Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW?
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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.
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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.
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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?
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Here is the best (admittedly sketchy) wiring diagram we have so far:
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.
On Thu, 21 Mar 2013 03:08:09 +0000 (UTC), Bimmer Owner wrote in Re Root cause insight into the common BMW blower motor resistor failures:
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This is a design feature of BMWs. It's one of many whose purpose is to economically support BMW dealers with $$ from the fools dumb enough to buy a BMW.
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.
Here are pictures from the last half dozen who tried that approach:
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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?
This is an interesting approach, given that the vast majority of bimmer owners do NOT replace the blower motor - they replace the FSU.
While the blower motor replacement procedure is a major PITA, one 'can' test the leads from the FSU harness connector pins #5 and #1 which are power and ground respectively to the blower motor.
Again, we don't have a circuit diagram, but it has been said that the blower motor takes about 6 amps (variously, depending on the speed) but it would take a test jig to test that in operation.
To my knowledge, nobody has created that test jig (although I know of only one attempt, which failed):
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It's easy enough to test the resistance of the blower motor though, and those results have come out at about 0.4 to 0.6 ohms.
It would be expensive to change a blower motor on a whim, so, how would YOU suggest the blower motor be tested in situ?
It 'can' be done, but would require a test jig inserted inline as the FSU is deeply ensconced under the dash while the blower motor is even more deeply so.
While that preventive work might be prudent, the sheer effort to remove the entire dash simply to access the blower motor would be problematic.
Still, if the problem is that the blower motor is merely using more current as it gets older, why wouldn't a NEW FSU burn up within a few weeks of insertion?
FSU failure that plagues almost every 1997 to 2003 BMW?
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Also, doe s anyone have an idea HOW TO TEST a "repaired" FSU? The "blower motor resis tor", 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.
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ad.php?t=143393 The problem with replacing this ~$100 part is that the ne w 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.
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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?
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Here is the best (admittedly sketch y) wiring diagram we have so far:
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My guess is this is the motor speed control and that it is ANALOG. This mea ns the resistance of the transistor varies with the speed. It gets hotest w hen the transistor is somewhere between full on and full off. At full on or full off the resistance of the transistor is either nearly infinent or clo se to being a short. In these conditions not much heat is disipated by it. I would design a pulse width modulated controller. This controls the curr ent to the motor in a digital fashion by switching the power off and on to the transistor is either in the fully off or fully on state. I have seen su ch circuits on the internet just by googling PWM motor controller. They are very simple to build with a big power FET and an NE555 oscilator also very cheap. Adapting it to the existing controls would be your problem. The goo gled articles will also tell you a lot more about how and why they work mor e efficently than an analog controller than I am willing to here.
If the unit is near its limits it might just get very hot and parts start aging very fast -> semiconductors will just fail after a little while. The new unit should fail sooner than the old one but who knows who soon, maybe after a few years. Peugeot's ( and Citroen) used just one huge pnp darlington which failed quite often , it was working too near its operating limits.
These are kind of sloppy jobs. But it's clear there are two TO-3 devices there which have been removed in all of those photos.
Well, has anyone got docs on that mystery IC there? It's from Elmos Semiconductor, but it's not a standard Elmos part number on it, so it's almost certainly a custom, since it doesn't look like anything in their standard book.
I don't know the wiring diagram on that particular model, so I don't know where you would need to break in to measure just the current of the blower without anything else. But it does not seem terribly insurmountable, especially seeing that BMW is very good about breaking everything out into connectors all over.
Well, that's partly why I drive a 2002 and an E28, everything is much easier to get to.
Of course, it's impossible to turn everything off, as the computer won't go to sleep for 16 minutes after the car is shut and armed, but, still - with the blower consuming something like 5 or 6 amps, we should be able to measure at least that.
The problem, even with the car's additional electronics powered up, is HOW MUCH of a difference are we expecting between an older blower motor current draw and a new one?
Are we looking for a 1 amp difference, for example?
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. :)
If one replaces one of these perhaps it would be advisable to put a fuse in the blower motor lines (it sounds like those can be gotten to easily unlike the motor itself). If it's being blown by intermittent high current draws the fuse could protect the $100 FSU. Another option would be, at least for those who can live without the highest blower setting and who think excess current draw is the culprit, would be to put a power resistor in the blower motor line to limit the current a bit. On the cheap pedestrian cars I drive the whole speed control is just a trio of power resistors placed in the air flow to help cool them. If they burn out (which is rare) they can sometimes be fixed with a pop rivet.
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