How To Install Air Horns On Your Truck – Part 2

By Reg Robinson


explanatory. I strongly urge some mean
of deenergizing the system. I have a
momentary switch for the horn and another, which interrupts power to the
pressure circuit. This prevents the
compressor from starting when I don't want it to, like in the middle of a cold
night or when the engine is cranking. An
illuminated switch was used here so I am reminded of the system's status. The horn switch is the black one to the right
in the photograph below. Note the fused
power supply.

Your looking
at the top edge of the fuse cover.

These are the basic parts. Now on to the installation.

deciding on the type of horns deciding on where they should go is the next
step. Do you want the horns hidden or
out for all to see? Because of their
size and because I wanted them visible I mounted my horns on my forward
rack. I have sliding racks on the bed
rails so it was a logical place. I left
enough hose coiled between the bed liner and the side to allow for a few feet
of rearward movement of the rack without removing the horns. Because of the design of the racks should
removal of the horns be required I need only undo two bolts. Mine are also train horns and quite honestly
when is the last time you a train with its horns beneath the locomotive?

Benefits of mounting topside include visibility for
aesthetics, protection from road debris, not having to deal with an already
crowded engine bay and less lines near hot or rotating components. The dollar investment demands thought in

The downside of topside
mounting? They are visible and
many know their worth, which might attract the wrong attention. Also due to their weight they need a sturdy
mount and if you don't have racks then you must fabricate some sort of base
plate. With the heavier horns sheet
metal is not an option for support.

Mounting location is also dictated by truck style. Look closely at what you have. Just make sure the horns are firmly in place,
out of harm's way and if possible tilted slightly downward to promote
drainage. You are responsible for
anything that falls off your truck and one of these bad boys bouncing down the
road is going to do damage.

mounting, front bumper or under carriage is popular. It's your choice. 

Where to
put the tank or bottle:

Probably the easiest place is in the bed, inside of a
toolbox. There the tanks and compressor
are protected, accessible for maintenance and inspection and secure. Make sure that the compressor is not touching
the tank and has space around it to allow for air circulation. Keep hoses clear of the compressor and sharp

I chose to install my tanks and compressor beneath the cab
next to the frame rail. Here's where a
knowledgeable salesman paid off. He
shipped tanks that would fit between the frame rails of my F 350. Shopping

Beside the frame rail is a popular and generally a logical choice for the tanks.

I mounted the tanks, compressor and relay on a skid I
fabricated from square metal stock and thick sheet metal. All found at the local home improvement
store. This allowed me to do much of the
assembly in the comfort of the garage at my workbench. Doing this also allowed me to do a temporary
assemble of all parts, pressure test the tanks and sound the horns. That way I knew everything worked before
crawling under the truck. 

Pressure check all you can off the truck. It's just easier. Air leaks will cause excessive cycling of the
compressor, which causes premature failure and may void the warranty. It will for the Viar. 

The smaller the tank capacity the faster the tank will
depressurize for any given size leak and horn usage. This is another reason I went with twin
tanks. It means the horn is more likely
to be ready when I jump in. It takes my
compressor about 11 minutes to raise the tanks from 0 to 140 psig. Most of the stores I visit are within 5
minutes. I can make it downtown and park
in twenty minutes. You get what I am
saying. Make your setup as leak tight as
possible so it stays ready. 

Fancy quick disconnect fittings came with my kit but I
prefer standard compression fittings. They're cheap, familiar and easily found. Three of the quick disconnect fitting in my kit
failed. One completely separated around
15 psig. Imagine that at 150 psig!

Make sure the nuts can't back off. Use a medium strength thread locker or
whatever you like. I prefer nylon
inserts. Don't use lock washers.

Try to keep the tank out of harms way. Keep the length of the discharge hose " the
hose from the valve to the horn short but allow for movement and body
flex. Remember this hose is only under
pressure during horn operations so danger from whip is minimal. Keeping the hose shorter will make for less
sound time delay. I purposely left a few
feet coiled between the bed liner and truck to allow movement of the racks as
mentioned earlier but also to delay the sound ever so slightly. I am not only after the volume but am also
trying to better mimic the sound of a locomotive. The extra hose acts as a surge volume
allowing the horns to sort of wind up like they would when an engineer
throttles open the manual valve in his cab. We've all heard the low to high pitch of a train horn as it approaches a
crossing. I have another SOV to be
installed in the future in parallel with the current one. This second valve will have a throttled ball
valve in line with it. Wired to the
second contact on the current momentary switch I will be able to sound a
sustained low blast and then switch to full volume. As mentioned the larger the
tanks then the longer the note and the better the sound recreation.

