G3408C and G3412C Engines Caterpillar

Detonation is engine knock that occurs after the air/fuel mixture has been ignited by the spark. The excessive mechanical stress and thermal stress can reduce the service life of the engine. There are several possible causes of detonation. Examples are a rich air/fuel mixture, overload, a high compression ratio, and high inlet manifold air temperature.

Combustion of the air/fuel mixture prior to the spark is a premature ignition. This is usually caused by a hot spot in the combustion chamber. Possible sources of premature ignition are an incorrect spark plug, an incorrectly installed spark plug, and deposits in the combustion chamber. Detonation can be the result of premature ignition. The premature ignition has the effect of advanced ignition timing.

Although a sensor may indicate the presence of detonation, the problem could be a premature ignition. An indication of detonation can also be caused by excessive mechanical engine noise. For these types of problems, refer to Testing and Adjusting, "Troubleshooting".

Two different types of detonation sensors are available. One type is a sensor assembly. The other type consists of two parts: buffer module and accelerometer.

Illustration 1	g00942676
Sensor assembly

The sensor assembly is attached to wiring and to a connector for the engine harness.

Illustration 2	g00818378
Two-piece detonation sensor (1) Buffer module (2) Accelerometer

The two-piece detonation sensor consists of buffer module (1), accelerometer (2), and wiring. The engine harness is connected to the buffer module.

Inspect the components. Make sure that the connectors and the wiring are in good condition. When you install a detonation sensor, tighten the sensor or the accelerometer according to the engine's Specifications.

The operation of the detonation sensor can be easily tested.

Use a screwdriver to tap the engine block next to the accelerometer. If the detonation increases, the sensor is operating properly.

Monitor the level of detonation while you wiggle the wires for the detonation sensors at the ends of the engine harness. When possible, wiggle the wires for the detonation sensors throughout the engine harness. If the level of detonation changes, there is a problem with the wiring.

If the detonation sensors generate a diagnostic code for a problem with the electrical circuit, refer to the engine's Electronic Troubleshooting manual.