BenSTi
10-15-2015, 07:58 PM
Hey Guys, First post here.
I recently purchased a 2009 535i used with ~49k on it. My friend I purchased it from is a mechanic and told me that the water pump would fail between 60-70k miles, "sure" I said. Well, at 64k it did, and it sucked. I had to limp the car home making it about two miles at a time before pulling over and letting it cool off.
As we all know, when the water pump fails, it's immediate and without warning. I decided to tear down my pump to figure out what had failed on it and if there was any way to detect the failure or even prevent it. Below is what I found.
Here's the pump taken apart, it comes apart in three pieces, the injection molded top plastic, cast aluminum electronics carrier, and the housing with stator (motor coils). The electronics plate is connected to the stator housing with the three motor phase busbars which are ultrasonic welded together. You'll need to pry hard enough to break these to get them apart. Otherwise it's just sealed and held on with an O-Ring and a few screws. The plastic top part has the pins moulded through it. This was likely a post-process done with ultrasonics after assembly.
https://farm1.staticflickr.com/593/22016206038_db4a541784_k.jpg
Once it's all seperated we can get a good idea of the beautiful electronics inside. It's a really impressive three dimensional design, stuffed into a tight package!
https://farm6.staticflickr.com/5800/22191262262_039781342d_k.jpg
https://farm6.staticflickr.com/5692/22203960675_1216e61e34_k.jpg
https://farm1.staticflickr.com/730/21581200174_1b362add09_k.jpg
On the "top layer" there is a very large electrolytic capacitor and an inductor. The capacitor is a TDK part, EPCOS B41695-S6228, datasheet here (http://en.tdk.eu/inf/20/30/db/aec_2013/B41695_B41795.pdf). It's interesting to note the rated lifetimes according to TDK, at 125C it's rated to >3000hrs, and at 85C it's rated to >8000hrs. Since the pump is directly in contact with coolant (~100C) we can assume the cap gets to this temp. That would put the rated lifetime at ~6000hrs. At an average speed of 25mph, that's 150k miles.
https://farm1.staticflickr.com/661/22177848596_c4497c09c8_k.jpg
This assembly seems to be a filter to reduce back EMI onto the vehicle 12V buss. This is typically tuned to the switching frequency of the inverter. Doing some quick math, this supports a switching frequency of ~1500hz (dcR of the system seems to be ~50mOhm)
If we cut the three busbars and remove this assembly, we discover a very cool inverter below.
https://farm6.staticflickr.com/5617/21582942293_0946b48ecb_k.jpg
On the left side, there is +12V and Ground Input, plus gate drive and sense lines. The capacitor seen is between +12V and Ground. On the right side, there is the three phase outputs to the stator. Going forward, I call these PH1, PH2, PH3 from bottom to top in the photo.
The six large transistors are IGBTs, not sure of the manufacturer. Going clockwise from the left top: PH1+, PH2+, PH3+, PH3-, PH2-, PH1-. There seems to be very small capacitors connecting each phase together and to +12V/Ground, interesting...
You'll notice that both PH3 IGBTs have failed catastrophically. When measured +12V to PH3out, I get ~15mOhm, same with Ground to PH3out. This shows that both have lost isolation. Without this phase, the motor simply cannot work. This likely was a cascade failure that happened in one of two ways:
Possibility One: One IGBT shorted, when the control board commanded the other IGBT in that phase to turn on, it created a very low resistance short between +12V and Ground, frying both parts.
Possibility Two: The control board commanded both IGBTs in the phase to close at the same time, same result as above.
It seems more likely to have one of the IGBTs short, this is a common failure mode, and because failure of any one part causes system failure, it is much more likely to happen.
So there you have it, the pump failure in my car was caused by shorted IGBTs on one phase resulting in sudden failure.
A couple other observations of note:
The rotor is over-molded with plastic, this is cracking and almost ready to come apart!
https://farm1.staticflickr.com/594/22203981165_5f1d62aa90_k.jpg
The rotor and inside plastics on the stator are actually flooded with coolant. This avoids the need for any rotating seals. Clever.
https://farm1.staticflickr.com/622/22203983335_f988550538_k.jpg
The control board is constructed on aluminum nitride with wire-bonded components and an exposed die, very cool. It's encapsulated in a soft silicone material.
https://farm1.staticflickr.com/744/22017052919_8f308d603d_k.jpg
There is also a board which I will call the "input board" This is where all of the signals come into before they are broken out to the inverter board and control board. It also transmits signals between the inverter and control board. It includes even more capacitance between +12V and Ground, +12V and Chassis, and Ground and Chassis. It's worth noting the entire circuit seems isolated from the case, the only ground path is through the connector, not the enclosure like many other components. This could be because it has low-side switching or current sensing on the ECU end.
https://farm1.staticflickr.com/767/22177820376_612f50274a_k.jpg
TL;DR: IGBTs shorted, caused catastrophic failure of both IGBTs on one phase. Stuff doesn't work anymore. No way to detect failure prior to happening, no way to prevent it from happening on existing pumps, no way to repair after it happens.
