Re: Cause of BMW blower Motor RESISTOR Failure?

Yep. A brush straddling two commutator segments can cause a very large surge.

Just buy an Infiniti ;-) ...Jim Thompson

Generally if the module is a resistor, there is one position where the motor is fed directly. So when the resistor goes open, your only speed is full blast or off.

If you have more speed values than could be explained by taps in a resistor, then you have an electronic control of some flavor. Usually when the electronic control fails, you have no fan at all.

I've replaced these in VWs, Mercedes, and Infiniti. The reality is the expensive cars have just as s***ty fan controls as the cheap cars.

Exar has/had a group that did nothing but reverse engineer the electronic modules for replacement parts suppliers. You can't believe the crappy designs used by the manufacturers. It was always a problem if the replacement parts some use good engineering, or just copy the moronic designs of the OEM.

I used to think they potted all these modules to protect the circuitry from the elements, but in reality they were just kind of embarrassed at the poor engineering in the design.

Most of the time the bipolar designers would spot circuitry that had potential to damage on start up. As you probably know, breakdown voltages in bipolar can be a function of pins being floated or shorted to other pins.

Resistors or electronic controls need heat sinks, so I don't think you can tell based on the outside of the package.

The replacement part for my Infiniti blower control was a Delphi make in Mexico! The original part, which looked a bit different (bigger heat sink) was made in the USA. The Infiniti part isn't potted. I saved it just to open it up and see if anything smoked. It looks clean. It is a PCB with 4 semiconductors, one power device, and what looks like a surge suppressor. The PCB has Picotech on it. It has a 0.004ohm current sense resistor on. I suppose I can salvage it for yucks. The board is conformal coated, which makes it hard to read chip numbers.

If I had to fault them on anything, I would say too much heat sink grease. It is oozed all around the power device. [3 pins, so MOS or BJT].

Tom

Blame it on Jim, his rates are high. :-) Mikek

All we know is that some component inside a sealed unit seems to have a high failure rate. It may (or may not) be correlated with motor wear or impending motor failure.

Some diagnostics need to be done (by the owner of a BMW and some simple test gear). PWM or not. Condition of motor seizing bearings, arcing commutator, etc.). And what inside the FSU(?) is failing. Open a bad one up, reverse engineer it and find out which component is puking.

If this is a common enough failure of a high cost part, I'd consider designing a better one and marketing it (or a kit) to BMW clubs.

My A/C blower motor controller design didn't use a dropping resistor... it's a 3-phase EC motor, speed controlled using a variable frequency control oscillator. Chip dissipation is zilch. PowerMOS dissipation is only 3W (each device), and they're heat-sinked to the motor housing (Aluminum).

BTW, It was a family rule, beginning around 1968... Chrysler products no longer allowed in the family. ...Jim Thompson

Sounds like a poorly protected implementation of PowerMOS used to switch in series resistors. ...Jim Thompson

That doesn't need to be the case. The cause can be something simple. In my car the resistor module has a fuseable link which corrodes and makes the module fail. Ebay to the rescue...

Could be. Perhaps they need to add a snubber to protect against the dv/dt transients from the commutator. Particularly in a worn-out motor.

This could be something an enterprising person could put together and sell as an add-on to the BMW crowd.

Perhaps someone here has a wiring diagram for such a vehicle? ...Jim Thompson

There was one posted earlier in this thread. Unfortunately, the offending module is just shown as a black box. Not sufficient to understand the circuit topology.

Yep :-( ...Jim Thompson

The original thread had six links from 6 people that had attempted to remove the potting. All ended in failure.

One used a hammer & a cold chisel. Others used dremmel tools & dental picks.

Some of them nevertheless succeeded in removing enough of the potting to make a repair. It would be interesting to reverse engineer the whole thing. But someone seems to have done so already as there are second source units available. Meanwhile, time is running out. These cars from the nineties will not last much longer I suppose.

petrus bitbyter

Hey! I drive cars from the '70s. They run just fine. And upset the enviro-weenies, since they are exempt from emission testing.

My '79 Porsche central warning computer is getting a bit flaky. If I can locate the correct connector, I was thinking about building a replacement. Using a microcontroller instead of discrete components.