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Thread: E32 Sword Repair Success

  1. #1
    Join Date
    Dec 2010
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    Billerica, MA
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    e39 530i, e28 535i, e32

    E32 Sword Repair Success

    I just want to report success repairing the sword on my 1990 735i.

    Just as with my experience disabling the deadlocking system there was a lot of information out there but much of it was on bimmerboard which being a nonlinear thread system was difficult to follow for me.

    Shogun had suggested on one thread link that an upgraded MOSFET could be used in place of the factory part. The upgrade part number is BUZ11 and is currently available from Digikey for $1.01/ea. Four MOSFETs are required, making it a cheap repair.

    I was a little concerned that no one on that thread had reported back with success in using the upgraded parts. It's always nice when people report back. Well, the stock BUZ74 parts are no longer available and the upgrades were cheap, so I installed BUZ11 in mine. It works perfectly.

    I did not take any photos during the process since the procedure is well documented here. Keep in mind that for sword removal if your car's build date is earlier than 6/90 you do not have a microfilter. This makes removal much easier and the screws are philips for early cars. It took me a little while to figure out why the Torx driver wasn't working. Mine has a build date of 10/89 and does not have a filter. The sword can be removed in 5 minutes with the glove box in place. It's not as easy for those with later cars.

    The change I made from the guide on that site was to use a mounting and standoff system from advice on another thread. I used:
    4x 0.25"L 8-32 round brass threaded standoffs, Digikey #1692AK-ND
    8x 0.25"L 8-32 stainless steel panhead screws
    The advice also suggested the use of washers. I omitted them because on two of the board side holes the washers would cause an electrical short between traces. The fit was perfect without washers.

    The PCB is single sided with non-plated through holes so it doesn't matter how wide you drill out the holes for the standoffs. There is exposed copper on the outside that must make electrical contact with the bottom of the screw. I found that the mounting hole on the BUZ11 was slightly too small for a #8 screw so I had to drill it out slightly to fit. I simply clamped the MOSFET in an old fashioned wooden clamp from Harbor Freight ("Handscrew Clamp" P/N 6987 - photo attached) and drilled away. Only a very small amount of material had to be removed.

    I also did not find that a drill press was necessary for the job. I simply held the sword in my hand while knocking off the rivet heads. The staking material on my part was a bit different than in those pictures. It looked like amber and was transparent. I used a pair of transverse end cutters (photo attached) to grab onto the glob and it pulled free in seconds with no damage to the board.

    To seal it up I used a quality RTV injected through the factory fill hole on the metal plate. This fully encapsulated the leads to prevent moisture intrusion. A skim coat over all exposed copper finished the job.
    Attached Images Attached Images

  2. #2
    Join Date
    Sep 2012
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    5
    Hi.
    I am another E32 owner having repaired the heater sword on my 1987 E32 735ia. Worked fine for a year and now the mosfets are burned again. Not visible on the sword but fan runs on full blow even when switched off. What are the preferred MOSFET types? Mine says buz71A, V530.
    And. Having studied other heater resistors it would appear that the newer E39hedgehog resistors have same contact/wiring? This resistor does obviously not fit in the slot where the original sword is but does it provide correct resistance for adjusting fan speed?

  3. #3
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    e39 530i, e28 535i, e32
    My BUZ11 based repaired sword is working well still. The BUZ11 MOSFETs are rated to handle much more load than the BUZ71A currently in your unit which may be the cause for their early demise.

    The BUZ71 is rated at only 13A (vs 50A for BUZ11) and also has a much higher drain to source resistance when on (0.1 ohm vs 0.03 ohm). The higher resistance will cause more significantly more internal heating in the MOSFET and can hasten its quick demise since it is not well heatsinked. To pick reasonable numbers, at 14v and 10A each BUZ11 MOSFET would have to dump 3W of heat due to I^2*R heating. The BUZ71 generates 10W of heat under the same conditions.

