Root cause insight into the common BMW blower motor resistor failures

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In the past i replaced the transistor with some irf mosfet .they were powerfull but didnt work properly .now i know you have to put low voltage and hi amps mos.
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snipped-for-privacy@gmail.com wrote:

If bias voltage does not match close enough heavier one won't function properly. Is there a way to improve cooling by heat sinking or applying proper thermal paste?
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The fsu from behr doesnt have that resin and it was easy to open.the new mo dule has the transistor on a copper plate to make the heat transfer fast bu t the contact with the aluminium its not so good.I did lost the waranty whe n i did this,but i'm not giving up in building an fsu from scratch.I have s ome pictures of the fsu.
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miercuri, 5 februarie 2014, 01:43:49 UTC+2, snipped-for-privacy@gmail.com a scris:

but the contact with the aluminium its not so good.I did lost the waranty w hen i did this,but i'm not giving up in building an fsu from scratch.I have some pictures of the fsu.
https://www.dropbox.com/sc/lfsinwz9sh48jid/W--GuYxNWN with this new fsu i can see the circuit bord and i can make an ideea about elmos 10901d and find a replacement
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https://www.dropbox.com/sc/a9bmsmsoagh1bzt/6KAIZ_tgiJ
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And this is the heatsink i want to use and maybe add a little fan which will run when hvac comes on but i need to find a +12v from hvac control https://www.dropbox.com/sc/h14pr26wq0xgd63/-Zrd9vszho
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On Tuesday, February 4, 2014 7:27:52 PM UTC-5, snipped-for-privacy@gmail.com wrote:

Good pictures.
It looks like the chip, the transistor and a bunch of caps, resistors, diodes. Anyone know what that funny looking copper strip is with glue or whatever under it is?
I would think that adding a little fan to the heatsink might be the most productive approach. If you can find a power source that's on when the HVAC on that would be great. But you could also power it from anything that has 12V when the ignition is ON, which could be a lot easier to find. Any chance of tapping into any of the blower airflow to cool it somehow? And is there even room in there for the additinal fan? Where the part goes under the dash, I can't tell what room there is behind it, but I would doubt there is enough for a fan.
One difference I see with your new part and Sitronic one from BMW is that your Behr one uses an aluminum housing all around the part, while the BMW part uses plastic. I noticed that difference in some of the pics before, but couldn't tell which ones were all aluminum and which just had aluminum for the heatsink. The all aluminum should be better because it can help conduct more of the heat away.
Another perhaps minor point. IDK how the thing is fastened into your car, but on the X5, it kind of snaps in, but from the removal instructions, there are supposed to be two small bolts that hold it down. The one here had been replaced previously and had no bolts. I can see why, given where it goes it would be hard to get to the bolts and the clip seemed to hold it well enough. But.... If it's bolted to metal and it's all aluminum like yours, that would help take some more of the heat away. But then it looks to me like most of the ones out there have plastic frames anyway, so in that case I don't think bolting it down is going to make much difference in transfering heat, as plastic doesn't conduct that well.
I also don't know how productive figuring out what's hooked up to what on that board is going to be. No one has been able to find a datasheet on that Elmos chip. No one is even sure what it really is. If the intention is to build a new design, my approach would be this. There are only 5 wires to this FSU.
12V GND Signal ( very small wire) Motor + Motor -
The missing piece here is what's on that control signal wire that sets the speed? If I was going to make a new design, I'd put a scope on it and find out if it's an analog signal, digital, etc. If it's just analog and varies with the set speed, then you can design a new circuit. Of course getting it into the right form factor, etc is going to take a lot of work. IMO, it's not worth it.
Oh, and regarding that single wire control, they use a similar single wire to control the variable speed auxilliary fan for the radiator. That fan has electronics in it too that can vary the aux fan speed, based on a control input. Simnple on/off like a billion other cars have was too simple. And it's in the worst place possible right next to the radiator.
The reason I bring that up, that fan fails frequently and can drain the battery. The car here, the fan stopped working, but the AC still works fine except when stopped in traffic. I was worried about it draining the battery, which they are known to do when they fail. So, I unplugged it. It's just 3 wires, 12V, gnd, control. And..... the AC no longer worked at all. The only way I could explain what was happening was that somehow the HVAC control knew the fan was disconnected, leaving me to wonder if that single wire control to it was bi-directional. IDK why anyone would design it that way, not 100% sure it's done that way, but thought I'd mention it in the sense that the single wire into the FSU might not be so simple.
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Hi.I managed to fit the fsu right behind the glove compartiment.Added a little fan ,but the hvac doesn't have a +12 so i made a circuit with an 10k ntc that allows the fan to start only when it gets hot The copper strip is just a bypass for the +12 of the blower . The radiator that i used is all copper and a had to isolate it because when it touches the ground(-) the blower comes on at full speed. I made the test on low speed half hour and it stays warm. https://www.dropbox.com/sc/bkpt1joem68b4u6/kv5qGP_CAL
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The fsu gets really hot at low speeds so the air flow its not enough to cool it down.At hi speed its cold .So that why i decide to add the fan .A cpu cooler will do just fine. I did drill a hole were the transistor has that hole to be placed on a heatsink and added a screw so that the heat transfer is better in that area.
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On Wednesday, February 5, 2014 11:47:31 PM UTC-5, snipped-for-privacy@gmail.com wrote:

