Take another look at the original sentence:
You wanted to fix it this way:
A perfectly good method of fixing a fused sentence is to separate the two complete sentences with a comma and a coordinating conjunction like but. When you use this method, however, the revision must make sense. But is a contrast word, and the two sections of the fused sentence above are not showing contrast.
You might want to consult the rules for fixing comma splices and fused sentences.