Root cause insight into the common BMW blower motor resistor failures

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Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW? http://www.bimmerfest.com/forums/attachment.php?attachmentid 6060&d94115994
Also, does anyone have an idea HOW TO TEST a "repaired" FSU?
The "blower motor resistor", which also goes by FSR (Final Stage Resistor) or by FSU (Final Stage Unit), is known to fry itself in almost every single E46 (3-series), E39 (5-series), and E38 (7-series) BMW. http://www.bimmerfest.com/forums/showthread.php?t 3393
The problem with replacing this ~$100 part is that the new replacement FSU fries itself just as often as the old one did, so you end up repeatedly replacing your fried FSU every few years or so. http://www.bimmerfest.com/forums/showthread.php?tR8566
That's fine for most people (although the DIY is a PITA) - but I ask this newsgroup whether anyone has any insight into WHAT is actually breaking - and - why? http://www.bimmerfest.com/forums/showthread.php?t09399
Here is the best (admittedly sketchy) wiring diagram we have so far:

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Blower motor drawing too much amperage taking it out. Change the blower motor anytime?
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On 03/21/2013 06:55 AM, the will wrote:

My thought as well. Have you measured current draw on a new blower motor and compared it to one that is installed in a car where the FSU has failed? that would tell you whether there's any merit to this idea or not.
nate
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My inclination is to do exactly the same thing I do with the cooling system issues: blame German engineers who seem to believe that their climate is typical of the entire world.
I don't see why it is so hard to unpot one of these things and repair them directly, especially if it's a semiconductor failure. Put a bigger transistor in there. --scott
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On Thu, 21 Mar 2013 09:47:31 -0400, Scott Dorsey wrote:

Here are pictures from the last half dozen who tried that approach:

Most who try to unpot fail, mainly due to damage caused to the surface-mount circuit board during the initial mechanical degooping step.
Those deft few who avoid knocking off the surface-mount components with the debriding chisel, are left with a badly bruised board, where some have said they've resoldered solder cracks (see pics).
One problem with "put a bigger xtor" is that nobody on this planet has produced a decent circuit diagram of the FSU.
Does anyone here have access to an FSU circuit diagram?
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On Thu, 21 Mar 2013 08:14:12 -0700, jim beam wrote:

To be clear, that's what 99.99999999% of the BMW owners do. But that's not the point of this thread.
The point of this thread is to get a handle on WHY they are all failing.
Specifically, how to figure that out is the question.
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On Thu, 21 Mar 2013 16:00:13 +0000 (UTC), Bimmer Owner

The simple answer is that they are under-designed for the conditions under which they apparently are regularly subject to.
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That isn't necessarily the case. For example,t hey could be correctly designed, rated for the application, etc but have a manufacturing defect in just one of the components.
A better questions is why BMW apparently doesn't give a damn to do the failure analysis to find out what's wrong. I have a friend who has an X5 and had this problem with the blower resistors. Even worse, the only symptom was it was draining the battery and it took a huge number of hours to track it down.
While you're all wondering about that problem, might as well add the fancy aux radiator fan to the list. This car had that go and now the replacement one has failed again. And the symptom there is, again, it drains the battery even when the car is off. That fan is a real POS. Instead of just a simple fan motor, it's a fan that's variable speed, driven by a PWM signal. So, instead of just a motor, that fan sitting in front of the hot radiator has electronics in it. A real genius of a design. And for what? Like the fan can't just be on or off? Only reason I can think of is that they want to save a few watts of power to try to get better fuel economy. And for that their customers get to shell out $500 for a new fan every few years.
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In message

If by PWM you mean pulse width modulation, then it would allow for variable speed, but a DC motor is an inductive load and is not sensibly controlled by such a system unless there is something in the circuit to allow the peak voltage generated by the motor at pulse cut of to be shunted to earth.
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simple

That "something" could be as simple as a diode. PWM is commonly used to vary the power to a motor. BMW, for example, uses it on the aux fan motor of the X5. And I would suspect that it's also used for the blower motor because you wind up wasting a lot less power that way. And every little bit of power saved adds up and effects MPG.
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In message

I agree with all you say.
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wrote:

PWM is the most common method of controlling the speed of DC motors - a flywheel diode is part of the "system" to handle the inductive kick-back. Virtually all battery operated variable speed power tools use PWM. So do virtually all electric bicycles with brush motors and the vast majority of electric forklifts.
In fact, just about any application of a brush type DC motor that requires reasonable speed control has switched to PWM control of some sort over the last 20 years, including power wheel chairs (except those using 3 phase brushless motors)
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On Tue, 26 Mar 2013 14:23:08 -0400, Michael A. Terrell wrote:

Here's what the heatsink looks like when I cut into it today:

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Ha, keep going!
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On Tue, 26 Mar 2013 14:46:25 -0400, tm wrote:

Well, I only had lunchtime, and it got uglier & uglier as I went!

Unless there is some chemical way to remove that black/gray rubbery (like a tough pencil eraser) gunk, there will be no way to read the numbers on the two transistors.
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On Tue, 26 Mar 2013 19:24:08 +0000, Bimmer Owner wrote:

I only found 1 transistor.

This is what is suggested in this quote below:
[QUOTET0iman]There is only ONE mosfet in the FSU and I would forget about trying to replace it even if you had one. Also, we have no reason to believe the mosfet has caused any FSUs to fail. I have yet to see the resistors in any of the picture shown anywhere in this thread. Can someone circle the resistors for me? I would think they would be fairly decent wattage so they would be very easy to see, but I don't see any resistors.[/QUOTE]
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I think he is incorrectly calling the SMT IC a mosfet.
What went in the other three holes on the PC board? What was under the two spring clips on the heat sink?
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On Tue, 26 Mar 2013 19:41:47 +0000, Bimmer Owner wrote:

I 'think' (but I'm not sure) that these are the resistors in series:

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irly

If they are resistors, I've never seen any that look like that. Also, given that you want to thermally bond any components that generate major heat, why are they not heat sinked? With any power design I've seen, the key components, eg the transistors are directly bonded to the heat sink.
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" snipped-for-privacy@optonline.net" wrote:

You've never seen strips of nichrome in a space heater?
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