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  1. #1
    Jeff
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    Blower motor fan will not turn off ? '88 735

    Fan stays on medium or high only. Tried a different controll panel. No change. Soldered sword. No change. Ideas anyone. I also changed my log-in name and it lets me in now.I guess they don't like business names posting here. Thanks for the help anyone.


  2. #2
    Max Fretter (NZ 87 735i)
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    Re: Blower motor fan will not turn off ? '88 735

    Hiya Jeff:<p>You might want to check further down for a post from either jaya or mannyk - one of them (I think!) had a shorted MOSFET in the sword, so the fan always ran...?<p>Max<p><br><i><br>Fan stays on medium or high only. Tried a different controll panel. No change. Soldered sword. No change. Ideas anyone. I also changed my log-in name and it lets me in now.I guess they don't like business names posting here. Thanks for the help anyone.<p></i><br>

  3. #3
    Jaya Perera
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    Sword Repair

    <i><br>Sword Repair:<br>The reason for repair Blower Motor (BM) turning continuously at a fast speed when the starter key is in position 2 and also when the engine is running. This will cause noise and of course the BM will run down and waste. Another possibility is that BM does not work when the central dial of HVAC is turned on, but it will work full speed at the extreme end when the micro switch is engaged.<p>Remove the larger screw and four smaller screws from sword to expose the circuit board. The four MOSFETs are on the long circuit board. <br>1. Heat with the stem of a soldering iron the epoxy blobs on the location of the FETs and remove by keeping a chisel and tapping with a hammer. This will expose the FET terminals, G=Gate, D=Drain and S=Source). <br>2. Check with a multimeter for continuity between terminals. Locate defective FETs if there is continuity (000 ohms) between G, D and S. If all the FETS are ok there is a resistance between G, D and S. The values are as follows:<br>G&S=58.2 ohms, G&D=106.6 ohms, D&S=48.5 ohms in circuit without connecting to power.<br>The Brand name for the FET is BUZ71A.The 8 pin IC on the circuit board is LM2903N. It is a Low Power Offset Voltage Duel Comparator (Voltage Comparator).<br>There is a relay on the board. It is normally open. It catches on at the extreme position of the centre dial on HVAC. The BM will turn at maximum speed at this setting. The intermediated speeds are governed by the FETs. If the FETs are defective the BM will turn at a fast speed without being able to engage intermediate speeds or BM will not turn at all. It will operate only at the extreme end of the dial with the micro switch engaged and the FETs bypassed.<p>3. Remove the defective MOSFETs and re-solder good ones. Exercise care when handling FETS. Take anti-static precautions. Do a test run by putting back the connector and power on at setting 2 of the starter switch. If all goes well<br>4. Cover with epoxy glue the soldered terminals, the idea being to prevent possible short-circuiting and? moisture settling on the terminals from the blast of the BM. The BM cools the aluminium heat sink, which is the long plate and that is why the Sword is located at this seemingly inaccessible place. It gets heated in no time if operated out side its normal position. So, test only for brief periods. <br>5. Push back Sword to its proper location (I had to cut a portion from the top of the recess to insert the Sword). Fix it back with the two screws.<br>6. Put back the other things removed in the reverse order.<p> It is aid that the FET BUZ31L has better handling capacity as it can with stand a higher voltage and heat. <br> Do the above repair at your own risk.<br>Jaya Perera 735i/A 88 78k New Zealand<p></i><br>

  4. #4
    Patrick C 88 750iL
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    What kind of epoxy glue do you recommend?


  5. #5
    Jaya Perera 735i 88 NZ
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    Araldite quick setting or even Polyurithane

    <i><br> <p></i><br>

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