Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW?
formatting link
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.
formatting link
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.
formatting link
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?
formatting link
Here is the best (admittedly sketchy) wiring diagram we have so far:
that looks like a linear semiconductor controller - an incredibly antiquated concept for a modern car.
old resistor packs for fans were open wire that sat in the fan's air stream for cooling. they were generally very reliable if their alloy wasn't too susceptible to salt.
that unit looks like it still sits in the air stream with that honking great heat sink and i estimate it's trying to dissipate >100W. that can only mean it's a linear controller because a modern pwm device can control high motor currents with very little heat dissipation
if it were an adequately designed unit, it should tolerate that and worse.
analyze the actual problem - don't just waste money replacing stuff. a $30 dvm will diagnose this for you, and you should already own one if you have any ambition to repair any modern vehicle.
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.
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.
Did you have any suggestions for the OP, or did you just show up to snipe without contributing anything as per usual?
You do know that most electrical/electronic components have a maximum current rating, yes? And that electric motors tend to draw more current when the bearings are going or they are otherwise subjected to loads higher than that for which they were designed? Does any of this sound remotely familiar to you?
Really, what the will suggested seems to be a logical first step.
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???!!!
Here are pictures from the last half dozen who tried that approach:
formatting link
formatting link
formatting link
formatting link
formatting link
formatting link
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?
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.
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):
formatting link
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?
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.
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.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.