This is also an excellent read on the basics of avionics wiring. Greg Richter is a rock star when it comes to troubleshooting. Heres the link and the evil ground excerpt:
From Greg Richter, Blue Mountain Avionics
Power and Ground
First off, we’ve got to power the thing up, whatever it is, which brings me to Kirchoff’s
Law, which simply states that whatever electric current goes into something, has to come
back out and return to the battery. Which means anything you power up needs two wires
to make it go – one for Power and one for Return. This brings us to:
The Evil Ground
In a car or metal airplane some wise soul thought it would be a good idea to just use the
chassis or fuselage as a common Return wire and save a few bucks. Great idea, if you
want to spin motors and make lights blink in a Model T, but not so good for delicate
electronics like EFIS, Radar, electronic engine monitors or FADEC. Both Electronics
International and blue mountain avionics specify ungrounded EGT probes because most
homebuilt airplanes have grounds all over the place and the stray currents that go with
them can cause odd EGT readings.
“Bad Grounds” cause more problems than just about anything else.
So -- I offer the Zen solution of not thinking about things in terms of “grounds”. Ditch
the whole, outdated, tragically useless concept. If you stick to the two-wire rule, one for
power and one for return, you’ll never have a ground problem, and everything you
connect will work the first time. Remember, “Ground”, in this context, is a convenient
shorthand for the return lead in a circuit. It doesn’t really exist as Earth Ground for us
aerospace-types. This brings us to our second rule:
All circuits are wired with a power lead and a return lead of the same size.
Power is color-coded yellow or red, return is color coded black.