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Thread: Power seat parasitic draw.

  1. #1
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    Power seat parasitic draw.

    Hi everyone. First time posting. I've had my 89 635csi auto for about a year now. Keep running into a dead battery issue since I had the car and replaced the battery. Was able to trace the problem to fuse 26 for the power seats with memory. When I unplug the power to the memory module, draw is normal at 0.41 mA. When plugged in it jumps up to 1.33mA. Both power seats work in all directions, and memory functions on all 3 buttons can be set and work fine. No malfunction warning lights either. Unplugged and checked all wiring to the switches, motors, and module and all is good. I ran through the diagnostics in the electrical manual also and everything checks out and all values are in the required ranges. Not sure what could be drawing that much power. Key position makes no difference and the above ranges are with ignition off and key out after about a couple of hours. Nothing else on that circuit either. At a loss to what else to check. Anyone experience such gremlins?

    Thanks!

  2. #2
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    Normal draw would be around 50mA. 1.33mA is too much draw. You can checkout the module to find
    the problem or leave the memory unplugged.

    Just my $.02

  3. #3
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    I just checked out the ETM, there's a ton of diodes in that module! I'd be taking it apart and testing them methodically one by one. Shouldn't take more than an hour to locate one. The schematic looks very discrete, except for the memory. I'd presume that memory is a likely culprit, since it requires battery power to keep the seat positions in memory.

    I couldn't find a lot of info out on the web, but I did bump into a slightly related post from Shogun, about the later E34 modules causing troubles.

    I have suspected for quite some time that Bosch is the spiritual successor to Lucas...why can't our Bavarians match up to the Japanese in general electronics quality?

  4. #4
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    Quote Originally Posted by DesktopDave View Post
    I just checked out the ETM, there's a ton of diodes in that module! I'd be taking it apart and testing them methodically one by one. Shouldn't take more than an hour to locate one. The schematic looks very discrete, except for the memory. I'd presume that memory is a likely culprit, since it requires battery power to keep the seat positions in memory.

    I couldn't find a lot of info out on the web, but I did bump into a slightly related post from Shogun, about the later E34 modules causing troubles.

    I have suspected for quite some time that Bosch is the spiritual successor to Lucas...why can't our Bavarians match up to the Japanese in general electronics quality?
    Thank you desktopdave. Very helpful. As I'm still learning the nuances of this car, I did notice that the STOP button for the memory seats seems to have no effect when depressed. However when I disconnected the switch, it made no difference on the draw either. When I got the car the seat would not move front or back. Turned out there was a piece of glass in the track. PO was stumped as to why it wouldn't move, and the headrest was off its hinge and a couple of cables needed repaired as well. Finally resolved all those issues and have a fully functioning driver's seat. Just surprised this is the end result. I'll do some more testing and poking. If the module needs to be disassembled I'll make sure to take some pics. I can put a kill switch under the seat to disconnect power, when not needed, but I would just prefer to fix whatever is wrong. Thanks again.

  5. #5
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    Update: Pulled the seat module and took it apart. Found this mess. Any suggestions on how to clean it up? It's only on the bottom. Probably someone's latte. 15304061316104808823536158132341.jpg15304061316104808823536158132341.jpg15304061736408927485676421349732.jpg15304062102783705883193326000205.jpg15304062446231306685075693517201.jpg15304062990301861214578821414325.jpg

  6. #6
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    That looks ugly, but it might be salvageable. It'd probably be easier to find a replacement unit though. I'd presume that the E23 and E28 have similar units under their power seats. You might get really lucky with an early-production E34 or E32 that has a compatible version.

    Minor water incursion causes that sort of damage, but there must have been a pool of water. I've seen a lot of BMWs with soaked sound insulation, under the carpet. Perhaps that wicked up into the controller under the seat? I'd use a toothbrush and hot soapy distilled water, dry it off nicely with a hair dryer or air gun, then see how much damage you have.

    Once it's clean you can see how much was shorted out and damaged by arcing. The only thing I'd really worry about is any ICs or transistors (like that one with the big heat sink). The bigger stuff usually isn't effected by the water.

  7. #7
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    I have one if your interested.

    Shoot me a PM,

  8. #8
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    Just a follow up, after cleaning the board the memory part was completely dead. Installed a new module and everything works great. Thank you 1986series6 for helping out.

  9. #9
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    Thanks for posting the result. Glad that it solved the issue.

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