Root cause insight into the common BMW blower motor resistor failures

A properly working blower motor does not need extra torque to start at low RPMs.

Torque is only needed when RPMs increase and mass air flow is increased, thereby, putting a strain on the motor. Basic resistor systems will vary in speed if air pressure isn't constant, and in most cases it isn't..

When there is no air flow or the flow has been restricted somehow, there is little to no torque demand, other than mechanical of the blower blades and those should turn easy, sine bearings and balance permits this.

PWM is just a cheap way of speed control, it does not mean it's better, in fact in some ways it's not, due to over head in noise..

A linear control with feed back will provide the needed torque but they do tend to run hot when throttled back, because of the resistance being present between the 12V and the motor terminals. THis is where PWM comes in a winner but then you need the added cost of noise reduction engineering.

I can only assume the linear module at least uses a feed back to maintain output voltage, if it is so cheap that it does not even do that, then maybe they are trying to emulate a real resistor or they are just shitty engineers or tightwads.

Jamie

you're right, except that there are more variables. very cold days, very windy days, blown snow powder, leaves, all kinds of things can mess with the motor starting at a low speed.

from what i can see, the /only/ drawback is noise. power efficiency, controllability, speed consistency, and yes, sometimes price, all are wins for pwm.

well, they're clearly failing at something if they're trying to provide an engineering solution. if however they're providing a financial solution with a per-determined failure rate, then they're right on target.

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

Yup, and I see the legs of the transistors (two). So those R's are in line with the emitters where they joint.

Most likely thermo stress cracks due to the potting restraint.

Jamie

THey make power resistors in T0-220 style cases, for heat sink mounting however, I doubt those are that.

Jamie

fantastic. could you condescend to help these guys wit their project then? or are you just here to whang your donger around?

then you're a complete genius because other people find it hard/impossible.

yeah, a schematic doesn't exactly mean much if you don't even know what the components actually are. [see above]

expensive? you're not kidding. and failure analysis is a whole lot easier if you know what you're looking at in the first place.

yeah, if you know precisely where you're drilling, in three dimensions. and you have dil/soic [etc] type chips. anything bga or similar, and you're sol.

and you're not doing that with a black box and getting meaningful data out of it.

ok, but you're missing one simple thing - this is a bunch of guys with a dremel and a dental pick. money/time are limiting factors, even if there were logic to reverse engineering, which i don't think there is in this case.

no, what you're experiencing is her "disappointment" at discovering that you are an anosognosic retard.

didn't know.

which would be another factor in favor of pwm...

that would be my guess.

i guess that's another part of what we're looking at here.

thanks for the feedback!

If that being the case, I guess we now know why the module keeps burning out :)

But there is factor that maybe you have forgotten or didn't know, and that is, the resistance of the DC motor. stall torque can be limited to what the DC R value is, in otherwords, this value forms a voltage divider and thus low voltage at stall current could seem like no voltage and not turn.

DC PM/SHUNT motors attempt to compensate when load is dragging it down in speed, that also includes a slow start. So as long as the speed control can maintain a low voltage set point even when the motor is calling for high amps - the motor R, it'll still start. However, there is another factor, the speed control may not be performing armature feed back and simply supplying current only.. If this is the case, then the motor will stall at low speed demands in conditions that make it hard for the motor to start.

I've also seen them allow the motor to run in torque mode to adjust for air density. It'll simply self adjust naturally, and in those cases you do not want armature feed back but torque feed back. Of course, this will cause heating of the module when torque demand is low..

Jamie

No, I'm not like you. I have worked in Electronics for 52 years. I could draw the schematics, but I would need a handful of bad modules and the time to do it right. Just like the kU band microwave audio, video, and data terminal hardware I worked on that's in orbit aboard the ISS. It takes hands on effort to reverse engineer a design.

Bomarc has reverse engineered a lot of automotive modules:

The one you want might be on their lists.

Dental picks are probably expensive. I picked up a nice 4 piece pick kit somewhere a couple years ago for maybe 5 bucks. Just came in handy for digging out a crumbling plastic hose nipple threaded into an aluminum intake manifold. My Dremel kit was pretty expensive, but hey.

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?

Bosch has done the nichrome insert power resistor for many years; I know that they used them in the turn signal flasher in the late seventies when they first went electronic. They aren't really very good resistors but they are very cheap.

--scott

Okay, there is a TO-220 package that is split open there, with the backplate on the right and the cover on the left. Can you chip off enough of the araldite from the cover to be able to read the numbers on it?

Or, could you get a good picture of the die which is left on the piece on the right? We might be able to identify it with a sharp photo of the die... although from what I can see from the fuzzy photo it does not look like a very happy die.

If it is actually a MOSFET it will look like this:

(That's a package that is a little bigger than the TO-220, but you can still see the channel down the middle of the FET and the overheating damage to the source.... the three leads have been torn off in the unpotting process though.)

--scott

Of all people, sears has one for less than $100 that does DC current

as i recall, i think harbor freights clamp meters are AC current only (like most are)

I am a fluke man too. Own at least 5 DVM's of various types. Big fan of the 70's series. Tough, last forever if you take care of them

bob

so what you're saying is that you're just here to piss and moan because you know how to help, but won't.

and credentials don't work on usenet - they're completely uncheckable and many are bogus. what matters is whether you can walk the talk.

as for having stuff in orbit, i don't have anything, but two of my best friends do. the difference between them and you is that they're not jaded and they're actually helpful.

that at least is vaguely useful, but you could have come up with that 20 posts ago.

Yawn. I don't have access to any defective modules. I don't know anyone at the local BMW dealership to ask for failed units and I'm not going to buy a new module to destroy for a whiny piss ant like you.

You can't even crawl.

Then tell them to reverse engineer it for you. Or can't they 'walk the talk', either?

And you couldn't so you piss and moan. I couldn't remember their name, since I hadn't seen one of their print ads for over 20 years. I looked them up, when I did.

i don't know who you think you're talking to, but i have consistently advocated /not/ deconstructing this unit. it's not worth it when cost of replacement isn't that high or you can build an alternate controller that will be more reliable.

at least i can follow a thread without being a crotchety old fart.

i don't /want/ to reverse engineer it any more than i want to repair broken light bulbs. you were the one bragging about how easy it was. i said it wasn't. and when it comes down to walking the talk, you won't.

i guess we should be grateful...

Apparently I'm talking to a blowhard troll.

Ass. The first step in designing a replacement is to understand what it is supposed to do, and how the original performed that function.

Show us.

Send me some defective modules, or quit trolling.