Root cause insight into the common BMW blower motor resistor failures

Page 10 of 14  


Okay, notice on the lefthand photo, there are two sets of three solder joints on the lefthand side of the board, which show signs of recent rework. There are TO-220 transistors attached to them, which are behind the board and you cannot see.
On the righthand photo the board is reversed... you can see two sets of three holes on the righthand side of the board which is where those TO-220 transistors were attached. You can see that they overheated the board and lifted pads in the process too.
But you cannot in either of these photos see the transistors or what the part number on the face of them is. Knowing what kind of transistors are used will go far toward explaining some possible failure modes. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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wrote:

Yes, what Scott said. Where are the two transistors that were soldered in the pair of three holes on the right of the right photo.
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On Mon, 25 Mar 2013 18:12:53 -0400, tm wrote:

I always thought those were just "posts" tying the circuit board to something inside the FSU - but I do agree that all the FSU autopsies show those posts being unsoldered to remove the board. Here's the quote that came with this picture, for example:

[QUOTE=sudnut]Ok, here are a couple of pics of my FSR from a 2002 E46 which is identical to FSR's in many other models and series of BMW. The first is what you see when you scrape off the epoxy-like filling covering the PCB. Using a hairdryer or better still a heatshrink gun carefully without overheating. If you smell burning its too hot. You should get a faint hot plastic odour that's all. I used one of those cheap sets of precision flat screwdrivers, they can be sharp so again be careful. You can see I gouged and scratched the circuit board a little in places, just make sure you don't slice through any tracks or lever off any components. The second picture is a zoom of the 2 groups of 3 pins which I believe are the cause of my FSR going crazy. If you look closely, all six pins have cracks in the solder around the pins, most noticeable are pins 1, 4 and 5 from left to right. My repair was simply to remelt and resolder these pins with a little more solder using a hot and fine tipped soldering iron, until I felt that the lead had flowed through the board and hopefully to the other side where the components/resistors are. If this repair doesn't last, I may remove the board fully and have a better look on the other side.[/QUOTE]
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On Mon, 25 Mar 2013 14:55:02 -0400, Scott Dorsey wrote:

Ah, I see what you're talking about and I've circled them in this photo:

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?
Since I have an FSU in my possession, I will try to lift the board. The FSU that I have, DOES have two sets of these "spikes" sticking up at those very locations, so, clearly "something" is there.

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.
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No, they are two TO-220 cased transistors. they are most likely attached to the heat sink somehow. Like was mentioned before, finding out the part numbers on those transistors will reveal a lot.
tm
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On Tue, 26 Mar 2013 00:39:23 -0400, tm wrote:

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.
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On Tue, 26 Mar 2013 05:00:41 +0000, Bimmer Owner wrote:

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).
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On Tue, 26 Mar 2013 04:22:30 +0000, Bimmer Owner wrote:

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:

[QUOTE=olivier577]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[/QUOTE]
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On 03/25/2013 09:40 PM, Bimmer Owner wrote:

"sophomoric" indeed.
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fact check required

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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)?
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On 03/25/2013 10:02 PM, Bimmer Owner wrote:

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.

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.
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On Mon, 25 Mar 2013 22:23:22 -0700, jim beam wrote:

Ah, so it's a plain old voltage divider?
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Bimmer Owner wrote:

I think the IC is a dual, high side motor driver IC and each transistor has a large emitter resistor with different values. That is the metal strips on the pins by the sets of three holes. That would give three speeds & off. Like a three way lamp with 50 + 100 Watt elements. Turn both on and it draws 150 Watts. This would explain the failure of the solder joints on the transistors since they are used in linear mode.
00 would be off 01 would be low speed 10 would be medium speed 11 would be high speed
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That's a good theory except why would the resistors not be attached to the heat sink?
There are still open questions that until answered we are just speculating with educated guessing.
1) What exactly does the control signal look like on a scope?
2) The two devices, mosfets or transistors? Any part numbers?
3) Waveforms on the motor power line for the different speeds.
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Because they draw no current, they only set the base bias of the power transistors.

It's a variable voltage.

Still unknown.

Reported as DC. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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wrote:

Are you sure? It seems it was measured with a DMM, not a scope. A PWM signal would just look like DC to a DMM. Since it was reported as "2.0 to 7.8" volts, you would not get those significent figures with a scope.

Any evidence it was checked with a scope? Again, a PWM signal will just look like a DC level to a DMM.
Let's see what the autopsy shows next :).
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On Tue, 26 Mar 2013 14:56:21 -0400, tm wrote:

Yes.
This quote below is verbatim from this location: http://www.bimmerfest.com/forums/showpost.php?pe36514&postcount 1
[QUOTE=olivier577;6536514]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[/QUOTE]
http://www.bimmerfest.com/forums/attachment.php?attachmentid06784&d 25522788
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http://www.bimmerfest.com/forums/attachment.php?attachmentid06784&d 25522788

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.
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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
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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.
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