As for compressed bottles I have generally seen larger ones
placed horizontally in the bed of the truck. Be sure they are properly and legally secured. You will be sacrificing bed space if they
can't fit under the toolbox. 

A note on
hoses and wires:

Whether hoses or wires find whatever entrance works for
you. Most trucks have plugged knockouts
along the belly and on the firewall. Ensure anything entering the truck doesn't create a path for water. Create loop drains or have lines enter well
above the ground.

Tuck wires out of the way. Think about road debris like branches or anything that could grab
dangling items. Use dielectric grease
and heat shrink splices and connectors where possible. Put protective sheathing on external hoses
and wire runs. The tubing in the
plumbing department of the local home improvement store works well. You can run the wires and lines inside or do
as I have just install everything then split the tubing and push it over the
wires/hose like the foam insulation for water pipes. I prefer splitting because water will always
get inside so give it a way out. Size the
sheathing so it fits snuggly. Splitting
it and pushing it over after things are installed is easier. Tubing especially the clear stuff doesn't
slide well and you really don't want to start messing with lubricants and such.

All that
black is tubing protecting the wire
runs. This is the underside of the skid below compressor. The other is the ubing protecting control wires passing through the firewall.

Below is the schematic of my setup. Note both power supplies are fused. A Bosch relay is a standard relay found in automobiles. As mentioned the switch upstream of the pressure switch allows total disabling of the system. The compressor will not start with this switch open regardless of tank pressure. The line side to this part of the circuit can be any 12V hot lead. Just ensure the existing circuit can handle the additional load. As shown in my starter relay picture all power is pulled directly from the battery to avoid problems with existing wiring.


compressor can go anywhere you can fit it. It needs to be able to cool itself and be out of danger's way. The compressor's intake must draw on clean
and preferably dry air. Unless you live
in the desert and never leave the garage this mean routing an intake line away
from moisture and dirt. 

Since the
cab is climate controlled I routed my intake there. The Viar came with a filter and housing that
fits on the end on the suction tubing. Find a knockout somewhere on the fire wall and feed the tubing
through. I routed mine through the same
knock out as the gauge line. The place I found that works is on the firewall
below the glove box to the far right. Watch for sharp edges that could do damage via vibration. My filter is tucked high above the front
passenger's feet. I rarely have someone
with me so this works out. I placed it
so someone would have to really kick around to hit it. 

Here's the
gage and compressor inlet lines

through a knockout. Protect these

hoses from
damage where they enter. 

compressor can draw lots of current. Pull power directly from the battery " not from any existing circuit.
Route it through a relay controlled by the pressure switch. Use the largest wire you can and keep the run
short to mitigate the voltage drop. I
found the wire I needed at Lowes. The
lead for my compressor was supplied. It
is without question the thickest wire I have ever seen on a vehicle. It is
almost the size of a fuel line. It is as thick as any of my house wires. Any significant voltage drop must be
avoided. You'll smoke the compressor and/or
blow fuses.

Power is
pulled directly from the battery. I use the

relay hot side. The blue wire feeds the

switch circuit and the black with the fuse

 is the compressor feed. As you can see this is a

terminal in my truck.

All power
feeds must be fused. Power for the
pressure switch and horn can be piggy backed on the hot side of existing fuses
using fuse taps but it still must have over current protection. Avoid using the
existing fuse for protection. The fuse box cover may or may not re-close. Fuse taps can be found at electronics stores.
Splicing is also an option.

The feed for everything in my system comes straight off the
battery. Not only because the fuse box
cover would not re-close but mainly because it created a cleaner
installation. I don't like tampering
with existing wiring. What is there was
designed for a particular purpose and load. Just like your house that breaker was design for a predetermined
load. Adding to that circuit just
reduced the distance to tripping or blowing a fuse.

Try to mount the relay in a dry location and orient it as so
as to drain should water enter. Mine is mounted below my skid but I have taken
precautions to protect it.


As mentioned earlier if it's an SOV then it can be mounted
anywhere. Try to keep the orientation
correct. The colder the climate the closer to a heat source it needs to be
mounted. Another option is to buy an SOV
rated for your temperature. Again watch
the run to protect it from vibration and missiles.

