Root cause insight into the common BMW blower motor resistor failures

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On Tuesday, February 4, 2014 12:02:11 PM UTC-5, Ralph Mowery wrote:

Yes, I hear what you're saying. I assume it's being used like a switch, all the time being switched on and off in a PWM fashion, like a light dimmer. But while going from off to on, in the middle, it will generate more heat. That's why electronic light dimmers get warm too. And I guess if something is wrong with the motor, like your bearing example, it could generate more heat in the FSU. I guess a way to look at it is like what would happen if you put a 1500W load on a triac based AC light dimmer rated for 700W. At full power, it would probably be fine. At half power, it could get so hot it fails.
If the bearings are going bad or there is some other reason the motor

Not so far because there isn't an easy way to do so. I don't have a clamp on DC ammeter and with the connector they use, no way to get something into/onto the circuit. It hasn't blown the blower fuse. I guess I could stick the wire with a pointed probe, but even that isn't easy because there isn't much access to the cable.
There is one more avenue being pursued. This car uses a cabin air filter and it's fairly dirty and needs to be changed. Thinking that might have some effect on it, but on the other hand AFAIK, it only filters the outside incoming air, which is like 20% max and the dash controls allow that to be shut off, or automatically controlled, etc. So I would think the blower should run fine with just the cabin air, especially at lower speeds, but who knows. So one of those is on the way.
After that, I think next is going to be to get another FSU, from another manufacturer and try it. The BMW one was $125, there are Behr ones available for $45. As you can see from the thread, these things are poorly designed and have a high failure rate. They've been around since 2000 and they still all suck and fail. Folks can't even agree if any of the various manufacturers are better. They have changed the design. The original ones had less heat sink pins, but they were larger. New ones have at least 2X the pins, like 40 of them, so clearly they've changed the design because of heat issues. Because they all suck is why the guys here have been trying to reverse engineer it and see if they can do something to make it work. I think just a larger heat sink would probably solve it, but no easy way to do that I can see.
Still you think a new one from BMW would work out of the box, but who knows. The other sad news is that to get to the blower requires removal of the whole dashboard :(
If you want to see a pic of what one looks like, just google BMW FSU.
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On Thu, 21 Mar 2013 03:55:47 -0700, the will wrote:

This is an interesting approach, given that the vast majority of bimmer owners do NOT replace the blower motor - they replace the FSU.
While the blower motor replacement procedure is a major PITA, one 'can' test the leads from the FSU harness connector pins #5 and #1 which are power and ground respectively to the blower motor.
Again, we don't have a circuit diagram, but it has been said that the blower motor takes about 6 amps (variously, depending on the speed) but it would take a test jig to test that in operation.
To my knowledge, nobody has created that test jig (although I know of only one attempt, which failed): http://www.bimmerfest.com/forums/showthread.php?tg8534&highlight=fsu
It's easy enough to test the resistance of the blower motor though, and those results have come out at about 0.4 to 0.6 ohms.
It would be expensive to change a blower motor on a whim, so, how would YOU suggest the blower motor be tested in situ?
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Static resistance doesn't tell you anything, but operating current measured with a DMM would tell you a lot.

I've never tested one, but I put a drop of turbine oil on the motor bearings every five years or so. I do the same on the window and seat motors too.
I'd imagine if you listen carefully and have good hearing you can tell if the motor is binding at all, but many people do not. --scott
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On Thu, 21 Mar 2013 11:45:54 -0400, Scott Dorsey wrote:

It 'can' be done, but would require a test jig inserted inline as the FSU is deeply ensconced under the dash while the blower motor is even more deeply so.

While that preventive work might be prudent, the sheer effort to remove the entire dash simply to access the blower motor would be problematic.
Still, if the problem is that the blower motor is merely using more current as it gets older, why wouldn't a NEW FSU burn up within a few weeks of insertion?
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On 03/21/2013 06:04 PM, Bimmer Owner wrote:

If the unit is near its limits it might just get very hot and parts start aging very fast -> semiconductors will just fail after a little while. The new unit should fail sooner than the old one but who knows who soon, maybe after a few years. Peugeot's ( and Citroen) used just one huge pnp darlington which failed quite often , it was working too near its operating limits.
(just my 2 cents) ismo
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I don't know the wiring diagram on that particular model, so I don't know where you would need to break in to measure just the current of the blower without anything else. But it does not seem terribly insurmountable, especially seeing that BMW is very good about breaking everything out into connectors all over.

