Re: [VAC] Ground Wires in 12 Volt Circuits

["Dr. Gerald N. Johnson" <geraldj@xxxxxxxx>]

Running a ground circuit would be ideal, though it will require
modifying each light to make a ground wire connection.

In the Airstream we are lucky in having a shell assembled from bare
aluminum with rivets in freshly drilled holes so have a pretty good
ground in the shell. All those SOB with prepainted aluminum sheets
should almost have to run a wire ground circuit to get the lights to
work the first time.

The common Grote and Peterson fixtures we can get and that have been
used depend on a mounting screw or two to supply the ground. Worse that
ground screw pressure depends on a piece of PLASTIC to maintain the
pressure. Clearly the manufactures neglect knowing that the word PLASTIC
means it deforms under pressure. All my '68 Caravel marker lights are
pop riveted in place and the plastic has relaxed causing most of the
marker lights to not work.

A day or two ago I espoused adding a ground pigtail under the plastic
base. Now that I've looked a new light or two, I can see that is NOT
going to work because there's no space for that ground pigtail.

A technique that might work and be easier to fix than my pop rivets is
to remove the pop rivets doing the grounding and replace it with a
suitable stainless steel self tapping screw. When installed that screw
should be equipped with a stainless steel internal or external star lock
washer and be coated with aluminum wire contact grease. The stainless
steel screws are easy to find, the stainless steel internal or external
star lock washers are hard to find, though I do find them in the
McMaster-Carr catalog. About $18 per thousand and sold in boxes of 1000.
This technique won't be forever perfect, I don't think but at least a
screwdriver can take up the flow of the plastic each time the light
fails to light.

I'll be looking at this more. I still prefer the copper wire pigtail
with a crimp lug or two, that's screwed directly to the aluminum shell
with the stainless steel screw (stronger than aluminum self tapping and
easier to be conductive) and star lock washer. Then the connection is
NOT dependent on pressure through plastic. I'd probably solder the
pigtail to the ground strap (right about the 234 part number) unless I
use a brass self tapping screw into that thin ground strap. The
fundamental problem with the pigtail is that there's no room behind the
plastic for it. So what I'm thinking now is to take the marker light
assembly off the trailer (by drilling out the pop rivets which I presume
are 1/8" diameter and will leave a decent hole for the #8 x 1/2" self
tapping screws that I picked up today). Then I'll open up a large enough
hole in the plastic (maybe 1/2") to allow reaching the aluminum trailer
skin with drill, #6 x 3/8" stainless steel self tapping screw, and
pigtail with crimped (probably not insulated) lug to fit that screw. The
wire will be stranded, probably 18 gauge. I don't know for sure without
soldering (which is not reliable under vibration, solder wicks into the
strands and makes a stiff spot to break) how I'll connect that pigtail
to the ground circuit. Perhaps with a brass screw into the ground strap.

When I assemble things I'll use aluminum conductor contact grease
(available in the electrical department at home centers and good
hardware stores, required for using aluminum wires in panels) where
aluminum is involved and Ford silicon dielectric grease to slow
corrosion where copper and brass are involved. 

I'll use the same process when I install the new Grote 6000 license
plate fixture I picked up today to replace the blob of rust that used to
be the Airstream's license plate lamp. I did find that NAPA has a
replacement license plate lamp they claim to have been original
equipment on Airstreams, but I decided $6.85 in hand was a lot better
than ordering the NAPA fixture for $35.00.

If I can't work in the pigtail the screw with lock washer and aluminum
contact grease is my best suggestion this week. I think the pigtail
through the half inch hole will work decently unless the 32 year old
plastic shatters when I try to drill the hole. It might be that a
forstner bit will make a better hole with less damage than a standard
metal drilling bit. I have only ten marker bases to drill to find out...

Gerald J.