There are a number of tools that help find the problem, but one tool close to my work bench is the book, "Troubleshooting Old Cars by Ron Bishop". As it says in the introduction; "Troubleshooting is a process of elimination to establish the facts, find out whether or not a suspected trouble actually exists, and to then track down the source in a logical manner." Ron goes on to say "I recommend that you do not make any adjustments or repairs until a complete analysis of the trouble is complete and that you are positive the source of the trouble has been located. Working slovenly, trying this and that, while hoping to stumble on the cause of trouble could result in changing the effect of the trouble and thereby entirely disguise the actual cause of the trouble."
Bill