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Interior Blower Fan Sword Repair
Our interior (coupe) fan started to only work at full speed (setting 4) and sometimes at 3, 2 or 1. Reading a lot about this over the years I knew it was probably time to do the "sword" repair so I started by removing the glove box and side panels on the right (passenger) side in the car. The glove box is very easy to remove. So is the 4 other panels I needed to remove to gain good access to the sword.
Once the panels were disassembled I could see a few connectors all the way forward towards the firewall and floor. One yellow, one blue and one brownish. The blue and yellow connect to the IHKA control unit, and the brown 5-pin one connects to the sword. I disconnected the brown connector and used a philips driver to remove the two screws that hold the sword into the blower assembly. Then just pull it straight out to the side.

Since I know it will take some time for me to order and get time to do the actual soldering and fixing of the sword I decided to figure out how I could at least get the blower fan to run or not when needed while the repair is taking its sweet time in my shop. This was actually very easy and I ended up soldering in 3 wires and connecting a relay instead of the sword. Here is how the wiring and function of the sword connector works:

 PinWire color
 Functional description
 Brown Grounding Directly and always connected to ground
 2 Black / White  Output to blower fan motor
 When fully grounded fan blows max speed. Allows speed change by adjusting the grounding level
 3 Black / Blue
 Full speed enable
 When speed knob is set to 4, max speed, this wire is connected to +B and enables a by-pass relay in the sword assembly
 4 Yellow / Green
 Ignition power
 When ignition is in position 2 this wire is connected to+B
 5 Forgot color Blower speed signal
 Variable voltage between 0 and +5V supplied from the blower speed knob. About 1.3V at 1, and 2.5V at 3. 5V at 4.
Please note that the above table is not quite finished yet, the pin-numbering and wire-coloring will be double checked and the table will be updated soon.
I bypassed the sword function and allow the blower to either not run, or run at full speed by inserting a relay that controls the connection between grounding and the output to the blower motor. This short table explains my temporary setup while repairing the sword:
Relay pin
Connected to Description
 30 Brown wire
 87 Black / White
 Output to blower motor
 86 Black / Blue Full speed signal, +B when full speed set
 85 Brown wire
To avoid destroying the sword connector, I just soldered into the wires just before the connector with the necessary 4 new wires and insulated the joints afterwords. Then I can just remove the relay later and connect the repaired sword. And if it ever breaks again, I can just put that relay back in.
The sword actually contains one relay to allow for the full speed setting (knob setting 4) that bypasses the electronics in the sword.
In my case one of the BUZ71S semiconductors probably failed and made the other 3 fail when they then became overloaded. These 4 BUZ71S semiconductors work in parallel and do the actual current controlling like an advanced resistor between grounding and the blower motor ground connector. They will now be replaced by new higher current capable semiconductors which probably will long outlive the life of the car.
Opening the sword is easy. 4 small philips screws hold two pieces of black plastic casing at the end of the sword. There is a small strip of weatherstrip insulation between the plastic case and the rest of the sword to avoid damp and moisture from entering the electronics. The whole PCB is also covered in a layer of laquer to protect against weather and moisture. You do not have to open the sword case to fix the BUZ71S. They are nailed and glued to the PCB and metal heat sink.
Before removing the BUZ71S test the NTC as below, and then use a knife and carefully pry under the big lumps of glue over the 3 pins that the BUZ71S are soldered to the PCB with. Just pry off this glue. Its relatively easy and just takes a few minutes of fiddling. Careful not to damage the PCB tracks in the process.
Then remove the small nut that holds the small NTC thermistor to the heat sink. This is an overheating protection that tells the electronics to cut out if the heat sink gets very hot. It should measure 300-330 ohms when at room temperature. And that resistance should drop when heated up. I used a lighter to slightly warm up the NTC to see if it worked. That might be a good test to perform before replacing the BUZ71S.

To remove the BUZ71S and replace them, I suggest using a 5mm metal drill and carefully drill out the nails from both the heat sink and PCB side. You do not need to drill far, just enough to remove the top part that sticks to either the PCB or heat sink. Do not worry about drilling into the PCB or tracks when you drill out the nails. There is plenty of PCB left to carry the current under operation later.
I then used some desoldering wire and desoldered the 3 pins of each BUZ71S. Then just pry the heat sink of the BUZ71S and the PCB. Careful not to bend the PCB too much so its tracks crack.
Then its just a matter of cleaning of some more glue and soldering in the new BUZ71S, assemble the heat sink and NTC thermistor. Use some spacers and bolts and nuts instead of the original nails to secure the BUZ well to the heat sink so that they can transfer as much heat to the heat sink and not burn out under load.
When all assembled I would have used to spray glue or clear paint to weather-proof the BUZ71S again.
Connect to your car and hope it all works well again.
Troubleshooting for finding whats at fault
If the blower fan does not work at full speed, then the relay in the black plastic casing is at fault. Also if the blower fan only works at full speed, I would suggest checking that relay first. All other intermittent speeds are controlled by the BUZ71S semiconductors and they will be at fault if 1,2 or 3 settings dont work.
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Long time no post... again, but I have been busy on several things. Today I finally finished one of my projects, a working BMW diagnostic system / PC.

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