Root cause insight into the common BMW blower motor resistor failures

Yawn. I worked with 16 layer boards at a factory troubleshooting defective, new boards that cost over $8,000 to stuff. The internal routing of a simple low frequency board is irrelevant for drawing a schematic. You can X-ray a board or mill it one layer at a time if you want to duplicate the routing. All you need to do for bais reverse engineering is to trace each with an ohm meter by probing every pad and termination to identify the signal path and draw a schematic from your notes. I've done this with four layer boards for 30+ years. "Specs on the chips" makes things easier but knowing who made it, and how it's connected will tell you if it is a custom part or just house numbered.

There are industrial solvents to remove any potting compound, but they aren't cheap or easy to buy. Failure analysis is a specialty in electronics manufacturing. It is expensive but gives you the answers you need so you can design out the problems.

A jig to hold the module in a CNC machine would allow you to cut accurate holes down to the right points to see if the IC was bad, without unpotting a failed unit. Then a test fixture with 'Pogo Pins' would allow you to see what was bad. If there were enough bad units, it would be worth designing and programming a computerized fixture.

Well, that's about what you need to know. Game over.

--scott

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Do you see the two groups of three empty holes on the right side? They look like cracked solder joints on TO-220 packages which could be semiconductors or power resistors.

You can see the leads in those holes, in this photo. They look cracked. That is a common failure due to stress caused by uneven heating of potted components.

I think the IC is a dual, high side motor driver IC and each transistor has a large emitter resistor with different values. That is the metal strips on the pins by the sets of three holes. That would give three speeds & off. Like a three way lamp with 50 + 100 Watt elements. Turn both on and it draws 150 Watts. This would explain the failure of the solder joints on the transistors since they are used in linear mode.

00 would be off 01 would be low speed 10 would be medium speed 11 would be high speed

'Reduction Of Hazardous Substances'

Crappy lead free solder that cracks with a lot less stress that leaded solder. Brought to electronics by European bureaucrats.

Sheesh indeed. That heatsink is small. Sure, it will work for a while, but it isn't designed for long life. Depending on forced air cooling when the air is either heated or cooled is just plain stupid.

Restriction of Hazardous Substances; that is, no lead (among other things.)

nate

Here's what the heatsink looks like when I cut into it today:

Because they draw no current, they only set the base bias of the power transistors.

It's a variable voltage.

Still unknown.

Reported as DC.

--scott

That's a good theory except why would the resistors not be attached to the heat sink?

There are still open questions that until answered we are just speculating with educated guessing.

1) What exactly does the control signal look like on a scope?

2) The two devices, mosfets or transistors? Any part numbers?

3) Waveforms on the motor power line for the different speeds.

Ha, keep going!

Someone had scoped the whole board, and it was DC voltage everywhere (according to that reference). It's pretty clear there is no PWM.

The 16-pin surface mount chip seems to be a automotive temperature compensated voltage regulator with a huge voltage range, according to a lookup of the part number on it.

Here is the Elmos 10901D chip of my FSU as I cut it open today.

I wish there was a way to get rid of that heavy fibrous plastic goop! (What is that black fibrous tough stuff anyway?)

Like I said before, don't drive one then. It's kind of like going on a date with that unbelievably attractive female type who is also smart, witty, fun to be around, actually seems to like you, and oh by the way is completely mentally unhinged.

Suddenly you find yourself putting up with all sorts of stuff that you wouldn't, otherwise... (now that said, touch wood, current ride has exhibited none of the known issues... which reminds me, I need to call and schedule the battery cable recall @ the stealership)

nate

Are you sure? It seems it was measured with a DMM, not a scope. A PWM signal would just look like DC to a DMM. Since it was reported as "2.0 to 7.8" volts, you would not get those significent figures with a scope.

Any evidence it was checked with a scope? Again, a PWM signal will just look like a DC level to a DMM.

Let's see what the autopsy shows next :).

LOL. There's more truth to that then you may think.

On Tue, 26 Mar 2013 14:09:11 -0400, Michael A. Terrell wrote:

It might be, but this Russian site intimates it's a temperature compensated voltage controller.

Translation below: REGULATOR ELMOS 10901D Found: 100 Showing: 1 - 10 Car Voltage Regulator Category: Car Source: Radioland country Electronics Temperature controller cabin air KAMAZ Category: Car Source: Plans radiokonstruktsy Simple Temperature compensated voltage regulator. Controller together with thyristor-transistor electronic ignition unit with a long spark, ensuring the rapid start-ups at various operating conditions, allowed to increase battery life of up to nine years. Category: Car Source: For the life of a soldering iron ... Regulator for automotive windshield Category: Car Source: MASTER KIT The controller measures the wiper-this control is designed to use regular mode switch blades and is contactless. Category: Car Source: For the life of a soldering iron ... Temperature compensated voltage regulator device in some ways superior designs. The controller can be used as a universal device is suitable not only for mounting on any car, but everywhere, where the generator rotor speed is variable (eg, wind power). Choose the appropriate control elements, it can be easily adapted to work with any voltage (up to 400V) and excitation current (tens of amperes). Category: Car Source: For the life of a soldering iron ... Voltage regulator 2012.3702, 22.3702, 221.3702 Category: Car Source: For the life of a soldering iron ... Voltage regulator 201.3702 Category: Car Source: For the life of a soldering iron ... Voltage Regulator 13.3702 Category: Car Source: For the life of a soldering iron ... Voltage regulator RR132A, 1112.3702 Category: Car Source: For the life of a soldering iron ...

Are those the two transistors sitting on top?

It's an epoxy resin filled with silica. It will come off with DMSO at three atmospheres or so. Sometimes a soak in DMSO for a couple weeks will make it peel off. This will also soften the PC board though.

--scott

I feel your pain. I have a 1980 MB 300SD. For the AC, MB used a part from Chrysler that they used on the Imperial. It's the size of a grapefruit and it controls the entier HVAC system. In that one widget you have a valve that's in the path of the engine coolant system that goes to the heater, about a dozen vacuum hoses that control the various flaps. You also have the electical outputs for the blower, AC clutch, etc. And all that is driven by a small electric motor that is inside the thing. The motor is part of a feedback servo system that moves in relation to the desired temp setting vs the actual. It goes from max cooling, to max heating. Oh, and a critical part of that sytem is the potentiometer inside that widget, the resistance value of which changes based on the current position of the motor that works the whole thing, moving it from max cooling to max heat.

So, you have hot engine coolant, vacuum, electrical, a potentiometer, all inside one widget. How smart is that? The typical failure mechanism is that the plastic housing cracks and it leaks coolant. They were up to $600 in the 90s for a new one. Some guy down in TN started a business rebuilding them. His uses an aluminum housing for the part where the valve and hot coolant are. I bought one 10 years ago for $500. It's been fine until recently. Now it's starting to have problems. Haven't had a chance to find out what's wrong.