Root cause insight into the common BMW blower motor resistorfailures

[jim beam]

There is something critical about those two sets of inline posts
because, as I dig deeper, I see others concentrated on them also.

For example, the quote below came with the picture below:
http://www1.picturepush.com/photo/a/12511554/img/12511554.jpg

"sophomoric" indeed.

 

[Bimmer Owner]

or google for this guy:
"ESI 695 80 Amps DC/AC Low Current Probe"

This seems to only be $107 but it doesn't say whether it works
with the Fluke 75 or not.
http://www.amazon.com/ESI-695-Amps-Current-Probe/dp/B000FN4IUK

 

[Bimmer Owner]

they are two TO-220 cased transistors.
finding out the part numbers on those transistors will reveal a lot.

I have an FSU in my possession, so I will dig them out & snap
a photo when done and post back the results.

To my knowledge, nobody has ever posted a photo of what those
two MOSFETS look like, nor the part number.

 

[Bimmer Owner]

The mesure of the DC voltage on those resistors can be used to
What does this sentence above mean?

Does it intimate the blower motor is actually drawing more
current as it ages (as someone had suggested prior)?

 

[Bimmer Owner]

This seems to only be $107 but it doesn't say whether it works
with the Fluke 75 or not.
http://www.amazon.com/ESI-695-Amps-Current-Probe/dp/B000FN4IUK

For the same price, it looks like we can get a separate unit:
http://www.westsidewholesale.com/gifts-more/tools-more-clearance/fluke/fluke-t5-600.html

Fluke T5-600, SKU: 133038, $109.95

 

[jim beam]

What does this sentence above mean?

with two resistors in series, the voltage across any one is a function
of its resistance and the current in the current in the other.

Does it intimate the blower motor is actually drawing more
current as it ages (as someone had suggested prior)?

good question - there might be a slight change in draw as the motor
ages, but unless the motor has serious bearing health issues or blows a
magnet coil, i can't imagine it will change by much. and other than
short circuit detection/protection, i have doubts about the extent to
which such a simple controller will need to know.

 

[tm]

Looking more deeply, I find ANOTHER reference to the two MOSFETS,
which, are clearly the two transistors shown in the wiring diagram
that you had surmised must exist (by detective work).

Here's another quote which went along with this photo below
that mentions the unknown-as-yet MOSFETS:
http://www3.picturepush.com/photo/a/12511566/img/12511566.jpg

As a general rule, MOSFETs are not used for linear current control. It is
more looking like this is a switcher (PWM) though I dont see an inductor.
Could be they just use the motor for that.

If you can see any numbers on the devices, it will help. Also, the solder
sure looks like RoHS shit tin.

 

[Scott Dorsey]

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.....

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

 

[Scott Dorsey]

ok, as i understand it, and as i said to scott earlier, this is a
problem because it mungs low speed motor start and low speed torque.

Not really, it's feeding the motor with variable DC from that integrator
stage. It's not much different than a big rheostat in terms of starting
torque. It's just more expensive and less reliable and quieter.
--scott

 

[Scott Dorsey]

I have a Fluke 75, so I'll have to see what clamp on DC current
probes fit it.

I don't think Fluke makes much in the way of current probes for DC with
appreciable sensitivity. The i1010 is sensitive enough for this sort of
job, but not for a lot of other things you might want. They do make some
fancy intelligent probes but they're all more expensive than just buying
a Fluke clamp meter like the 365.

That said, I have a cheapo Extech 380947 and it's not built like the Fluke
but it's sensitive enough to detect small ground leakage currents.
--scott

 

[Scott Dorsey]

Ah, I see what you're talking about and I've circled them in this photo:
http://www3.picturepush.com/photo/a/12511526/img/12511526.jpg

Like a good detective, you've seen something that I had not seen.
What you said makes sense. However, I have never heard of anyone
removing two transistors from that board. I wonder if those two
sets of inline pins are just the connections to the heat sink?

No, there are two transistors bolted to the heat sink. That's why the heat
sink is there, to cool those two power transistors.

But, what puzzles me is that nobody has ever mentioned removing
transistors from those two spots. Therefore, I suspect they're just
anchor posts, since the solder is clearly removed in the autopsies.

No, the spacing is consistent with a TO-220 transistor pair, and if it's
a linear pass regulator like it appears to be, there needs to be a big
transistor somewhere. Also, of course, there is the heatsink.

That IC is only control logic, it just takes some mystery input signal and
produces a variable voltage for the transistor base. Those two transistors
are doing all the hard work. BUT, if you want to replace the device with
a retrofit one, you need to know what that mysterious input signal is.
--scott

 

[Scott Dorsey]

As a general rule, MOSFETs are not used for linear current control. It is
more looking like this is a switcher (PWM) though I dont see an inductor.
Could be they just use the motor for that.

It's not a switcher. And those transistors may not be MOSFETs. But there
is no reason not to use mosfets in linear mode, other than the fact that
no two off the line have the same gain or transconductance.

