EDGE-CUTTING 4524147 - Caterpillar

Illustration 1 g00288430
Typical Square Wave Pulse Width Modulated Duty Cycle Signal
When powered up, a position sensor will continuously send a Pulse Width Modulated (PWM) square wave signal to the ECM. The frequency of the signal is most often 500 hertz. Some sensors operate at 5000 hertz. The operating frequency stays at approximately the same rate during operation. During operation, the duty cycle of the PWM sensor signal changes depending on the direction and amount of movement on the axis that is being monitored. The duty cycle is the percentage of time that the signal is high as compared to the total time for one cycle. The percentage of duty cycle signal from a typical position sensor that the ECM will recognize as valid is 10 5 percent to 90 5 percent at the extreme ends of the axis movement. An axis that is in the center or neutral position would result in a duty cycle signal of approximately 50 6 percent.The sensor signal can also be measured as voltage. Most PWM sensor signals will measure an RMS (average) voltage of between 0.0 VDC to 5.0 VDC depending on the duty cycle value. A low duty cycle results in a low voltage measurement. A high duty cycle results in a higher voltage value. A valid duty cycle range that the ECM will accept of 10 5 percent to 90 5 percent equates to a voltage range of 0.5 VDC to 4.5 VDC.In order to aid in ECM circuit diagnostics, all of the PWM sensor signal input circuits have an internal "pull up voltage". An above normal voltage is internally connected to the ECM sensor PWM signal input circuit through a resister.During normal operation, the sensor signal will hold the circuit at a certain signal amplitude, however, circuit conditions such as a loss of power to the component, a disconnection or an open circuit will allow the circuit to be pulled high by the ECM pull up voltage. This will result in an above normal voltage condition at the ECM contact. As a result, the ECM will activate a diagnostic code for the affected circuit. When the ECM detects an abnormal condition in the circuit, the ECM will activate a diagnostic code for the PWM sensor signal circuit. The ECM will most often monitor the circuit for the following FMI conditions:FMI 02 - Data erratic, intermittent or incorrectFMI 03 - Voltage above normalFMI 04 - Voltage below normalFMI 08 - Abnormal frequency, pulse width or periodFMI 13 - Out of calibrationNote: The ECM may activate diagnostic codes for other FMI conditions, however, the FMI conditions that are listed are the most common.When the ECM activates an FMI 13 diagnostic code for a particular sensor, a calibration procedure must be performed in order to maximize performance in the involved system.A calibration procedure for a PWM sensor enables the ECM to "map" a specific position for a cylinder or pedal to a specific sensor signal. This allows the