I have been chasing this exact problem on a 97 civic. Here's what I've done to date:

Problem: Alternator is not charging upon start up and at idle. Once engine RPM are increased above 2400 (in park/can not moving) the number 15 fuse in the under dash panel blows and the alternator charges erratically, rapidly alternating between 0 – above 19 volts.

Here are the steps we’ve taken to try and resolve the issue:

- Replaced alternator twice
- Replaced battery
- Cleaned all visible grounds
- Removed all non-essential fuses from both the under dash and engine - compartment and ran engine. Results were the same.
- Completely removed gauge cluster from the dash to eliminate a potential tach/guage short.
-Removed and inspected engine compartment fuse box. Inspected ELD and plugs in box.
-Inspected visible wires under hood and under dash for indications of shorts or breaks

We have checked ohms on a few fires and here are the results:
- Black and white wire side of number 15 fuse shorted to ground (key in off position)
-White and blue wire running to alternator (checked by unplugging harness from alternator) shorted to ground (key in off position)
-Unplugged harness from VSS and check each wire with the following results
o Black/Yellow 2M ohms
o Black 0 ohms
o Blue/White 1.8M ohms

Any ideas where we should go next?

Thanks!