  4. #4
    Join Date
    Oct 2014
    Location
    Rock Hill, SC
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    My Cars
    93 & 94 740iL
    Thanks for bumping, will have to do this soon.
    Current Collection:
    1993 740iL - daily driver; 1994 740iL - project #1; 1994 740i - project #2
    1991 Buick Roadmaster Estate Wagon - beast of burden & tool/parts hauler

  5. #5
    Join Date
    Sep 2012
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    Thanks a lot for your very quick response. Your suggestion makes sense as I could feel significant heating of the sword when testing the fan after repair before sliding the sword back into its slot. I'll go for the Buz11 MOSFETS this time.

  6. #6
    Join Date
    Sep 2012
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    Hmm.
    Have replaced MOSFETS with Bus11. Fan worked fine for a Very short time. (Like 1 min). Then back to "fan running on full speed all the time, switching fan off doesn't help."
    there's a slight increase in fan speed when switching to full speed but fan runs on almost full speed regardless of what other position switch is turned to, even when switched off. Getting somewhat frustrated of this sword problem. Can sword shortcirquit new Mosfets after just 1 min use?

  7. #7
    Join Date
    Dec 2010
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    Billerica, MA
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    e39 530i, e28 535i, e32
    It's possible that you have another issue that is causing the rapid failure such as that the blower motor is drawing too much current and burning at least one MOSFET out.

    Or when you installed the MOSFETS did you ensure that the backs of them lay perfectly flat against the sword and used thermal paste to ensure a good connection? If not each device will get far hotter than it's supposed to and fail to a short circuit condition which is what you're experiencing. The purpose of the sword metal is to conduct heat away from each MOSFET. It is intentionally placed into the air stream of the blower motor to provide some convective cooling. If there isn't good conductivity to the metal plate then the whole system isn't working as designed.

  8. #8
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    Sep 2012
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    The blower motor is making some squeaking notices so that could be the issue. Also, my late 87 E32 does not have a filter cleaning the air inlet. Could there be some dirt somewhere that's preventing cold air cooling the sword?

  9. #9
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    Yes, if your blower is making noises than there's an extremely high chance that you've found the source of your problem. The bushings are worn and are seizing, causing it to draw far more current than it's supposed to.

    It's possible but unlikely that there is enough debris in the system that the blower is starved for air. The air has to all go through the heater core which is where you'll see it accumulate. When you pull the sword out there may be some junk down there you can clean out.

  10. #10
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    Sep 2012
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    Thanks again. I started the fan with the sword out of its slot. There is plenty cold air flowing out of the sword-slot. Guess blower replacement or at least bushings replacement if possible is next project.

  11. #11
    Join Date
    Dec 2010
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    Billerica, MA
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    e39 530i, e28 535i, e32
    Quote Originally Posted by E32freak View Post
    Thanks again. I started the fan with the sword out of its slot. There is plenty cold air flowing out of the sword-slot. Guess blower replacement or at least bushings replacement if possible is next project.
    Unfortunately the bushings can't be replaced separately. The carbon brushes can, but if it's squeaking those aren't the source of your problem. The bushings usually go long before the brushes wear out.

    The only thing can can possibly be reused are the "squirrel cage" fan blades, but I honestly wouldn't bother. You can save around $40 by buying just the motor and swapping your blades on, but it's a lot of work and getting them perfectly aligned while maintaining the original balance can be tricky. A new motor with cages is only around $100. I've done the cage swap on an e28 but only because the motor alone was $25 (different part) so it was worth the attempt.

  12. #12
    Join Date
    Oct 2014
    Location
    Rock Hill, SC
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    My Cars
    93 & 94 740iL
    I did the blower motor recently - royal pain in the neck on the V8 engine, might be a bit easier on an inline 6 - If you have the V8... Make sure you disconnect the coolant hoses from the pump on the driver side - I snapped the connection to the aux water pump trying to reach the last bolt on the splash guard... Trying to be lazy cost me an extra $100 for a new pump...

    Aside from that it is easy - you do need a pick tool to remove the strap holding the blower motor in place, but it pulls right out from there.
    Current Collection:
    1993 740iL - daily driver; 1994 740iL - project #1; 1994 740i - project #2
    1991 Buick Roadmaster Estate Wagon - beast of burden & tool/parts hauler

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