Root cause insight into the common BMW blower motor resistor failures

Page 13 of 14  
Followups set to sci.electronics.repair .


At a guess, one or both of the big power transistors that are inside the FSU are failing. If the failure is that the blower motor doesn't run at all, they are probably failing open. If the failure is either that the blower motor runs at maximum speed, or a fuse blows, then they are probably failing shorted. The blower motor probably draws more current as it ages, and it may eventually be exceeding the power-handling capability of the transistor(s). When the motor is switched off, it may also generate a bit of a voltage spike, which may be above the voltage rating of the transistor(s).
A possible solution is to replace the transistor(s) with ones with a higher power rating in the same package. Another approach is to improve the heat-sinking, maybe by adding metal to the existing fins. Or, cut off the existing fins, bolt it to a huge slab of metal, and relocate the entire thing away from the blower duct.
As a crutch, you could drop the voltage to the FSU a little bit. This would slow down the blower, but also might tend to keep the voltages and currents down to what the transistors can handle. You would need to know the maximum current you would expect the FSU to draw; this probably happens when the charging system voltage is at is maximum, the blower motor is stone cold, and you turn it from "off" to "max". Then, buy a big rectifier diode with a rating of a few amps more than that, and splice it in to the power wire to the FSU. This will drop the voltage by a volt or two all the time. Or, you could put a power resistor in line instead; this will cause a variable voltage drop depending on how much power the FSU and blower is drawing at the time.
Keep in mind that in the winter, keeping the windshield clear is a safety function, so don't drop the blower speed too much.

The tricky part depends on the nature of the control signal to the FSU. If it's a simple analog voltage, that is easy to generate on the bench with a potentiometer. If it's some kind of digital bus (CAN?), it is *possible* to generate that on the bench, but it's probably easier to get the dashboard heater control out of a junked car and let it generate the signal.
To load the FSU, you can either use a power resistor that draws about the same amount of current as the blower motor on "high" (a headlight lamp might qualify), or an actual blower motor. The resistor will be "better behaved" than a real motor.
For a power supply, it depends on how much current the blower motor needs. You can get relatively inexpensive 13.8-volt power supplies in ranges up to several amps, designed for running "12 V" equipment on the bench. Samlex is one manufacturer but there are others. If it needs more than 10 A or so, it's probably cheaper to just use a real car battery and charge it when it's not being used.
You should probably arrange it so that there is some air blowing on the FSU under test. If you are using a real blower motor, you can make a duct out of cardboard. If not, use something like a 12 V computer case fan to move a little air across the FSU.
It may also be interesting to have some kind of thermometer on the FSU case while it is under test.
A good way to figure out what the FSU is actually doing is to probe a working car with an oscilloscope. This will show you immediately how the FSU is controlling the blower motor speed, as well as what the control signal looks like. You can stick a straight pin *through* the wire insulation as a test point, and then seal up the hole with electrical tape or silicone sealant.
As has been mentioned, it would be also interesting to cut one of the blower wires and measure the current drawn by the blower motor. For extra credit, do this on a new car (or a newly-installed blower motor) and then compare to a blower motor in a car that has just had its FSU fail.

On one hand, you would like stuff not to break. On the other hand, spending $100 every two or three years on a car that starts around $36,000 new is maybe not out of the realm of possibility. (Perspective: that's one tire or 25 to 30 gallons of gas.)
These cars are apparently sold worldwide. If the FSUs sell for cheaper in a lower-cost country, enough to offset shipping and taxes, import a box full of them and make money. :)
Matt Roberds
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On Thu, 21 Mar 2013 21:56:49 +0000, mroberds wrote:

Actually, I was remiss in not stating that the blower motor generally fails by acting weirdly, often said to "have a mind of its own", and, most often by a parasitic current draw overnight that kills the battery.
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r

i guess they call replacing a simple resistor with a bunch of transistors.... "progress"
i'd replace the module with a bunch of power resistors and maybe a PTC (self resetting) fuse
Mark
Mark
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On Thu, 21 Mar 2013 21:56:49 +0000, mroberds wrote:

That seems like an EXCELLENT idea, if we can put some kind of temperature indicator in the FSU tines, then we can observe what the temperature is in situ - which might tell us something about what is overheating these things (assuming heat is the culprit).
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On 03/21/2013 04:12 PM, Bimmer Owner wrote:

won't save the unit, just a frozen motor.

you can run the unit out of the fan housing - hold it in your hand and you'll soon find out if it's getting hot or not.
--
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d...

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le

U

I had a dodge caravan that fried its heater AC motor speed control resistor repeatedly....
the connector to the wiring harness detoriates from the high current and the voltage drop causes the connector to heat up and the entire assembly fails.
Oddly enough I repair roll laminators that apply plastic film to paper think menus:)
laminators experience similiar failures so I did the following.
Purchased a new resistor block, soldered wires on all the connectors putting a heavy wire on each one.... Put a pigtail on each one. Installed resistor block. Its screwerd to the fire wall.
Cut the plug assembly off the harness, stripped all wires, twisted them together and installed wire nuts on each one.
had the van for years with zero problems for this part:)
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Have you got a link to these "common problems" you keep posting about?

http://www.bimmerfest.com/forums/attachment.php?attachmentid 6060&d94115994
========

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wrote:

e

..

