance. More heat causes tempered spring connection to fail and become loose, creating more resistance.

I agree, you’d think with that much heat, to clearly melt the plastic connector, and also melt the solder off the circuit board, that the fuse would fail, that’s what fuses do, right? It did not, but the resistance at the connector was turned into heat (P=I2R), and what it did do was melt the wiring harness, effecting the need for a wiring harness repair. And replacement of the melted module, and the worn blower motor. Schematic diagram is attached for you adventurous sorts. I love German wiring diagrams, so logical, and…well, so German.

The Proper Repair

The only true and permanent repair is to replace the blower motor, control unit, and repair the wiring harness. It’s just not worth the risk, do it right the first time, and it’s good to go.

At Atlantic Motorcar, we’re all about providing our customers not only Great Service but also Value. We understand that “An ounce of prevention is worth a pound of cure.” (my mom would be proud I remembered that.), in other words, preventing, or catching problems like these brake lines early, can save you more than just money.

Questions, or if we can be of help in any way with service on your Audi, BMW, Mercedes, Mini Cooper or other European (and now Japanese) import, please contact us.

Our team of Service Specialists is here to help, for even the newest autos! (207) 882-9969.Knowing, not just “doing”, that’s the Atlantic Motorcar Center way.

The Atlantic Motorcar Center Service Team

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