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Intermittent (kind of) false alarm?

Discussion in 'E39 (1997-2003)' started by vtx, Jun 1, 2009.

    vtx guest

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    Alarm goes off in my car under very specific circumstances:

    - Ambient temperature is high (105F+);
    - The car was previously in the garage;
    - After ever a short drive, it's parked (no matter whether in the sun or back in the garage);
    - Within two to five minutes, the alarm invariably goes off.

    It doesn't matter whether the tilt and internal motion sensors are disabled, it goes off anyway.

    I'm trying to pinpoint the sensor that is being attacked by gremlins, I wonder what are the questions I should ask myself?

    Do I understand correctly that the alarm going off doesn't leave any traces in OBD? (did a reading hoping to find a code, no luck).

    What are the usual suspects?

    /me scratching head, investigating further
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    mooseheadm5

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    You need a real computer to read the codes. An Autologic or GT1 will be able to tell you what sensor is tripping the alarm. I have had to replace a hood switch for that reason on an E39, but without talking to the alarm system, replacing things is just a shot in the dark.

    vtx guest

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    Hmm... I went to Checkers and loaned their device - like I said, it came up clear, no codes were stored. Is it possible that some readers just won't see some codes?

    Interesting, so did I, back in about 2004 - but it was still under the warranty, so I didn't know any specifics.

    That's why it really puzzled me that the reading came up clean. But wait a sec, you said "talking to the alarm system", is that not the OBDII connector?

    /me away reading up on code readers
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    mooseheadm5

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    Like I said, you need a real computer. The auto parts store can only read emissions related OBD2 codes. You need a BMW specific computer like the GT1 or Autologic (the Launch X431 may work too.) Since the prices on these things are 20K, 11K, and 4K respectively, not much chance you will find one lying around at an auto parts store. Take it to an independent BMW specialist but make sure they have one of the 3 computers.

    vtx guest

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    Yep, the price is definitely beyond what I can afford.

    Just spoke to a tech in a dealership - he said that the most probable cause for this problem in this car is the hood switch - he says, they get flimsy and bend down with age, especially in hot climates. Gave me the way to verify that it's that switch, and told me about a cheat they use - install X5 switch instead of stock E39. Longer travel, less chance of getting flimsy - that's what I've figured from his explanations.

    I wonder, is there a reader usable from BMWs within $1000? That, I would just buy without hesitation - I'm sure it'll pay for yourself if not money saved then time saved (and time = money), really fast.
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    mooseheadm5

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    If there were such a thing, I would have one. Our shop had to pony up the $11,000 for the Autologic so we could diagnose and program the newer BMWs. Anything that is cheap will only read off emissions related codes and will not access the alphabet soup of control modules- ABS, DSC, IHKA, etc.
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    102929

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    A faulty sensor is one possibility - and it seems like the hood switch is the most common of those.

    BUT - yours sounds more like a low battery condition. A battery with one or more weak cells will have a sudden voltage drop shortly after being driven and that will set off the alarm because it thinks someone is tampering with the car.

    Cheers
    Jim Cash

    vtx guest

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    102929: sorry, forgot to follow up - it was the hood switch all right. Dealership folks say that they now install X5 switches instead of E39, but that one is shorter, and I didn't want to spend too much time on it so I bought the exact replacement - well, two hood switches in six years is not that bad.

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