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  1. #1
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    Battery light comes on when driving then drains it

    I purchased a 2002 X5 3.0. After 3 weeks the battery light came on while driving and eventually drained it. Had the alternator replaced and two weeks later it happened again. THis time BMW replaced the key and the immobilization system computer. Yesterday light came on again. It seems if I turn it off when I see the battery light come on and restart it the light goes off. If I continue to drive it with light on it will drain the battery and truck will die. To me something is telling the alternator not to charge the battery but restarting it seems to clear it.

    Any ideas? Already spent $1500.00 and still not fixed. Very frustrating.


  2. #2
    robertplattbell
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    Occam's Razor...


    Did you test the battery?

    Note that a dying battery may hold enough "surface charge" to start the car, but will drain down quickly. It's capacity is more Amp-minutes than Amp-hours. A dead battery can take out a brand new alternator in short order, too.

    While BMW batteries can last as long as 9 years, they also can fail in as little as 6 months (in my personal experience).

    Be thankful you have a 3.0 and not the liquid-cooled alternator of the V-8 models, which is a $1900 part.

    Also, check the battery connections for tighness and corrosion. Yes, this means removing the spare tire, etc.

  3. #3
    robertplattbell
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    Re-test alternator, too.


    The other day, I noticed the car was cranking a little slowly. After 30 years of dicking around with cars, I knew how this was going to play out. The original battery is 8 years old, a staggering amount of time for a battery to last, compared to the good old days.

    Two days later, at the liquor store, turn the key and nothing.

    Jump start it to get it home. Thank God for alcoholics. Always willing to give you a jump.

    I bought a new battery (Exide) from BJ's Wholesale for $79. They have a look-up catalog there and even had it in stock (Jacksonville location). The battery compartment has two mounting holes for the hold-down bracket - for larger and smaller sized batteries.

    The parts lookup book lists two batteries (different CCA) for the 2002 X5 3.0. BJ's only carried the smaller one (it has the correct vent tube, though). It fit the space, with the clampdown in the smaller battery position.

    Seems to be working fine now. We'll see.

    ALTERNATOR LIGHT: Traditionally, this light comes on when the system voltage drops below 13.5 volts or thereabouts. A working alternator should put out 14.5 volts. If you are getting less than 14, then something is wrong with the alternator.

    So if you are getting an alternator light, something is wrong in your charging system, possibly the alternator (new parts can and do fail right out of the box). Possibly a shorted cell in the battery.

    With the car off, the battery should read about 13.5 volts. Anything below that means a discharged or possibly defective battery. A car battery is basically dead at 11 volts or so. Most electronics on a BMW shut down at 10.5

    Again, a bad battery puts a lot of strain on the alternator. So bad battery problems can cascade into alternator problems. And if you try to recharge a flat battery with the alternator a few times, you can destroy a perfectly good alternator in short order. It overheats.

    I did this dance with my truck, frying two perfectly good replacement alternators, trying to recharge a dead battery that I thought was good. I had the battery tested, and it was "borderline good". That don't count, I discovered. I even did the "Gee, there must be a short somewhere!" distraction. There rarely is. Most charging system problems are battery/alternator.

    When I get a charging system problem, I first check the battery (you can take it to any part shop for testing on their machine). If it is more than 5-6 years old and suspect AT ALL, and particularly if winter is coming, I just replace it. $79 is not a lot of money and it is an easy DIY project.

    I used to use BMW batteries from the dealer, when they were $125 with BMWCCA discount. The last one I bought for a cabriolet (and those are different in six ways from the sedan and coupe batteries) it was like $170. Ouch. They do last as long as 9 years, though.

    But for my dog's car (the X5), a $79 Exide will do just fine.

    If the charging system still has problems, I test the alternator. Again, most auto parts places can do this, either with the alternator in the car or out (even Sears will test it in the car, for free).

    The alternator is not too cheap these days, but a local armature shop may be able to rebuilt it. The catalogs have these for about $200 to $400 rebuilt.

    So, if I was in your shoes, I would test that battery, replace it if it is at all suspect (borderline don't cut it) and then take another look at the alternator.

    Good Luck. Let us know how it works out.

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