QuestionRecently my 2004 BMW 7 series 760Li (vin ends in DK11218) has exhibited an interesting but annoying bobbing up and down under certain situations. When the car gets into this mode, a few other things start happening too. First, the transmission starts shifting abruptly and with some dramatic lag up to 0.5 seconds. Second, my aftermarket stereo amp starts motorboating (cycling on and off). Consequently I am suspecting an electrical problem resulting in interesting behaviors thanks to the various computers in the BMW perhaps resetting or disabling themselves.
I started monitoring the car's Voltage (using the press and hold odometer trick) and there are some correlations. Here are my observations:
1) The Bobbing only happens around 15 to 20 mph. It feels like the back of the car has a pogo stick for a suspension.
2) The bobbing mostly happens when doing turns, particularly a U-Turn. If it starts bobbing in a U-Turn, it keeps bobbing until I come out of the U-Turn.
3) I test drove in a parking lot just going in a left hand circle at 18mph. When the bobbing starts, it does not stop for 3 to 10 seconds.
4) It seems to happen more often when I have been driving many short errands in town. I think draining the battery?
5) Around the same time after running errands, my transmission starts shifting abruptly, and with some lag. Watching the Voltage Monitor, the voltage spikes down from 14.2V to 13.2V momentarily as the transmission downshifts. It does not spike down during downshifts at other times.
6) Around the same time after running errands, my after market amplifier will start motorboating (turning on and off). When the amp motorboats, the Voltage spikes down from about 14.2V to 13.5V.
I have since disconnected my aftermarket stereo amps to see if they were the cause. The car still gets into the bobbing in turns and hard transmission shifts after running errands for a while. A short 5 mile highway drive, and it all seems to go away again.
Some other potentially important observations:
1) If I put the key in the ignition, about 1 minute later, the instrument cluster shows a message "Charge Battery!"
2) I had the electrical system checked by the shop, and they said it was all OK other than "excessive diode ripple" from the alternator.
3) Right before starting the motor, the Voltage Monitor usually says around 11.6 Volts.
3) Right after starting the motor, the Voltage Monitor says 14.8 Volts for a while, until it ramps down to 14.2 Volts after many minutes. It seems to stabilize around 14.2 to 14.4 V except for the spikes noted in the above discussion.
Thanks for any assistance you can provide. The tranmission hard and laggy shift was a little annoying, but the bobbing is intolerable. Even my passengers have exclaimed "what was that!" after doing a U-Turn.
-Bob
AnswerBob, I can't even comprehend why you would install an aftermarket amp in a 760Li. The battery is toast. The question in my mind is that is it comming from a defective alternator Voltage regulator or is it because the car is staying awake because of another faulty module or the aftermarket crap. I would remove the stereo stuff so the dealer can warranty the battery or the alternator. Secondly what is meant by 'Bobbing' Is the RPM fluctuating? Transmission Control is most vulnerable to voltage variation. Tell the service advisor that you had jumpstart the car. That should get you a warranty battery replacement if the car's electrical system isn't too butchered for them to deem it warrantable. Ask if they can do a closed circuit current draw test to see if the car is going to sleep normally and not drawing too many amps. I don't know why they didn't replace the alternator if they got an excessive diode ripple reading. Some of those alternator's have bearing noise issues anyway. The tech can read out the driving profile out of the computer and tell you if you are not driving the car for long enough to charge the battery. Good luck.
-Evan
