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bet they are not anywhere as sensitive as race ignitions. and how many different filters are they using? how confined are they?

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I dont know about the filters but its the low voltage
stuff that your trying to protect.. your dealing
with 0-5 volts or even 0-2 volts and the auto companies
run the grounds as said above... are they keeping it
clean with filters or not.. I dont know... I know
I had to deal with millivolt stuff on the fuel tank
(mainly the filler tube when it was plastic)

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That is the way Toyota/Lexus vehicles get away with grounding most of their 20-30 separate ECU's to the sheet metal. Most of the sensing circuits operate on a 5 volt reference signal so a sheet metal body ground on even the most sensitive systems is adequate. I'm kinda ground crazy on all my components on the Savoy I run ground points all over and most all of them can be traced back to my ground strap from the battery.
All that being said I still ground my battery to the bar in the trunk

Gus