Root cause insight into the common BMW blower motor resistor failures

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:

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

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:

- 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]

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:

On the picture , one of the component is gone with the rubber foam : its look like transistor : black with 3 pins ( it 's not bipolar transistors ).

It happen to me also : the component gone so easily that I didn't realize it, maybe it is the issue for that FSU.

It is only while I compare to other pictures in the forum that there was a lack of component on my board.

fortunately I found it together with the rubber foam parts, so I will solder it back later[/QUOTE]

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.

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 s*it tin.

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

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

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.

Agreed.

--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.

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.

All I know is that the HVAC controller sends a 2.0 VDC to 7.8 VDC signal to one of the five pins of the FSU. I presume that directly corresponds to the desired blower motor speed - but that is conjecture on my part.

The originals fail within about 5 years. I've had my second one fail in

This is one of the classic failure indications! Very very very common! However, another classic failure indication, other than the dead battery in the morning, is a blower that has a "mind of its own".

Together, those two sets of symptoms account for 99% of the failures.

Of all the anecdotal evidence presented, I don't think I've ever heard of a failure being that the system was totally dead.

What that tells us, I don't know.

Exactly!

But is that a clean DC level or a pulse width modulated signal? If it was measured with a multimeter, you won't know.

Has anyone probed around the module with a scope? Is that possible?

That and the question about the TO-220 devices. I think it would be possible to make a better replacement if those questions were answered.

tm

Your idea about testing it is correct, however a bad one can work fine until it gets hot enough for the IC chip to fail. Hvac control voltage is

The load resistor should have a metal case mounted on a heatsink to dissipate the heat if you are going to run the test more than a few minutes.

My original 13 spike FSU was replaced twice under warranty, so the car had 3 FSU's. When the 3rd old style failed i replaced it with the new design FSU. That was 8-9 years ago, it is still working. One thing not considered in your post is how many did not use OEM units, the $75 eBay ones are junk. The $175 units at the stealer seem to hold up a little better.

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 don't even own a BMW. After this thread, I don't think I ever will.

Here is a picture of the BMW E39 HVAC/IHKA controller and sampling fan.

I'm googling for specs as we speak.

The other possibility is that it's a digital command signal of some kind. I don't know how they typically do that, but if a