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7 Series - E38 (1995 - 2001)

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  #1  
Old 08-09-2010, 11:08 AM
srecko711 srecko711 is offline
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99 740i - Electronics problem

Been a member for a while but thankfully I haven't needed much advice. Now I do and will appreciate suggestions.
I work overseas so I am gone for months at a time. When I return my 99 740i usually needs a jump. As usual it will act cranky until the battery reaches full charge then it's fine. This time it was different. I charged it for 24 hours because it was stone dead. After 24 hours it barely turned over. I assumed it was time for a new battery. I drove it home from storage (50 miles) and all problems went away. It fired up immediately and none of the typical low battery problems occurred. About 10 days later I drove about 80 miles on the highway. Parked it and when I returned 2 hours later it was dead. I got a jump and drove home. On the way home the dashboard looked like the Macey's Christmas parade. ABS and battery lights were flashing like crazy. When I got home the interior lights barely glowed. I charged it for a few hours expecting to drive to get a new battery but the problems went away again. It has now been sitting for over a month and it fired up right away. I don't trust it after the last trip out of town but I don't want to start haphazardly replacing electronic components. I get no error codes.
Any thoughts?
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  #2  
Old 08-09-2010, 11:52 AM
balance balance is offline
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When a car battery sits for a long time, it will lose voltage little by little until it is completely dead. Car batteries are designed to be constantly charged by the alternator. If a battery is allowed to die completely, there is only a chance that it will charge back up and hold voltage, but it is certain that the life of the battery has been shortened.

If you jumpstart a car with a dead battery, there is a good chance that it will start because the battery on the other end of the jumper cables will have enough voltage to turn the motor over, and the car will keep running because the alternator is pumping enough voltage for spark and other engine control systems. The problem is that this is very bad for the alternator, as it is not designed to be pumping out voltage constantly, and you will kill the alternator quickly by driving with a bad battery.

My guess is that on your car, the alternator is failing, and the car was running on just the battery, which is why your ECU didn't have enough voltage for engine management, which is why your dashboard, and probably every other engine management system went haywire. You should charge the battery and get it load tested, and you will probably need to buy an alternator either now, or soon.

My suggestion to you is to get one of those low amp chargers that plug into the wall that keep the batteries charged if the car is not driven for a while.
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Old 08-10-2010, 05:05 AM
srecko711 srecko711 is offline
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Thanks "Balance". Any battery brand suggestions?
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Old 08-10-2010, 03:08 PM
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CodyItaliano CodyItaliano is offline
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brand isnt so important as getting an AGM type battery is. AMG will be able to hold a charge and amperage much longer than a traditional wet cell battery.

as far as brands, Exide is one of my favorites. or if you can afford it, get a proper BMW battery, but they are quite expensive.\

Dealership says $260 for the correct battery for the E38.
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Old 08-10-2010, 03:18 PM
balance balance is offline
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I haven't had to change my battery yet, but from what I'm hearing, the original batteries last close to seven years, so I can tell you that from what I hear, they last a very long time, but they are expensive compared to others.

The only battery that has amazed me was the "yellow top, dry cell" battery from Optima. My brother had one on a car with a 1000 watt competition stereo system for close to five years before the alternator died and killed the battery. It stayed in the shed for close to a year with zero voltage. Before he threw it away, he tried charging it just for the heck of it and amazingly, it charged and passed a load test. The same battery is still in use in his car two years later.
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Old 08-13-2010, 07:53 PM
srecko711 srecko711 is offline
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Thanks for the suggestions.
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Old 09-16-2010, 09:50 AM
srecko711 srecko711 is offline
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Here's a short update to provide a solution to the problem I encountered. Apparently the alternator was operating properly when the engine was cold. It was putting out 80amps at idle. When the engine heated up the alternator shut down. Apparently the alternator is cooled by the engine coolant so there is a relationship between the engine temp and the alternator. Rebuilt alternator and the problem is gone.
Thanks for the suggestions and i hope this helps someone with the same problem.
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