I have a 1991 740 Turbo Sedan. About a month and a half ago the overdrive light started coming on when I started the car, but not every time. It was just before Christmas, so I couldn't afford to have it looked at. About 3 weeks ago, while i was driving, the dash lights dimmed, wipers slowed, and the stereo started shutting off intermittently. then I lost acceleration, and the car stalled out. a friend came and boosted me, and everything seemed fine after that. until 8 days later, when i was driving, the same thing happened with the stereo, so i tried the wipers, and they were really slow. i got the car home, and by the time i got a boost a few hours later the battery was dead. another 8 days later (today) the same thing happened. my husband's going to swap the battery with our other car, and get a boost and we'll see if that does anything. we're both suspecting though that the alternator is the issue, and it's not charging the battery enough. any ideas?

 

Measure the voltage across the battery terminals when the engine is running. It should be at least 14v. That will be a good indication of whether the charging circuit is working.

 

Got CAA to check the voltage coming from the alternator, and it read 13.9. We put a charge on a different battery from our other car, and it seems good so far. As simple as a dead battery, I guess!

 

I suspect the same will happen again. The only thing the battery does is crank the starter to get the engine to run. After that the electrical system and ignition system gets juice from the alternator. Electrical problems while driving down the road is not a battery issue. Does the 'BATT' light come on while cranking the engine?

 

No, the battery light doesn't come on... am i looking at a bigger electrical problem, then?

 

No 'BATT' light while cranking the engine indicates a problem with the alternator or the wiring to/from the alt. There is a little red wire on the back of the alt which supplies excitation voltage. At low RPM the alt requires excitation voltage or it won't put out. At higher RPMs the alt will usually self-excite. Did you measure the voltage across the BATTERY terminals with the engine at idle? That will tell you if the alt is putting out. It should be 14-14.5v or so.