Detecting an Object Through an Auto Firewall

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Detecting an Object Through an Auto Firewall

Post by PrIsMaTiC » Tue Jan 16, 2007 7:25 pm

I want to run a cable through an auto firewall. I found
an area on the engine side of the firewall that is free
of vacuum hoses, wires, etc. I have to drill through
on the cabin side, and that's the problem.

I can make some rough measurements on the engine
side where I want the cable to pass through, but there
is no way to transfer those measurements to the passenger
or cabin side. If I tried I would more than likely be off
by two or three inches. That would not be good since
I could easily damage something expensive!

What I need is some object I can tape to the cabin side
of the firewall, that would be detected on the engine
side. The firewall itself is heavy sheet metal with layers
of plastic, insulation, and who knows what else.

I was thinking about those small but very powerful
neodium magnets. Maybe if you taped one on the
cabin side of the firewall, it could be detected on the
engine side. Another possibility might be a simple
oscillator who's signal could be pinpointed on the
engine side.

I've always done some of the repairs and maintenance
on my own cars, but I'm not a pro mechanic. I don't
want to invest in an expensive instrument that would be
used once and put on a shelf for the next ten years.

When you think about all that steel and copper wiring
its hard to imagine what type of device I could build or
buy that would do the job.

Before someone asks the question, I have checked out
existing pass through points on the firewall. If I could
force the cable through a grommet or plug without
damaging something, I would probably have to use a
very long cable. For this project to work, the cable must
be six feet or less.

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Post by MicroRem » Tue Jan 16, 2007 8:50 pm

I vote for low tech! how 'bout a magnet on the inside of the car and some iron filings sprinkled over the firewall in the engine compt in the general vicinity?

If it doesn't work your not out much.


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Chris Smith
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Post by Chris Smith » Tue Jan 16, 2007 9:15 pm

Having done this a few thousand times, try a 1/8 inch drill, 1/4 inch long.

Patch the mistakes with silicone.

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Post by dacflyer » Wed Jan 17, 2007 3:51 am

i agree with chris also.. when drilling don;t let the drill bit go any farther than a 1/4 or so..

if you want to, goto a hobby shop get a ring coller with a set screw,, like they use to hold wheels onto axels of model airplanes,, slip this ring over your drill bit to the depth you want, then lock the ring in place with the set screw.

good luck !

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Post by PrIsMaTiC » Wed Jan 17, 2007 6:59 am

To tell you the truth guys, the last time I did anything
like this I was a teenager. My dad always had really
large four door sedans and wagons. There was always
plenty of empty space under the dash and around
the engine.

The firewalls on modern cars are packed with hoses,
wires, ducting, and almost anything the mind of man
can conjure up!

Chris, when you say you've done this sort of thing a
few thousand times I'm guessing you run wiring harnesses
for high end stereo gear. Let's say I choose a spot on
the cabin side that I think will break through where I want
it to be. Even if I use a small, narrow drill I could still
damage a wire or hose that's pressed up against the

I need some feeling of certainty about where that drill bit
is going to break through. I don't want a two dollar bit
to cause several hundred dollars worth of damage.

I thought of something that might work. Plug a pair of cheap
ear buds into a portable radio. Tape one of the buds on the
cabin side where you want to drill the hole. Using a mechanics
stethoscope, run the probe along the engine side of the
firewall. As you get closer to the bud on the cabin side,
the sound from the radio will get louder. The spot where
the sound is the loudest should be pretty close to where
the drill bit will break through.

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Post by SETEC_Astronomy » Wed Jan 17, 2007 7:34 am

I have had some luck with finding a landmark hole and measuring from there.
Can't you find a wire that pokes through and measure from it? I see where you can't measure from the edge of the engine bay to a spot and then measure from the other side because the shape of the car but if you can find a hole inside and out somewhere in the middle you should be able to figure it out.

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Post by Chris Smith » Wed Jan 17, 2007 8:49 am

If you fear damage this much, simply set the drill for 1 +1/2 times the depth of the fire wall metal, or less than 60 thou.

250 thou or 1/4 inch is normal considering the wiring is in a convoluted casing of plastic, which usually jumps away from a small drill hit.

Then the PVC coating on each wire is at least 50 thou, so just set the bit to a little over the thickness of the metal fire wall and remember hoses are even thicker.

And Yes, I have done dozens of stereos, wiring addititions, computer work, and a list of other things that others wont touch in spaces made only for a contortionist.

If you think the US has made it hard, try foreign.
They have always been small and tight.

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Post by Lenp » Thu Jan 18, 2007 5:06 am

If the firewall is double skinned, with a space between, drill from one side where you are sure it is clear then carefully insert an awl or thin rod and have someone tap it lightly while you try to feel the vibration on the other side. The fingers are quite sensitive to the impact point.

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