I have replaced or repaired about three 240s with this problem. My best results were to re-route the black (oil pressure) and red (alternator) wires back along the passenger side of the engins bay to the grey plug on the firewall. At the plug I cut the black and red wires and spliced in the new wires. Just leave the old wires in place.

The vacuum line which also runs under the front of the engine had no damage so I left it there.

I don't know how your voltmeter is wired in. Normally the voltmeter is wired to a spot on the fuse panel. If that red wire under the front of the engine is intermittantly shorting, it would cause the alternator to have problems and should make the warning light on the dash come on. In any case, you need to replace those two wires.

You may have other wiresd with bad insulation. I got by with repairing just the wires listed above. However, some have had to replace the entire harness eventually. Dave Barton, listed in the other message has a very good reputation and sells good used harnesses at a great price.