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Root cause insight into the common BMW blower motor resistorfailures

T

tm

Jan 1, 1970
0
Bimmer Owner said:
Someone had scoped the whole board, and it was DC voltage everywhere
(according to that reference). It's pretty clear there is no PWM.

The 16-pin surface mount chip seems to be a automotive temperature
compensated voltage regulator with a huge voltage range, according
to a lookup of the part number on it.

Here is the Elmos 10901D chip of my FSU as I cut it open today.
http://www4.picturepush.com/photo/a/12515632/img/12515632.jpg

I wish there was a way to get rid of that heavy fibrous plastic goop!
(What is that black fibrous tough stuff anyway?)

Are those the two transistors sitting on top?
 
S

Scott Dorsey

Jan 1, 1970
0
Bimmer Owner said:
I wish there was a way to get rid of that heavy fibrous plastic goop!
(What is that black fibrous tough stuff anyway?)

It's an epoxy resin filled with silica. It will come off with DMSO at
three atmospheres or so. Sometimes a soak in DMSO for a couple weeks will
make it peel off. This will also soften the PC board though.
--scott
 
Bear in mind that the $75 Sitronic Ebay FSU is known to be even
more faulty than the $175 Valeo FSU from the stealer.

I can think of nice test equipment to buy instead of a $175 FSU every
few years!

I feel your pain. I have a 1980 MB 300SD. For the AC,
MB used a part from Chrysler that they used on the Imperial.
It's the size of a grapefruit and it controls the entier HVAC
system. In that one widget you have a valve that's in the
path of the engine coolant system that goes to the heater,
about a dozen vacuum hoses that control the various
flaps. You also have the electical outputs for the blower,
AC clutch, etc. And all that is driven by a small electric motor
that is inside the thing. The motor is part of a feedback
servo system that moves in relation to the desired temp
setting vs the actual. It goes from max cooling, to max
heating. Oh, and a critical part of that sytem is the
potentiometer inside that widget, the resistance value
of which changes based on the current position of the
motor that works the whole thing, moving it from max
cooling to max heat.

So, you have hot engine coolant, vacuum, electrical,
a potentiometer, all inside one widget. How smart is
that? The typical failure mechanism is that the plastic
housing cracks and it leaks coolant. They were up to
$600 in the 90s for a new one. Some guy down in
TN started a business rebuilding them. His uses an
aluminum housing for the part where the valve and
hot coolant are. I bought one 10 years ago for $500.
It's been fine until recently. Now it's starting to have
problems. Haven't had a chance to find out what's wrong.
 
B

Bimmer Owner

Jan 1, 1970
0
Ha, keep going!

Well, I only had lunchtime, and it got uglier & uglier as I went!
http://www5.picturepush.com/photo/a/12515993/img/12515993.jpg
http://www4.picturepush.com/photo/a/12515997/img/12515997.jpg
http://www2.picturepush.com/photo/a/12516000/img/12516000.jpg
http://www3.picturepush.com/photo/a/12516001/img/12516001.jpg
http://www2.picturepush.com/photo/a/12516005/img/12516005.jpg
http://www1.picturepush.com/photo/a/12516009/img/12516009.jpg

Unless there is some chemical way to remove that black/gray
rubbery (like a tough pencil eraser) gunk, there will be no
way to read the numbers on the two transistors.
 
T

tm

Jan 1, 1970
0
Bimmer Owner said:
Any evidence it was checked with a scope?

Yes.

This quote below is verbatim from this location:
http://www.bimmerfest.com/forums/showpost.php?p=6536514&postcount=131

Hi, After soldering the lost/refound component,
remaking the joints of the 2 mosfet and testing the FSU alone with an
oscilloscope, here are my observations:

- the FSU works again

- there is no PWM , the gates signals are continuous voltage only , this
is the reason why it heat so much its aluminium box... In fact there is no
point on the board where square signals are present. Can somebody check
its own FSU if it's the same ?

- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the
currents between the 2 MOSFET and balance the power also. The mesure of
the DC voltage on those resistors can be used to evaluate the current of
the blower and its worn state.

- I guess the principal duty of the computer on the other side is
switching off the power transistors if the control voltage goes under 1V.

I put the FSU back in the car and it still work, I don't know if it will
last long. because of the heat...

