RE: Chasing down electrical problems....
The foregoing discussion re-emphasizes a hard rule of trouble shooting
electrical/electronic system: 80% to 90% of failures will be mechanical in
nature. That is, bad or broken connections, dirty connections, bad or
corroded wiring, etc.
In the 1950's the US Navy did extensive studies on the effect of their
standard 'Preventative Maintenance' procedures that applied to electrical
and electronic systems. These procedures included cleaning of relay contacts
on crossbar switches etc. They found that more failure occur ed WITH
maintenance than without. They also documented the high percentage of
failures that were due to bad connections, etc. Simple visual inspection and
continuity testing will find most electrical problems, regardless of how
complex the attached components are. Mechanical switches are next in line
for failure, then relays. Solid state equipment ends up at the bottom of the
list for 'most likely to fail'. Example: how many ECU's are we aware of that
actually fail vs. headlight switches.