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Old 04-24-2012, 11:18 PM
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2003 X3 strange electrical fault and won't start normally without jump starting

Hi All,


I have a 2003 X3 3.0i with Satnav, sunroof and xenon lights. It is doing bizarre electrical things and I am wondering if anyone can tell me if there's a computer module that could be the culprit.


Here is the symptoms of the problem:


1. When I turn the ignition key to start the car, the starter motor just makes one single click but there is no cranking, even though the battery is fully charged. I know all systems are good to go for the start because I can hear the usual electric whirring noises in the engine bay and all the usual warning lights come on the gauges, as per normal, just no cranking occurs.


2. I replaced the battery with a brand new one that has been fully charged, and same symptoms.


3. I attach jumper leads from my very large 4x4 battery directly to the battery in the rear, no difference.


4. Here's the curious thing - I attach the same jumper leads this time to the jump start terminals in the engine bay, and the starter does crank the engine to start successfully. I am able to stop and start the engine several times this way.


5. But when I disconnect the jumper leads, the problem returns.


6. Connecting the jumper battery to the front always starts the engine, connecting the jumper battery to the rear where the main battery is never starts the engine.


7. I drive around and after about 30 minutes the main battery in the back is flattened. I get all the usual symptoms of a low voltage problem. Warning lights randomly appear on the instrument panel like the ABS, then the Airbags, then the transmission goes into limp home mode. Yet, at no stage is the red battery warning light on.


8. I test the output from the alternator as the engine is running, and it is fine. Yet, it is not charging the battery in the rear.


9. I check all the fuses and they are all OK. I check the two mega fuses in the box near the cranking battery in the rear with a multimeter, and they appear to be fine.


What is going on with this BMW? I rarely drive it, and it's only got 110,000 kms on the odometer, always garaged.


I bought it as a used vehicle 12 months ago so I don't know what the previous history is.


Has anyone experience this sort of "weirdness"?


Is there some kind of a relay that connects the battery and the rear with the engine in the front that is perhaps is faulty?


Help!


Regards,


Lawrance
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  #2  
Old 04-25-2012, 02:14 AM
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So, basic electrical theory would say that there is no connection on the main power lead to the alternator and starter, the jumper connection is on that line.

But less basic is that BMW equipped the positive cable with a disconnect that separates in an accident. I hear it's near the battery.
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Old 04-25-2012, 04:00 AM
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Yes, I agree, there has to be some kind of a circuit from the cranking battery to the starter motor which it is somehow unable to access. How can I identify this circuit and troubleshoot it?

Re the pyro battery disconnect, it hasn't discharged, so the positive lead is very much still connected to the battery. I am aware of this feature, as it is also implemented in Range Rovers from when BMW owned Land Rover. But sadly, that's not the issue.
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Old 05-02-2012, 03:12 AM
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Thumbs up

Just wanted to close out on this issue, which has now been resolved.

My mechanic is a champ - he never gives up.

He had a hunch that the main wiring loom that connects the rear of the car to the front was somehow faulty. However to prove that, he did have to remove half the interior of the car to get access to the wiring loom.

Turns out he was right - one of the main wires in the wiring loom is faulty. But instead of replacing it with a new loom, he replaced the section that was faulty. That's because the genuine replacement wiring loom from BMW would take weeks to get in and would cost an absolute fortune.

All is good again.

According to my mechanic, it looks like the vehicle has had a rear-ender and the accident repairs did not include replacing the loom.

The problem isn't the safety terminal on the battery positive post. The faulty wire was a section of wiring that is not visible. Must have been a big crash previously! :eyecrazy Hope this car is not a dud.

Anyway, $380 later, the vehicle is back on the road, and I have a spare battery from my misdiagnosis. :rolleyes
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