I recently purchased a 2009 535i used with ~49k on it. My friend I purchased it from is a mechanic and told me that the water pump would fail between 60-70k miles, "sure" I said. Well, at 64k it did, and it sucked. I had to limp the car home making it about two miles at a time before pulling over and letting it cool off.
As we all know, when the water pump fails, it's immediate and without warning. I decided to tear down my pump to figure out what had failed on it and if there was any way to detect the failure or even prevent it. Below is what I found.
Here's the pump taken apart, it comes apart in three pieces, the injection molded top plastic, cast aluminum electronics carrier, and the housing with stator (motor coils). The electronics plate is connected to the stator housing with the three motor phase busbars which are ultrasonic welded together. You'll need to pry hard enough to break these to get them apart. Otherwise it's just sealed and held on with an O-Ring and a few screws. The plastic top part has the pins moulded through it. This was likely a post-process done with ultrasonics after assembly.
https://farm1.staticflickr.com/593/22016206038_db4a541784_k.jpg
Once it's all seperated we can get a good idea of the beautiful electronics inside. It's a really impressive three dimensional design, stuffed into a tight package!
https://farm6.staticflickr.com/5800/22191262262_039781342d_k.jpg
https://farm6.staticflickr.com/5692/22203960675_1216e61e34_k.jpg
https://farm1.staticflickr.com/730/21581200174_1b362add09_k.jpg
On the "top layer" there is a very large electrolytic capacitor and an inductor. The capacitor is a TDK part, EPCOS B41695-S6228, datasheet here (http://en.tdk.eu/inf/20/30/db/aec_2013/B41695_B41795.pdf). It's interesting to note the rated lifetimes according to TDK, at 125C it's rated to >3000hrs, and at 85C it's rated to >8000hrs. Since the pump is directly in contact with coolant (~100C) we can assume the cap gets to this temp. That would put the rated lifetime at ~6000hrs. At an average speed of 25mph, that's 150k miles.
https://farm1.staticflickr.com/661/22177848596_c4497c09c8_k.jpg
This assembly seems to be a filter to reduce back EMI onto the vehicle 12V buss. This is typically tuned to the switching frequency of the inverter. Doing some quick math, this supports a switching frequency of ~1500hz (dcR of the system seems to be ~50mOhm)
If we cut the three busbars and remove this assembly, we discover a very cool inverter below.
https://farm6.staticflickr.com/5617/21582942293_0946b48ecb_k.jpg
On the left side, there is +12V and Ground Input, plus gate drive and sense lines. The capacitor seen is between +12V and Ground. On the right side, there is the three phase outputs to the stator. Going forward, I call these PH1, PH2, PH3 from bottom to top in the photo.
The six large transistors are IGBTs, not sure of the manufacturer. Going clockwise from the left top: PH1+, PH2+, PH3+, PH3-, PH2-, PH1-. There seems to be very small capacitors connecting each phase together and to +12V/Ground, interesting...
You'll notice that both PH3 IGBTs have failed catastrophically. When measured +12V to PH3out, I get ~15mOhm, same with Ground to PH3out. This shows that both have lost isolation. Without this phase, the motor simply cannot work. This likely was a cascade failure that happened in one of two ways:
Possibility One: One IGBT shorted, when the control board commanded the other IGBT in that phase to turn on, it created a very low resistance short between +12V and Ground, frying both parts.
Possibility Two: The control board commanded both IGBTs in the phase to close at the same time, same result as above.
It seems more likely to have one of the IGBTs short, this is a common failure mode, and because failure of any one part causes system failure, it is much more likely to happen.
So there you have it, the pump failure in my car was caused by shorted IGBTs on one phase resulting in sudden failure.
A couple other observations of note:
The rotor is over-molded with plastic, this is cracking and almost ready to come apart!
https://farm1.staticflickr.com/594/22203981165_5f1d62aa90_k.jpg
The rotor and inside plastics on the stator are actually flooded with coolant. This avoids the need for any rotating seals. Clever.
https://farm1.staticflickr.com/622/22203983335_f988550538_k.jpg
The control board is constructed on aluminum nitride with wire-bonded components and an exposed die, very cool. It's encapsulated in a soft silicone material.
https://farm1.staticflickr.com/744/22017052919_8f308d603d_k.jpg
There is also a board which I will call the "input board" This is where all of the signals come into before they are broken out to the inverter board and control board. It also transmits signals between the inverter and control board. It includes even more capacitance between +12V and Ground, +12V and Chassis, and Ground and Chassis. It's worth noting the entire circuit seems isolated from the case, the only ground path is through the connector, not the enclosure like many other components. This could be because it has low-side switching or current sensing on the ECU end.
https://farm1.staticflickr.com/767/22177820376_612f50274a_k.jpg
TL;DR: IGBTs shorted, caused catastrophic failure of both IGBTs on one phase. Stuff doesn't work anymore. No way to detect failure prior to happening, no way to prevent it from happening on existing pumps, no way to repair after it happens.