Dan,

Don't be surprised at the responses you got from the tech reps - IMHO,
they're just as confused as you are (maybe "confused" is a bit too harsh,
but I've heard this song and dance before).

I worked in aerospace for over 40 years on several large programs (B-70,
shuttle, B-1 and more) in avionics systems design and it's my considered
opinion, from observation and experience, that wiring design for RFI (RF
interference) abatement, either radiated or susceptability, is a black art.
It seems, when all else fails, that good ol' "cut and try" becomes the
method du jour.

Often the information you get from the gurus is only a good starting point,
or baseline. Then when problems arise, you'll have to try something else.
Case in point, on my LNC2 (early 235) my wing-tip strobes had the high
voltage cable shields grounded at the power supply per manufacturer's
instructions. With the strobes on, my radios - comm and nav - and electronic
OAT gauge went bonkers. Disconnecting the cable shields from the PS case and
connecting the shields to the strobe light returns at the light killed all
those gremlins. Probably wouldn't work for anyone else's airplane, but it
sure cured that specific problem in mine. Go figure!

Bottom line, don't get too unhappy with the reps, most likely what they tell
you was just what worked for them on the last installation upon which they
worked. Start with what they recommend but be aware that you just might need
to engage in some "cut and try" before your gremlins are dead.

Dan Schaefer