My friends 1991 obd0 b18 civic 4 door was having probs today.
When we were cruising his lights get brighter at 4k rpm and up.
It only happens 4k and up. Everything would get bright like the cluster and headlights get very bright like its pulling a lot of power.

So we got the alternator replaced 3 times and even tested same prob still. Will use a volte meter and it idles at 14 volts. When idle goes to 4k voltage goes up to 18-19 volts. Could it be the alternator or something grounding.