TUBE 5003698 - Caterpillar

The Integrated Sensing Module (ISM) supplies 8 VDC to power the sensors. The detonation sensors provide electrical signals to the module that indicate mechanical engine vibrations. Each sensor outputs an electrical signal. The signal is amplified and the signal is filtered. The frequency of the signal corresponds to the mechanical frequency of the vibrations. The amplitude of the signal is proportional to the intensity of the vibrations.The ISM monitors the signals to determine the presence and the severity of the detonation. The ISM can retard the timing of the cylinders to limit detonation levels. The timing may be retarded for a single cylinder or for more than one cylinder. The timing may be retarded for all the cylinders, if necessary. If retardation of the timing does not sufficiently limit the detonation, the ISM will shutdown the engine.An ISM can retard timing by as few as three degrees for light detonation levels. The timing can be retarded up to six degrees for severe detonation. For most applications, the minimum allowable actual timing is ten degrees BTC (five degrees BTC for propane operation). A proportional strategy is used for advancing the timing after the timing has been retarded. The rate of advance is based on the level of detonation. The rate is faster for lighter detonation. The fastest rate of proportional timing advance is one degree per minute.The ISM will diagnose the detonation sensors for a signal that is shorted to the −Battery side, to the +Battery side, or for an open circuit. To avoid detecting vibrations that are not related to detonation, the ISM only monitors a detonation sensor when one of the pistons that is monitored by that sensor is between top center and 40 degrees after top center on the power stroke. Therefore, the “Block Tap” method of testing the detonation sensors cannot be performed on this engine.An input from a detonation sensor that is diagnosed by an ISM as "open/shorted to +battery" may measure 0 VDC on a voltmeter. This is caused by the lack of pull-up resistors in the detonation sensor circuit inside the ISM.The ISM also supports related event codes when the levels of detonation warrant a reaction from the ISM. If the timing has been retarded by the maximum amount and the level of detonation remains high, the ISM will shutdown the engine. Detonation protection is disabled when the engine speed is less than 250 rpm.Logged diagnostic codes provide a historical record. Before you begin this procedure, use the Cat Electronic Technician (ET) to print the logged codes to a file.The most likely causes of the diagnostic code are a poor connection or a problem in a wiring harness. The next likely cause is a problem with a sensor. The least likely cause is a problem with an ISM.The troubleshooting procedure may generate more diagnostic codes. Keep your mind on correcting the cause of the original diagnostic code. Clear the diagnostic codes after the problem is resolved.Note: The most likely cause of a