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 TM 55-1905-223-24-3
SECTION IV. UNIT MAINTENANCE TROUBLESHOOTING
2-13. General Engine Noise Diagnostic Procedures . When diagnosing engine noise problems, make sure
that noises caused by accessories, such as the air compressor and power takeoff, are not mistaken for engine
noises. Remove the accessory drive belts to eliminate noise caused by these units. Noise will also travel to
other metal parts not involved in the problem. The use of a stethoscope can help locate an engine noise.
2-14. Frequency of Engine Noise . Engine noises heard at the crankshaft speed (engine rpm) are noises
related to the crankshaft, rods, pistons, and piston pins. Noises heard at the camshaft speed (one-half of the
engine rpm) are related to the valve train. A hand-held digital tachometer can help to determine if the noise is
related to components operating at the crankshaft or camshaft speed.
2-15. Isolating Engine Noise . Engine noise can sometimes be isolated by holding down the injector plungers
one at a time (refer to paragraph 2-40). If the volume of the noise decreases or the noise disappears, it is
related to that particular engine cylinder.
2-16. Accessory Noise. Engine driven components and accessories, such as gear-driven fan clutches,
hydraulic pumps, belt-driven alternators, and turbochargers can contribute to engine noise. Use the following
information as a guide to diagnosing engine noise.
2-17. Main Bearing Noise (refer to Table 2-2, Item No. 34). The noise caused by a loose main bearing is a
loud, dull knock heard when the engine is pulling a load. If all main bearings are loose, a loud clatter will be
heard. The knock is heard regularly every other revolution. The noise is the loudest when the engine is
"lugging" or under heavy load. The knock is duller than a connecting rod noise. Low oil pressure can also
accompany this condition. If the bearing is not loose enough to produce a knock by itself, the bearing can
knock if the oil is too thin or if there is no oil at the bearing.
a. An irregular noise can indicate worn crankshaft thrust bearings.
b. An intermittent sharp knock indicates excessive crankshaft end clearance.
Repeated clutch
disengagements can cause a change in the noise.
2-18. Connecting Rod Bearing Noise (refer to Table 2-2, Item No. 35). Connecting rods with excessive
clearance knock at all engine speeds and under both idle and load conditions. When the bearings begin to
become loose, the noise can be confused with piston slap or loose piston pins. The noise increases in volume
with engine speed. Low oil pressure can also accompany this condition.
2-19. Piston Noise (refer to Table 2-2, Item No. 36). It is difficult to tell the difference between piston pin,
connecting rod, and piston noise. A loose piston pin causes a loud double knock which is usually heard when
the engine is idling. When the injector to this cylinder is held down, a noticeable change will be heard in the
sound of the knocking noise. However, on some engines, the knock becomes more noticeable when the
engine is operated at a steady speed.
2-20
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