Well, that's partly why I drive a 2002 and an E28, everything is much easier to get to.

You would expect that, indeed. --scott
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wrote:

Just measure the battery current with and without the blower running.
Turn every thing else off.
QED
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On Thu, 21 Mar 2013 16:59:01 -0400, tm wrote:

That's a pretty good idea.
Of course, it's impossible to turn everything off, as the computer won't go to sleep for 16 minutes after the car is shut and armed, but, still - with the blower consuming something like 5 or 6 amps, we should be able to measure at least that.
The problem, even with the car's additional electronics powered up, is HOW MUCH of a difference are we expecting between an older blower motor current draw and a new one?
Are we looking for a 1 amp difference, for example?
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wrote:

Just put your ammeter into the heater blower fuse connector and you get the current of the blower motor.
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On Thu, 21 Mar 2013 20:01:20 -0400, clare wrote:

That's an interesting idea.

The fuse for the blower motor is called the "infamous F76" for a reason. http://bimmerfest.com/forums/showthread.php?tg4612
It's a 40 amp fuse under the glovebox but it's in a really inaccessible spot; however, it's right side up, so, the wires going INTO it are visible from the tips of your feet under the glove box.
So that's a possibility; but you'd have to cut the wires.

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On Fri, 22 Mar 2013 04:31:00 +0000 (UTC), Bimmer Owner

Why would you have to cut the wires???? Simply remove the fuse and connect the ammeter. You guys make your lives so difficult
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On Mar 22, 3:04 pm, snipped-for-privacy@snyder.on.ca wrote:

It's kind of amazing the fuse is 40 amps, no? I mean a 1 hp motor draws ~1500W And this heater blower is 480W? Seems like a lot to me.....
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snipped-for-privacy@snyder.on.ca wrote:

Take a blown fuse and use it for a test connector with a cheap 50-0-50A meter. Then you can just plug it in in place of the fuse to make the test. You won't even have to worry about the polarity. You can use a high current shunt, & a digital meter if you want more accuracy.
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On Sat, 23 Mar 2013 22:27:41 -0400, "Michael A. Terrell"

Or just get the special tester that is made to plug into the fuseblock. Autel makes the MX101 and 201 (10 amp and 20 amp) units for the lighter duty stuff.
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snipped-for-privacy@snyder.on.ca wrote:

Is that cheaper than roll your own?
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On Sun, 24 Mar 2013 02:28:00 -0400, "Michael A. Terrell"

Definitely not, if you already have a multimeter - but it is easier for the guys who can't figure out how to do it without butchering the wiring harness.
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snipped-for-privacy@snyder.on.ca wrote:

That type shouldn't be allowed to own any tools. They generally do more damage than good. If they do get something to work, it rarely lasts because they have no clue what because the problem.
I've seen too many vehicles that some idiot cut and patched back together. One stepvan I bought years ago had a damaged harness and I talked them down almost $1000 on the price. It was coming off lease from a fleet, and they wanted to fix it themselves. i pointed out that if they could repair it properly, it wouldn't be in that condition.
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On Sat, 23 Mar 2013 22:27:41 -0400, Michael A. Terrell wrote:

That's an idea.
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On Fri, 22 Mar 2013 15:04:39 -0400, clare wrote:

The 40 amp fuse is barely accessible (as can be seen from the photos). It's not even easy to pull the fuse & even harder to replace it. So, all I was saying was that it's actually rather difficult to insert test leads into the empty fuse #F76 fuse holder.
I'd wager it 'can' be done - it's just going to take an hour or so to get the leads in place.
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So go back to measuring at the battery terminals. Do you have or can you borrow a clamp on DC ammeter? They are great for this sort of problem. You don't even lose any skin that way.
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