MG Car Repair: starter run-on, starter solenoid, intermittent faults

Question
I have a 1980 MGB, and the starter motor won’t disengage in about five
percent of my starts.  When I remove the third fuse down after
an unsuccessful attempt, it starts without a problem.  I’ve replaced the
starter relay, and I think it’s wired correctly. I’m about to install
a diode between the recently purchased hi-torque starter and the coil, and
I’d be interested in your thoughts.  Why do you think it’s
sporadic and not constant?  Thanks, John

Answer
Hi John,
One of the most difficult problems to solve is an intermittent fault because testing must be done in the failed mode and if you can't catch it in it's failed mode it is very difficult to locate. Your odd symptom of pulling the a third fuse is in my opinion useless, because since the fault is intermittent you can't be sure the removal of the fuse is changing anything.

Intermittent faults can be found by testing to see if the solenoid is being triggered at the time of failure. This is done by installing a test light on the trigger wire for the solenoid and monitor the test light each and every time the starter solenoid is used. If the test light is on when the starter stays engaged, then the problem is before the solenoid but if the light is off and the starter stays engaged, it is the solenoid.

With a test light to monitor in the correct place when the failure in in effect you can keep narrowing the problem down to where the fault is.
This type of testing should be done with the starter relay also as a sticking contact in a relay can also be a cause.

It is absolutely a waste of time guessing what is wrong by the symptoms.

Howard