Olivier

http://www.bimmerfest.com/forums/attachment.php?attachmentid=306784&d=1325522788

Ok, they are most likely transistors and the two straps are combining the
emitters. Is the center junction of the two straps connected to the motor?

Does the red jumper connect to the 40 amp fuse? If both of those are a yes,
then they are most likely NPN power transistors and the jumper ties the
collectors together. Or power darlingtons. And it is just a linear supply.

In your other pictures, the spring clips just hold the transistors against
the heat sink.
 
Any evidence it was checked with a scope?

Yes.

This quote below is verbatim from this location:
 http://www.bimmerfest.com/forums/showpost.php?p=6536514&postcount=131

Hi, After soldering the lost/refound component,
remaking the joints of the 2 mosfet and testing the FSU alone with an
oscilloscope, here are my observations:

- the FSU works again

- there is no PWM ,


You said you tested the FSU alone. If so, how can you
say there is no PWM signal between the car and FSU?


the gates signals are continuous voltage only , this is the reason
why it heat so much its aluminium box... In fact there is no point on
the board where square signals are present. Can somebody check its own
FSU if it's the same ?
- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the currents between the 2 MOSFET and balance the power also. The mesure of the DC voltage on those resistors can be used to evaluate the current of the blower and its worn state.

Say what? 10 milliohms is .01 ohms. How could that
possibly balance the power to a motor in a 40 amp circuit?
- I guess the principal duty of the computer on the other side is switching off the power transistors if the control voltage goes under 1V.

Which makes no sense at all.

I put the FSU back in the car and it still work, I don't know if it will last long. because of the heat...

Olivier

http://www.bimmerfest.com/forums/attachment.php?attachmentid=306784&d....

It seems they last at least a few years. Maybe it's like
gas. You buy it and you go so far. I think you're in way
over your head here;
 
Hi, After soldering the lost/refound component,
remaking the joints of the 2 mosfet and testing the FSU alone with an
oscilloscope, here are my observations:
- the FSU works again
- there is no PWM , the gates signals are continuous voltage only , this
is the reason why it heat so much its aluminium box... In fact there isno
point on the board where square signals are present. Can somebody check
its own FSU if it's the same ?
- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the
currents between the 2 MOSFET and balance the power also. The mesure of
the DC voltage on those resistors can be used to evaluate the current of
the blower and its worn state.
- I guess the principal duty of the computer on the other side is
switching off the power transistors if the control voltage goes under 1V.
I put the FSU back in the car and it still work, I don't know if it will
last long. because of the heat...

http://www.bimmerfest.com/forums/attachment.php?attachmentid=306784&d....

Ok, they are most likely transistors and the two straps are combining the
emitters. Is the center junction of the two straps connected to the motor?

Does the red jumper connect to the 40 amp fuse? If both of those are a yes,
then they are most likely NPN power transistors and the jumper ties the
collectors together. Or power darlingtons. And it is just a linear supply..

In your other pictures, the spring clips just hold the transistors against
the heat sink.- Hide quoted text -

- Show quoted text -

The main points of his post, like the "two bridges that are .01 ohm
resistors, that "balance" the motor don't
concern you in the least?

Blind leading the blind
 
T

tm

Jan 1, 1970
0
Hi, After soldering the lost/refound
component,
remaking the joints of the 2 mosfet and testing the FSU alone with an
oscilloscope, here are my observations:
- the FSU works again
- there is no PWM , the gates signals are continuous voltage only , this
is the reason why it heat so much its aluminium box... In fact there is
no
point on the board where square signals are present. Can somebody check
its own FSU if it's the same ?
- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the
currents between the 2 MOSFET and balance the power also. The mesure of
the DC voltage on those resistors can be used to evaluate the current of
the blower and its worn state.
- I guess the principal duty of the computer on the other side is
switching off the power transistors if the control voltage goes under
1V.
I put the FSU back in the car and it still work, I don't know if it will
last long. because of the heat...

http://www.bimmerfest.com/forums/attachment.php?attachmentid=306784&d...

Ok, they are most likely transistors and the two straps are combining the
emitters. Is the center junction of the two straps connected to the motor?

Does the red jumper connect to the 40 amp fuse? If both of those are a
yes,
then they are most likely NPN power transistors and the jumper ties the
collectors together. Or power darlingtons. And it is just a linear supply.

