Root cause insight into the common BMW blower motor resistor failures

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On 03/21/2013 06:47 AM, Scott Dorsey wrote:


that's not going to fix it though. and the germans sell a LOT of these vehciles in the middle east - it's a good deal hotter there than here. they know exactly what they're doing.

unpotting is a nightmare - it will take much less time to build your own pwm controller. who knows, maybe you can switch the existing unit???!!!
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Unpotting is fun, it's a nice change in the day to just sit down for a couple hours with a dremel tool and a dental pick.
But I agree, building an aftermarket controller replacement would not be a tremendously difficult thing to do, and it might be a highly profitable one. --scott
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On 03/21/2013 12:05 PM, Scott Dorsey wrote:

i'd rather repair light bulbs.

well, it cost me about $18 in parts to retrofit the linear dash light dimmer on my civic with a pre-built arduino unit. a custom unit could come in substantially less than that, if in sufficient quantity.
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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 03/21/2013 08:04 AM, Bimmer Owner wrote:

if people priced their time and ignored the damage in which attempts to unpot invariably result, it's cheaper to just buy a new one.

you don't need it any more than you need the circuit diagram of a chip's internals - all you need is its function parameters - which you pretty much already have.
you might be able to pwm the unit itself thus pretty much removing the heat component thereby prolonging its life [literally] exponentially.
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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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On 03/21/2013 06:01 PM, snipped-for-privacy@optonline.net wrote:

but that's not going to apply to multiple different unit manufacturers, over long periods of time.

um, because it's a profit center? either they charge you $100 for a $6 unit, or you get fed up with the vehicle and buy a new one. that latter is the psychology of their target market.

indeed - a very good point. which begs the question, if they can pwm the aux fan, wtf can't they do it with the blower fan???

bmw are designed, root and branch, to be expensive to maintain and own after the warranty period. they spend a lot of money on r&d to achieve that. and even more on advertising to convince their target that the extra cost is justified for membership of the "ultimate marketing tagline" club.
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In message

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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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

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On 03/23/2013 03:48 AM, Clive wrote:

ancient news - controller designers and semiconductor manufacturers have been on top of this from day 1.
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wrote:

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 03/21/2013 09:00 AM, Bimmer Owner wrote:

i already told you - it's overheating. semiconductors don't like heat.

knowing how the light bulb blew doesn't fix it.
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On 03/21/2013 10:07 PM, jim beam wrote:

Yeah, we know that.
*why* is it overheating?

But it may make the replacement last longer.
nate
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On 03/21/2013 07:23 PM, Nate Nagel wrote:

because it's linear, retard. if you don't know what they means, fuck off until you find out.

putting facts in front you dumb ass all day long doesn't make you any smarter.
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On 03/21/2013 10:25 PM, jim beam wrote:

so "because it's linear" it by nature overheats to the point of failure? Odd, I'm pretty sure that that controller worked initially on, well, all of the vehicles in which it was originally installed.
The question is, is it overheating to the point of failure because the designer cut things too fine (in which case designing a better part would be the right approach), or is it because there's another issue somewhere else *causing* a part that would otherwise have acceptable service life to fail (in which case replacing it with a stock replacement and fixing the underlying issue would be the most economical thing to do)?

IKYABWAI.
It would really be just like you to spend all day designing a more robust controller, building it, watching it too burn up, and then realize that the problem was something else, like a chronic problem with dry fan motor bushings, windings dragging on the case, something like that.
But sure, don't check the obvious stuff, just go into your long-winded sometimes technically correct and sometimes not babble, I know that anything I say won't stop you anyway.
nate
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Says the idiot who says it overheats "because it's linear", as if proper design (adequate heat-sinking) can't prevent overheating in a "linear" design. Sheesh.
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dizzy wrote:

Sheesh indeed. That heatsink is small. Sure, it will work for a while, but it isn't designed for long life. Depending on forced air cooling when the air is either heated or cooled is just plain stupid.
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