Truck Fleet Maintenance |
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Brake Systems
Air escaping from pressurized systems generates ultrasonic frequencies which can be easily detected.
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.
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.
Tires
Road calls can often be avoided when leaks are found before a unit leaves the yard.
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.
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
Sound generated by leaking valves is transmitted through the manifold
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.
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.
Problems with air induction systems can be pinpointed.
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.
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.
