Root cause insight into the common BMW blower motor resistor failures

Page 9 of 9  



LOL. There's more truth to that then you may think.
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On 03/26/2013 03:00 PM, tm wrote:

Sadly, I speak from experience, in both respects.
nate
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
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Scott Dorsey wrote:

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!
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50?

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.
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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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On Tue, 26 Mar 2013 15:51:52 -0400, tm wrote:

The red jumper was a hack added by one user to fix the solder cracks, I think.
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I am aware of that. It looks like it ties the two collectors together. But does it go to the contact that connects to the fuse? The main point is "do the collectors (center pin on transistor) connect to the 12 volt input to the module"?
Thanks, tm
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On Tue, 26 Mar 2013 15:51:52 -0400, tm wrote:

This implies two transistors (although I only found one).
I will dig through the mess again - but I think I was too fat thumbed when I cut it open, and may have destroyed the evidence.
I do have a second FSU (since two failed on me) though ... but I want to try to FIX that one (instead of destroying it).
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131

t,

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 ?

urrents 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 bl ower 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?

ng off the power transistors if the control voltage goes under 1V.
Which makes no sense at all.

last long. because of the heat...

.
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;
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wrote:

/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 ?

/Say what? 10 milliohms is .01 ohms. How could that /possibly balance the power to a motor in a 40 amp circuit?
Not to the motor idiot. To the transistors.

/Which makes no sense at all.
Certainly not to you.

/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;
But not yours?
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No, because I'm not the one trying to reverse engineer an electronics module in a car, that contains among other things, an unknown 16 pin chip, without benefit of any of the necessary tools. You guys don't even know what the interface to the car is, whether it's analog or digital, etc. And you don't even have an oscilloscope to look at anything with.
Oh, BTW, if you're all so smart, how come I was the first one to find out for you that the 16 pin chip number you had is a vaild one for a real chip?
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snipped-for-privacy@optonline.net wrote:

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.

Resistors are used in the emitter path when combining two or more to the same circuit. This is needed to insure both transistors share in the load when it comes to biasing... Otherwise, you'll get one that favors beta and the other will sit back and snooze. Call it a ballast R if you wish.. These R values are generally low.
Jamie
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