It’s one of my favorite literary lines ever, from Michael Crichton’s The Andromeda Strain. The scientist in question is pondering whether successful treatment of the symptoms displayed by his patient means that the disease causing them is actually cured.
So it goes with electronic diagnoses involving logic boards. Stray voltage at one pin results in a strange signal at another one. The voltage can have three sources, but which one is it?
For example, a artificially high (500rpm) tach reading on an F650GS that remains after shut off for several seconds. Coupled with poor running/starting. Symptoms occur only when bike is started wet after cold rain. Brought up to running temp and restarted, the problem is gone. Signal path is a single star circuit of the ECU, the #1 coil, and the tach signal feed. Diagnosis starts with check air and fuel to rule them out for the poor running aspect. Both are fine. Second step – pull plugs. As expected the #1 plug is funny looking, not bad, but a bit pink on the insulator. The primary resistance on the coil is a hair high when the coil body is wet.
Is this a symptom, or is it the disease? I’ll find out when I pull the tach apart. I hope.