Root cause insight into the common BMW blower motor resistor failures

Bimmer Owner · 2013-03-21T03:08:09+00:00

Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW? 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. 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. 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? Here is the best (admittedly sketchy) wiring diagram we have so far:

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trader4 13 years ago

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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Bimmer Owner 13 years ago

Well, I only had lunchtime, and it got uglier & uglier as I went!

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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.

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Bimmer Owner 13 years ago

Actually, it's the same transistor, which broke in half while I was attempting to get the black rubber eraser stuff off of it to read the numbers.

It's really going to be HARD to read those numbers now...

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Bimmer Owner 13 years ago

Yes.

This quote below is verbatim from this location:

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[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]

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Bimmer Owner 13 years ago

I only found 1 transistor.

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This is what is suggested in this quote below:

[QUOTE=540iman]There is only ONE mosfet in the FSU and I would forget about trying to replace it even if you had one. Also, we have no reason to believe the mosfet has caused any FSUs to fail. I have yet to see the resistors in any of the picture shown anywhere in this thread. Can someone circle the resistors for me? I would think they would be fairly decent wattage so they would be very easy to see, but I don't see any resistors.[/QUOTE]

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tm 13 years ago

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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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tm 13 years ago

I think he is incorrectly calling the SMT IC a mosfet.

What went in the other three holes on the PC board? What was under the two spring clips on the heat sink?

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trader4 13 years ago

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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trader4 13 years ago

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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Nate Nagel 13 years ago

Sadly, I speak from experience, in both respects.

nate

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Bimmer Owner 13 years ago

I 'think' (but I'm not sure) that these are the resistors in series:

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Bimmer Owner 13 years ago

I'm still trying to figure out if there was only one or two:

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trader4 13 years ago

If they are resistors, I've never seen any that look like that. Also, given that you want to thermally bond any components that generate major heat, why are they not heat sinked? With any power design I've seen, the key components, eg the transistors are directly bonded to the heat sink.

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tm 13 years ago

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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tm 13 years ago

/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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tm 13 years ago

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.

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Michael A. Terrell 13 years ago

I was never crazy about that style of heatsink.

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Michael A. Terrell 13 years ago

You've never seen strips of nichrome in a space heater?

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Michael A. Terrell 13 years ago

Two.

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Jamie 13 years ago

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

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