Saw your pics and sounds like you have a good design there that should work. The idea of just making the fan go on when it gets hot was a good one, so you could then just tie it to the 12V that is already there. If you tied the fan to the 12V for the blower, you might want to put an inline fuse in. That blower circuit is 30A? I've never seen a CPU fan like that cause a problem, but just a thought. The fan may have some kind of protection of it's own though, so manybe it's not an issue.
Good idea to screw the transistor down. I noticed that in the photo, it wasn't clear exactly how they had it secured down. And it's also near one edge of the heatsink, instead of being in the middle. Before you modified it, was there thermal paste between the back of the board and the heatsink?
This is just a really bad design. If they had correctly bonded the heatsink directly to that transistor where all the heat is being generated, I bet it would work fine.
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The original fsu board was tied down only on the edges by that black plasti c which was clipped by some small fins of the aluminium radiator .It did ha ve a thermal paste i also used a paste . The +12 i took from a main +12 and used a 2.5 amps fuse . The transistor i think disipates about 50w to 80w of heat at low speeds tha t makes the heatsink to small to dissipate that heat and the airflow of the blower at that speed is too little.Its true the transistor is made for 250 degrees but lets be true they dont make like they used.
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On Thursday, February 6, 2014 11:04:17 AM UTC-5, snipped-for-privacy@gmail.com wrote:

have a thermal paste i also used a paste .

he blower at that speed is too little.Its true the transistor is made for 2 50 degrees but lets be true they dont make like they used.
I think it's probably generating more heat than that. The transistor is rated at 300W. In any case, I don't think it's the power transistor's power handling capability that is the problem. It's that the heat causes other parts to fail. I'm sure you've seen pics of ones where the solder joints came undone. IDK what causes the one here to basically shutdown once it gets too hot, but suspect it's something to do with the Elmos chip, either by intentional design or that it just shuts dowm by malfunctioning from the heat. Whatever the max operating temp is for that Elmos ASIC, I'll bet it's being exceeded when these things fail.
The overall design is probably just enough if everthing works like it should. If the blower winds up pulling some more current than normal, for whatever reason, then it pushes it over the edge. I'm thinking as Ralph suggested, it's probably the blower that's the root cause here. And to fix that requires pulling the dashboard in the X5. Another great design. Not something I wanted to do 30 years ago, but today with all the air bags and God knows what else crammed in there, it's got to be a lot worse. Wish I could get some oil to those motor bearings and see what happens.
Going back to what Ralph said about bearings, I'm not convinced that the blower is putting out full RPM even when it's connected to 12V directly. There ia a lot of air coming out, but I think it may have been even more previously, hard to tell for sure. But if it is putting out less at full, then that could be a sign of the bad motor bearing thing. Another sign I think is that it seems even when you first start it up, with a cold FSU, it seems to me that the airflow at say mid speed is more like airflow at 1/3 speed, etc.
Good luck with your project at let us know how it works after you've had it running for awhile.
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The heatsink of the unit needs to be where air can circulate past it. The fan is a good idea. I am not sure how the transistor is fastened to the heat sink. It needs to make good mechanical contact with it.
When you use the thermal paste just use the smallest ammount you can. The paste is not really that conductive of heat. It is just beter than air. You want the transistor to make good mechanical contact with the metal of the heat sink. There are very small inperfections in the metal that has air voids in it. The object is to just fill the air voids. Too much paste is worse than none at all.
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I know that it needs a space with more air thats why im thinking of rellocating the fsu in the engine compartiment,but im waiting to get warmer outside. The transistor is now fastened very good not like the original were it made contact only on the margins of the copper plate
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On Thursday, February 6, 2014 2:30:05 PM UTC-5, Ralph Mowery wrote:

