Root cause insight into the common BMW blower motor resistor failures

I must be lucky. My 2000 323 has had only one of the above "failures"

- leaky power-steering hoses.

Reply to
dizzy
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ok, as i understand it, and as i said to scott earlier, this is a problem because it mungs low speed motor start and low speed torque.

this seems to be the problem. you can get good efficient control at a few hundred hz, but it's super noisy. if you take the frequency up above audio, bigger motors seem to be more of an issue.

you got that right - and it would go a lot further past its warranty if it was better rated.

Reply to
jim beam

as said by others, buy a clamp-on current meter. it's not like you'd never use it again.

Reply to
jim beam

honda use a "plastic cooling system". it's not infallible, but you can get 15+ years out of the first one. there's no reason bmw couldn't achieve the same - if they wanted to.

that is the catch - bmw target a certain type of buyer that typically won't keep a car more than 3 years. after that, they don't care and bmw can safely switch to "maximize parts sales/write off the old cars and sell new ones" mode.

in europe, bmw have been aggressive leaders in "recycling" and spend a lot of money advertising the fact. in practice however, it means that they buy used cars back and have them scrapped, thereby keeping used parts off the market - the mba's have done their math.

it depends on perspective. i agree that some appear to be extremely rudimentary, but that is contradicted by the fact that they spend a huge amount of money on r&d, and have boatloads of phd's on staff. they were also rescued by a bunch of mba's in the 70's and have had a root and branch focus on design life ever since.

call me when you have 100k miles on it...

Reply to
jim beam

THe problem is eddy currents at high pulse rates. Between the wire used and the core you can get heating in the motor..

Most inverters or pulsers for industrial drives tend to operate in the 8kHz range, that seems to be a good compromise.

Another way to do this, is to have an inductor on board with the speed control circuit. You would PWM that inductor in series to a filter cap on the output which will then give you a clean variable DC. THe inductor will be doing all variable voltages. This is a form of a PFC type of supply and the only heat you get is from the DC coil R in the inductor and the heat from the switching MOSFET, which should both be rather low. BY doing it this way, you could operate a switcher at lets say up in the 100kHz or more range and this would keep the size of the caps and inductor small.

It is cheaper to use the motor as the inductor for PWM control but, it also can radiate noise on the lead wires. I guess one could actually mount the speed control directly on the motor using PWM and the noise problem should be minimum.

Calculating the cost between the two, BMW most likely decided to go with the basic linear type, Because why should they care? It'll most likely make it past it's warranty!

Jamie

Reply to
Jamie

I have a Fluke 75, so I'll have to see what clamp on DC current probes fit it.

Reply to
Bimmer Owner

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

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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.

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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.

Reply to
Bimmer Owner

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:

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[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]

Reply to
Bimmer Owner

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

Reply to
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:

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[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]

Reply to
Bimmer Owner

cheaper to buy a whole new meter!

or google for this guy: "ESI 695 80 Amps DC/AC Low Current Probe"

Reply to
jim beam

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:

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[QUOTE=olivier577] The 2 MOSFET drain and source are tied together but the gates are differents

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]

Reply to
Bimmer Owner

"sophomoric" indeed.

Reply to
jim beam

This seems to only be $107 but it doesn't say whether it works with the Fluke 75 or not.

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Reply to
Bimmer Owner

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.

Reply to
Bimmer Owner

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)?

Reply to
Bimmer Owner

For the same price, it looks like we can get a separate unit:

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Fluke T5-600, SKU: 133038, $109.95

Reply to
Bimmer Owner

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.

Reply to
jim beam

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 shit tin.

Reply to
tm

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

Reply to
Scott Dorsey

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