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| Truck
Fleet Maintenance |
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ULTRASONICS:
no longer a
space-age technology |
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Brake
Systems
When properly used, an ultrasonic detector can help pinpoint leaks quickly
in both vacuum assisted and air brake systems. In many cases, leaks can
be detected from a considerable distance. A systematic procedure usually
works the best in this application.
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Air escaping from pressurized systems
generates ultrasonic frequencies which can be easily detected. |
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After
building up full air pressure, shut off the engine to eliminate
interfering exhaust and mechanical noise. A casual sweep of the
vehicle from one end to the other using the detector with a hand
held microphone can often locate the leak. When necessary, headphones
can be used to reduce noise interference from within the shop.
If the first sweep does not locate the teak, trace out the entire
air system piping at close range (one to two feet) with the hand
probe. Once the general area of the leak has been determined,
the sound concentrator or localizer can be used to pinpoint the
exact location of the leak. The sound concentrator is usually
a small rubber funnel which reduces the sensitive area of the
probe to a circle of about 1/4-inch (6.4-mm) diameter. The device
also sharply reduces the effect of ultrasonic noise originating
away from the area of interest.
For effectiveness, be sure during the entire search that pressure is maintained
in the brake system. If no leaks are discovered, check the shutter-stat, windshield
wipers, air horn and other air-actuated equipment. Leaks in these accessories
can produce abnormal gauge drop.
When it's necessary to perform tests with the engine running or in the presence
of other sound sources which interfere with the use of the open hand probe, the
sound concentrator in most cases locates the leak by shutting out most of the
interference. It does, however, require much closer and more careful tracing
of potential leak points. |
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Tires
For heavy trucks, many
mechanics check tires for adequate pressure by striking the
tire with a hammer and noting the rebound and sound. This
procedure doesn't find leaks. It merely finds soft tires.
It's possible that a leak may have just occurred or the tire
has just been inflated and hasn't had time to become soft.
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Road calls
can often be avoided when leaks are found before a unit leaves
the yard. |
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| A sweep with the hand probe may remedy the situation. The detector
normally finds leaks of sufficient magnitude before they cause
on-highway downtime later. The detection of even one leak may,
very possibly, save the cost of original investment into the equipment.
Facts which bolster this belief are savings realized by preventing
the loss of a tire (on duals, a second tire), possible damage to
driveline components and road service costs to replace the tire. |
Leaking valves
Leaking intake valves can be
checked for defects by using the probe and the headphones
with the engine running. All valves should emit a sound
in a very close meter and sound pattern when the probe
is placed on the intake manifold opposite the intake port.
The valve or valves out of pattern can then be positively
checked, when the engine isn't running, by bringing that
particular cylinder to full compression and placing the
probe on the valve stem. The "hiss" of escaping
gas across the valve seat will be distinctly audible on
the leaking valve. Exhaust valves can be checked by using
the same procedure. However, signal intensity will be much
greater in them. |
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Sound
generated by leaking valves is transmitted through the
manifold |
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| Cooling System Aeration |
Efforts to determine the presence of air
in the cooling systems of fleet vehicles is receiving more
and more attention, particularly in diesel engines. Air
can cause hot spots resulting in burned liners and heads
in a very short time. Since all cooling systems are under
pressure, coolant must go out in order for air to get in.
Aeration detection equipment gives the signal that air is in the
system but not the location of the leak. Using an ultrasonic probe
in suspect areas such as the head gasket, around the water pump and
injectors will usually detect the exact source of the leak. The "bubbling" of
air in a cooling system is very noisy and usually enables the exact
determination of the problem.
Being able to single out a certain unsuspected trouble point can
save a lot of unnecessary labor. All such tests should be conducted
while the engine is idling. Internal leaks may also be pinpointed
with the probe immediately after engine shutoff when internal Pressure
is at its highest point. Headphones should be used for this. |
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Exhaust
system
With the engine
idling, pulsation in the exhaust system can be detected
with the hand probe. Any leak points normally give off
a much louder and sharper sound. If required, the sound
concentrator can be used to locate the exact leak point.
Again, headphones are essential for this procedure. |
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Problems
with air induction systems can be pinpointed. |
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