The power transistor is not fastend to the heat sink at all. If you look at the pics, it's on the top side of the PC board. The bottom of the PC board has copper over all of it. Then the bottom of the PC board contacts the heatsink. So, you have the PC board material in between, which is not a good heat conductor. And the transistor is closer to one side of the heat sink and no bolt through the usual bolt location provided.
And to top it off, these things have been failing for 10+ years and not one of the many suppliers has seen fit to make some of the simple changes that would fix it.
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Its directly on the copper plate.The pcb has a rectangular hole .its tinned
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Unlike the older models when the transistor heated up it transmitted the heat to its pins ,now the heat transmits to the whole pcb thanks to genius who decided to glue the pcb to the copper
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On Thursday, February 6, 2014 4:29:23 PM UTC-5, snipped-for-privacy@gmail.com wrote:

I just discovered something important that I haven't seen mentioned here or in any other threads on the BMW blower motor resistor (aka final stage unit) problems. This module is mounted in a location where the heat sink fins are directly in the blower motor airstream. It relies on the blower air to keep it cool. From where it's located and how you remove/install it, it's not obvious that the heat sink fins wind up in the blower air, because you can't see where they actually are.
When I replaced it, to make sure it worked before putting it together, I just left it hanging for testing. It overheats as soon as you slow the blower to about 80% of full speed and stops working. Once I stumbled on the fact that if it's mounted it gets air flow to cool it and I mounted it, it's apparently working. Just thought I'd pass that along, as it seems like something that could trip up a lot of people.
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On Tuesday, February 4, 2014 11:11:36 AM UTC-5, snipped-for-privacy@gmail.com wrote:

Out of curiousity, how did you open it up? The ones I've seen are potted in 1/2" deep epoxy, with no obvious way to get that out without destroying what's inside....
Also, Ive been helping a friend with the FSU problem. His blower went from working fine to not working at all. Being concerned about all the apparently crap ones out there, bought one from a BMW dealer that is marked Sitronic. It appeared to work, but..... With the blower on high it will run all day, no problem. You can back the blower off to about 80% and as you do so, the heat sink starts to get very hot, but the blower still runs at 80%. If you back the speed down anymore, it gets even hotter and then starts pulsating the blower, finally stops all together. I tried dipping the end of the heatsink rods into a container of water when it does that and within a couple seconds the blower starts working again fine. So, it's definitely heat related.
Any ideas on what's wrong? First impression would be that it's the new FSU because the blower works on full or near full speed. But I guess the other possibility is that the motor has a problem? Seems odd though that a single winding type DC motor could run fine at full speed, but then cause the FSU to overheat at lower speeds.
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Could be the motor. While not exectally correct, the motor requires a cretain ammount of power. When running at full speed the transistor is acting more like a swithc. At slower speeds it is acting more like a resistor. Any power that is not used to get the motor to speed is converted to heat in the FSU. The slower the motor is ran, the hotter the FSU will get. If the bearings are going bad or there is some other reason the motor is drawing more than the rated current the FSU will heat up more than normal, especially at slower speeds.
Have you checked the current the motor is drawing and made sure it is getting the voltage it is suspose to at full speed ?
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