In your other pictures, the spring clips just hold the transistors against
the heat sink.- Hide quoted text -

- Show quoted text -

The main points of his post, like the "two bridges that are .01 ohm
resistors, that "balance" the motor don't
concern you in the least?

Blind leading the blind
=============================================

It is common to place low value resistors in the emitters to balance the
current in two paralleled devices.
0.01 ohms would be very realistic in a 40 amp system.

As to blind, you exhibit that trait much better than anyone else involved
with this thread.
Asshole.
 
T

tm

Jan 1, 1970
0
Jamie said:
Actually that looks good and should work just fine.

That type of probe is self contained, meaning it does
not depend on the DMM input impedance for proper match.

BUt 80 Amps is kind of small I think, I have a AC/DC clamp
that does 800 amps, but to do low current readings of less than 1
amp becomes a problem with AC. DC I can zero it.

Jamie
You can increase the sensitivity by winding multiple turns around the clamp
on core.

Two turns makes it a 400 amp meter and so on.
 
J

Jamie

Jan 1, 1970
0
Bimmer said:
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


Actually that looks good and should work just fine.

That type of probe is self contained, meaning it does
not depend on the DMM input impedance for proper match.

BUt 80 Amps is kind of small I think, I have a AC/DC clamp
that does 800 amps, but to do low current readings of less than 1
amp becomes a problem with AC. DC I can zero it.

Jamie
 
J

Jamie

Jan 1, 1970
0
jim said:
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.
A properly working blower motor does not need extra torque to start at
low RPMs.

Torque is only needed when RPMs increase and mass air flow is
increased, thereby, putting a strain on the motor. Basic resistor
systems will vary in speed if air pressure isn't constant, and in
most cases it isn't..

When there is no air flow or the flow has been restricted somehow,
there is little to no torque demand, other than mechanical of the blower
blades and those should turn easy, sine bearings and balance permits this.

PWM is just a cheap way of speed control, it does not mean it's
better, in fact in some ways it's not, due to over head in noise..

A linear control with feed back will provide the needed torque but
they do tend to run hot when throttled back, because of the resistance
being present between the 12V and the motor terminals. THis is where
PWM comes in a winner but then you need the added cost of noise
reduction engineering.

I can only assume the linear module at least uses a feed back to
maintain output voltage, if it is so cheap that it does not even
do that, then maybe they are trying to emulate a real resistor or
they are just shitty engineers or tightwads.

Jamie
 
J

jim beam

Jan 1, 1970
0
jim said:
On 03/25/2013 05:57 PM, Jamie wrote:

jim beam wrote:

Another way to do this, is to have an inductor on board with the
speed control circuit. You would PWM that inductor in series to a
filter
cap
on the output which will then give you a clean variable DC. THe
inductor
will be doing all variable voltages.


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.


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.
A properly working blower motor does not need extra torque to start at
low RPMs.

Torque is only needed when RPMs increase and mass air flow is
increased, thereby, putting a strain on the motor.

you're right, except that there are more variables. very cold days,
very windy days, blown snow powder, leaves, all kinds of things can mess
with the motor starting at a low speed.

Basic resistor
systems will vary in speed if air pressure isn't constant, and in
most cases it isn't..

When there is no air flow or the flow has been restricted somehow,
there is little to no torque demand, other than mechanical of the blower
blades and those should turn easy, sine bearings and balance permits this.

PWM is just a cheap way of speed control, it does not mean it's
better, in fact in some ways it's not, due to over head in noise..

from what i can see, the /only/ drawback is noise. power efficiency,
controllability, speed consistency, and yes, sometimes price, all are
wins for pwm.

A linear control with feed back will provide the needed torque but
they do tend to run hot when throttled back, because of the resistance
being present between the 12V and the motor terminals. THis is where
PWM comes in a winner but then you need the added cost of noise
reduction engineering.

I can only assume the linear module at least uses a feed back to
maintain output voltage, if it is so cheap that it does not even
do that, then maybe they are trying to emulate a real resistor or
they are just shitty engineers or tightwads.

well, they're clearly failing at something if they're trying to provide
an engineering solution. if however they're providing a financial
solution with a per-determined failure rate, then they're right on target.
 
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