If you can see any numbers on the devices, it will help. Also, the solder
sure looks like RoHS shit tin.

Agreed.
--scott

 

[jim beam]

Not really, it's feeding the motor with variable DC from that integrator
stage.

i understand that - and variable voltage is the problem. the secondary
[bordering on primary in some applications] advantage of pwm is low
speed start and torque. if a motor starts at low dc voltage, not only
is the start speed inconsistent, it has little torque. pwm can start a
motor slower and at much higher torque. it's a big deal.

 

[jim beam]

I don't think Fluke makes much in the way of current probes for DC with
appreciable sensitivity. The i1010 is sensitive enough for this sort of
job, but not for a lot of other things you might want. They do make some
fancy intelligent probes but they're all more expensive

way to understate - those things are insanely expensive. otoh, there's
almost no competing product, so they have the ability to leverage pricing.

<http://www.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=614-1090-ND>

that's why i settled for the esi unit. not as good as the fluke, but
has similar sensitivity in a package robust enough for automotive use.

 

[jim beam]

For the same price, it looks like we can get a separate unit:
http://www.westsidewholesale.com/gifts-more/tools-more-clearance/fluke/fluke-t5-600.html

Fluke T5-600, SKU: 133038, $109.95

if you want your purchase to be useful for automotive, you want
sensitivity. and ability to hook it up to a scope is a huge plus.

 

[jim beam]

As a general rule, MOSFETs are not used for linear current control. It
is more looking like this is a switcher (PWM)

there's been another post - the output has been scoped. it's linear.

besides, to-220 isn't reserved for fet's.

<http://www.digikey.com/product-detail/en/BUV26G/BUV26GOS-ND/1476296>

though I dont see an
inductor. Could be they just use the motor for that.

now you're guessing.

If you can see any numbers on the devices, it will help. Also, the
solder sure looks like RoHS shit tin.

you're not telling the difference from a simple visual. sorry.

 

[[email protected]]

No, there are two transistors bolted to the heat sink.  That's why the heat
sink is there, to cool those two power transistors.


No, the spacing is consistent with a TO-220 transistor pair, and if it's
a linear pass regulator like it appears to be, there needs to be a big
transistor somewhere.  Also, of course, there is the heatsink.

That IC is only control logic, it just takes some mystery input signal and
produces a variable voltage for the transistor base.  Those two transistors
are doing all the hard work.  BUT, if you want to replace the device with
a retrofit one, you need to know what that mysterious input signal is.
--scott

One key question in all this is if the interface from car to
"resistor block" is some kind of simple digital interface,
ie it sends some bits that get interpreted as "go to speed 3"
or does it send a PWM signal. My guess is the latter.
That's my understanding of what BMW does with the aux
radiator fan in the X5. Another German electronic miracle
that fails and in doing so, mysteriously drains the battery.

Someone should put an oscilloscope on this and find
out what the signal looks like. If it sends a digital code,
then making a replacement from scratch is a big
hurdle. If it's sending a PWM signal, they you could
build an equivalent from Radio Shack parts. It still
seems like more work than it's worth.

How fast are these things failing for those that want
to make their own? 2002 X5 here and it's only had this
problem once, about 2 years ago and replacement one
is still working. And another data point. The failure on
that X5 resulted in the blower draining the battery when
the car was off. Blower ran fine. Only odd thing in
retrospect was that when the car was off, a couple times
I heard a faint noise. In retrospect, it was probably
the blower getting just enough current to start to turn
then stop. And only noticed it a couple times. The
bad thing with the failure mode of this and the AC fan
is that both were draining the battery and both were
very hard to pinpoint, resulting in huge labor charges.

 

[bonny doon]

To my knowledge, nobody has ever posted a photo of what those
two MOSFETS look like, nor the part number.

The semi-conductors circled are not the MOSFET you are looking for
information on. The Mosfet is 14 pin and what is circled is something
different.

I think if these Mosfets were in 1000 BMW owner's hands, maybe one person
out of 1000, if that, would have the skills to install in the carcass we
have left after removing all the potting material. How are we determining
the MOSFET is bad in any situation? Again, even with a data sheet which
we will never get because these Mosfets are supposedly "special" to BMW
(just like they try to hide their MAF sensor). Few would have the skills
to test a 14 pin Mosfet. I believe the FSUs that have been "repaired" are
again the usual "open" circuit BMW seems to like so much (can we say ABS
module).

 

[Bimmer Owner]

with two resistors in series, the voltage across any one is a function
of its resistance and the current in the current in the other.

Ah, so it's a plain old voltage divider?

 

[Bimmer Owner]

BUT, if you want to replace the device with a retrofit one, you need to
know what that mysterious input signal is.

All I know of the HVAC input signal is that it's a 2.0 to 7.8 VDC
signal from the HVAC controller, presumably to correspond to the
various levels of the fan blower motor speed.