If you
want to go the manual route more power to you. You'll have to figure out the best way to feed the air lines into your
truck. You'll also want the valve
somewhere near the driver's armrest. 

is also where water intrusion is a concern. Contamination can cause these valves not to seat. At best this will result in system
leakage. At worst a horn with no means
to turn OFF! I can't stress it
enough. Keep the system clean. Use clean, dry and preferably filtered
air. Drain the tanks periodically. 

Since an SOV is an electrical/pneumatic/mechanical device it
can and will eventually fail. The odds
and time of failure will greatly depend on its quality, the climate and the
care you take with wiring and installation. As always you get what you pay for and reap whatever level of effort
goes into the job. Invest in both

Once everything is hooked up do a slow pressurization. If using the installed compressor ensure the
engine is running prior to running the compressor. I have a normal 3/8th
female fitting installed on the rear most tank pointed to the tail end. This comes in handy for allowing tie in with
my shop compressor and for easy depressurization. If you do install such a fitting make sure it
faces towards the rear and cover it if possible to eliminate catching dirt and
water that could interfere with connecting or enter the system. 

At about 20 psig or so stop the pressurization and do a leak
test. A weak soap solution will do or
you can buy leak detection liquids. Take
your time and wait for the bubbles. Tighten the leaking fitting and move on. Make small adjustments maybe 1/16th of a turn at a time. 100% leak tight is probably not going to
happen but get as close as possible. You
should not be able to see the gage move nor hear any air escaping. Teflon tape works well but never use any on
the intake side of the compressor. If
you do use the tape use it sparingly. The horns diaphragms have a small orifice, which could become fouled by
tape shreds that became dislodged. 

Keep after the leaks. When satisfied start the engine and allow the compressor to bring the
system up to operating pressure. Keep an
eye on the gage to ensure the compressor stops when it should. When the compressor cuts off do another leak
check and tighten fittings as necessary. Be careful hoses aren't twisted as you tighten.

Ensure all are clear or warned, get inside, roll up all
windows and sound the horn. Do a quick

If all has gone well to this point drive the truck a short
distance and check that all is as it should be. Drive a mile or so and do another check. Look closely at the hardware.


Dos and

sound these horns near those without ear protection. I can't overstress this point. The driver
you're blasting is inside an insulated car and probably farther away from the
horn than you so what he hears is going to be quieter than what you hear. My horns, even being mounted topside, are not
very loud with my windows closed but are ear splitting loud to those outside on
two feet. In other words you may piss off a lot of innocent pedestrians and the
guy in front of you with his cell phone and music may not even notice. At close range you can cause damage to
someone's eardrum. Once I forgot I had
both passenger side windows down about 6 inches when I sounded the horns. For the next couple of hours hearing in my
right ear was dulled. As mention above
these things put out 100+ decibels at 100 feet. Think of these horns as acoustic
howitzers. Pause to think before you

Don't allow
any lines to rub or dangle.

Don't pull
compressor power from an existing circuit.

Don't allow
excessive leakage. It will kill the

Don't mount
any parts near rotating components or high heat sources. This goes especially for pressurized lines.

joining dissimilar metals if possible. Having different metals touch causes electrolytic corrosion. Add moisture and you have basically made a
weak battery.

Do use nonmetallic washers or gaskets between the component
or mount and the truck body. This will
help dampen vibration and prevent metal-to-metal contact and thus electrolytic

overwork the metal if you do fashion brackets or mounts. This creates stress that will relieve itself
through accelerated corrosion and cracking. 

All brackets here were fashioned on my anvil from steel
stock. Rubber washers are fitted at all
bracket to skid and bracket to body interfaces to dampen vibration and prevent
metal-to-metal contact. 


You can see the shine of the rubber

washers between the skid and bracket.

Do fuse all
power leads.

Do protect
the system from road hazards.

Do draw on
clean dry air.

Do drain
the tanks periodically especially for those like me in humid climates.

Do keep all
wiring as short as possible and use wire rated for the load.

Do ensure
all wiring and air lines are grease and gasoline resistant. 

Do wire as
to prevent the compressor from starting when unattended. Use a switched circuit
at a minimum. That is, when the ignition
is OFF so is the control circuit.

periodically check the system for loose hardware as well as leaks and damage.

I hope this
helps those in search of information on mounting the larger air horns on their

A special
thanks to Mr. Marks for his invaluable input on compressed bottles.

I welcome
all feedback especially on actual installations.

The best
of luck to you. If you're ever in the
Richmond area and see a big black F350 dually with triple horns topside give a
honk it's probably me. Just roll your
windows up so I can return the hello. 

Click here for part 1.

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