Another archive from an old Rennlist post, as this one keeps coming up again and again.
If your voltage stays low when you start the car (alternator not charging) and then jumps up to a normal charging voltage when you hit 2500 or 3000 RPMs, it’s the alarm. Yes, the alarm.
Somewhere in Germany in the 80s, some engineer said to himself “ve shall make ze alarm control ze DME relay AND ze alternator field current.” No one knows why. But it happened. And if your alarm is broken or removed, you or someone before you probably only bridged the wires to allow the DME relay to be energized (pins 1 and 4 on the alarm box connector), started the car and said “good enough.” Well, it wasn’t. Now go bridge pins 7 and 8 and your alternator will get +12v to the blue D+ (field) wire when you start the car, which will allow the alternator to start working at low RPMs. Without that, you need to get the RPMs up high enough to “auto excite” the alternator. The better condition your alternator is in, the lower that number will be.