[Dekuiper]

When my 1981 is running at idle, a multi-meter on the battery terminals reads 13.88. When I turn on the lights it reads 13.64. Will this eventually kill the battery if I drive a long distance at night? Or is the 13.4 still enough?

If I am right, shouldn't this read 14.4 all the time? So is the alternator going bad? Can these be rebuilt?

 

[ghnl]

What does it read at about 2,000 rpm? And does it then stay steady as speed is further increased?

Alternator output will be lowest at idle. By 2000 it should be around 13.8V - 14.2V and not increase dramatically at higher speeds.

And certainly they can be rebuilt. I think the regulator can even be changed with the alternator in the car. Maybe?

 

[Tifosi]

14.4 is peak output and you won't see that til the alternator is up to full speed, not idle.
13.64 is plenty to keep the battery charged as it peaks at around 12.3~12.5 when fully charged up and in good heathly condition.

EDIT:
There I go, slow again

EDIT EDIT:
I've changed my internal regulator while still in place.
But getting the snorkel duct off can be a bit of a hassle.

 

[Dekuiper]

It's at 13.6 with the lights on and at 2000 - 3000 RPS. 13.8 with lights off and in same RPM range. Looks like maybe a rebuild is in the future but not an immediate step.

 

[ghnl]

That is likely to be fine. Make sure the alternator belt is properly tensioned. About 1/2" deflection (with one finger's pressure) along the longest span.

 

[papajam]

...shouldn't this read 14.4 all the time?

It's a fairly rare occasion for the Bosch alternators in our Spiders to get to 14V let alone 14.4 (which borderline high anyway).
What is the output voltage at the alternator B+ terminal?