!- Hide quoted text -

I don't have a link, but we had the blower resistor widget go on an X5 here. And the aux cooling fan motor has gone twice. There are plenty of threads online about many people having those problems. Oh, and don't forget the nice X5 feature where the cable that they use to hold up the windows snaps, sending the window crashing down inside the door, breaking it into a million pieces. Had that happen twice too, once while the car was just sitting in the driveway. Other time was driving down the highway.
Then there are their defective rubber parts. Like the boot on the intake manifold that cracks in just a few years. Or the CV joint boots. I've had lots of cars with CV boots and only on the X5 do they fail every 20K miles. I've seen Honda CRVs that went 200K miles with no failure.
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On 03/23/2013 04:33 AM, snipped-for-privacy@optonline.net wrote:

serious question - why did you buy it?
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On Sat, 23 Mar 2013 07:20:44 -0700, jim beam wrote:

As for me, I fell sway to all the people saying how great the bimmer was.
It was only after I owned it, that I realized that BMW engineers knew how to design a suspension and a drive train, but they had no idea how to build a machine.
To their credit, some people say it's not the engineers fault as they probably know by now that every single Bosch 5.7 ABS control module fried in every one of the vehicles it was placed in, and that the final stage unit cooked itself to death in every single BMW it was ever placed in, and that the 2-bar plastic cooling system sprang a leak on almost every single BMW ever built, etc.
In fact, there's absolutely NO WAY BMW can't know about these egregious engineering flaws. So, the common conclusion is that their customers don't care - so why should they.
To me, it smacks of 3rd-grade engineering from BMW, so, that's why I, for one, am amazed (being an owner myself), how sophomoric BMW engineering really is.
Disclaimer: Yet, the drive train is phenomenal!
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On Mon, 25 Mar 2013 16:54:13 +0000 (UTC), Bimmer Owner

If the engine desnt burn to the ground due to oil leaks, or overheat or run out of oil, because of all the problems with "ancilliary systems"
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On 03/25/2013 09:54 AM, Bimmer Owner wrote:

honda use a "plastic cooling system". it's not infallible, but you can get 15+ years out of the first one. there's no reason bmw couldn't achieve the same - if they wanted to.

that is the catch - bmw target a certain type of buyer that typically won't keep a car more than 3 years. after that, they don't care and bmw can safely switch to "maximize parts sales/write off the old cars and sell new ones" mode.
in europe, bmw have been aggressive leaders in "recycling" and spend a lot of money advertising the fact. in practice however, it means that they buy used cars back and have them scrapped, thereby keeping used parts off the market - the mba's have done their math.

it depends on perspective. i agree that some appear to be extremely rudimentary, but that is contradicted by the fact that they spend a huge amount of money on r&d, and have boatloads of phd's on staff. they were also rescued by a bunch of mba's in the 70's and have had a root and branch focus on design life ever since.

call me when you have 100k miles on it...
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On Fri, 22 Mar 2013 23:42:00 +0000, R. Mark Clayton wrote:

Of course. Nothing I've ever written is not well documented.
I'm not sure "which" common problem you're asking about, so, here is just a sample of the most-common problems that afflict almost every BMW E39, E38, and E46 (I'm sure there are others but I'm only familiar with those models that use the M54 engine).
http://www.bimmerfest.com/forums/showpost.php?p `97104&postcountF
Behr cooling system leaking Plastic DISA valve breaking & destroying the engine Cluster pixel tape lifting MID pixel tape lifting Hella PBT headlight adjusters breaking Power steering cap & hose leaking Trunk wiring loom fraying CCV clogging Bosch ABS control module frying Cupholders breaking I6 VANOS seals deteriorating V8 valley pan gaskets leaking GKR/BMW/Valeo FSU/FSR dying Thrust arm bushings leaking Ambient temperature sensor breaking Window regulators breaking Vapor barrier adhesive leaking Jack pads falling off Windshield cover molding crumbling Driver's seat control switch breaking Rear center brakelight socket melting Seat cables fall out causing seat twist Windshield washer tanks & pumps leaking BMW roundel paint chipping Vent trim corner cracking & wood trim varnish cracking Rear center brake light socket melting
Note: The reference above has detailed links to EACH of these topics above.
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I must be lucky. My 2000 323 has had only one of the above "failures" - leaky power-steering hoses.
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On Mar 25, 2:55 pm, snipped-for-privacy@panix.com (Scott Dorsey) wrote:

..

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Maybe it's time to look at this thing like gas and brakes... You put gas in, you go so far. You put one of these blower modules in and you go for a few years again. It's not like they are a $500 or $1000 puter. Don't they cost like $50? I mean how much is time worth trying to reverse engineer it.....
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This is how people are. At some point intellectual curiosity takes over.
In this case it might actually be worth it, because of the sheer number of the things out there that are failing.... one person figuring the failure mode out might save a lot of people that grief.
But mostly it's just intellectual curiosity. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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On Tue, 26 Mar 2013 08:56:22 -0400, Scott Dorsey wrote:

Exactly!
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I don't even own a BMW. After this thread, I don't think I ever will.
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On 03/26/2013 12:35 PM, tm wrote:

Like I said before, don't drive one then. It's kind of like going on a date with that unbelievably attractive female type who is also smart, witty, fun to be around, actually seems to like you, and oh by the way is completely mentally unhinged.
Suddenly you find yourself putting up with all sorts of stuff that you wouldn't, otherwise... (now that said, touch wood, current ride has exhibited none of the known issues... which reminds me, I need to call and schedule the battery cable recall @ the stealership)
nate
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LOL. There's more truth to that then you may think.
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