> My manual says to run the negative lead directly to the battery,
> particularly with todays unibody construction.
>
> All it takes is a weak weld giving way or a rusty joint causing the path
to
> change
>
> At least they are no longer welded up on an assembly line of people. I
> worked on one of those over 40 years ago and I can testify there were
> certainly more than a couple of bad welds that went through. <:-))
>
When I was trying to install my screwdriver antenna on a 2001 Passat, I was
astounded at the lack of electrical "unibodiness" (and for that matter, the
amount of structural composites, insulating members, etc.). Even the bumper
(the steel one under the plastic one that you see) is not electrically
connected to the rest of the car, because the spring loaded dampers that let
it tolerate the "5 mph crash" are insulators. I certainly wouldn't count on
any particular "metal looking" part on that car being connected to any other
part, and certainly not connected (reliably) to the battery.
Many of the sheet metal panels appear to be bonded to other sheet metal
panels by adhesives, as opposed to welding. From a structural and assembly
standpoint, this makes sense. Modern adhesives are more than strong enough
for the application.
I'd also venture to guess that there is no metallic component exposed in the
passenger compartment that is "grounded" (except, perhaps, the cigarette
lighter outside terminal). Heck, there's not that much metal exposed in the
first place.
Interestingly, there are a number of new cars coming out with a body on
frame sort of construction, as opposed to full unibody (monocoque) design.
The ability to put vibration isolation between the passenger compartment and
the part connected to the engine and wheels (the frame) makes for a quieter
ride. One assumes that they have a bonding wire somewhere that electrically
connects the two.
And, if you talk about a car like the Saturn, which has plastic body
panels....
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