PIN AS 9R4494 - Caterpillar

Use this procedure to troubleshoot the electrical system if a problem is suspected with the primary engine speed/timing sensor or with the secondary engine speed/timing sensor. Use this procedure to troubleshoot the electrical system if any of the diagnostic codes in Table 1 are active or easily repeated.
Table 1
Diagnostic Codes Table
Code and Description Conditions which Generate this Code System Response
190-8 Engine Speed Sensor abnormal frequency, pulse width, or period The signal from the primary engine speed/timing sensor is intermittent or the signal has been lost. The code is logged. If a valid signal is not received from the primary engine speed/timing sensor, the Electronic Control Module (ECM) will default to the secondary engine speed/timing sensor. The engine will shut down only if the signals from the primary engine speed/timing sensor and the secondary engine speed/timing sensor are abnormal.
342-8 Secondary Engine Speed Sensor abnormal frequency, pulse width, or period The signal from the secondary engine speed/timing sensor is intermittent or the signal has been lost. The code is logged. If a valid signal is not received from the secondary engine speed/timing sensor, the ECM will default to the primary engine speed/timing sensor. The engine will shut down only if the signals from the primary engine speed/timing sensor and the secondary engine speed/timing sensor are abnormal. The engine uses two engine speed/timing sensors. The secondary engine speed/timing sensor monitors the camshaft gear and the primary engine speed/timing sensor monitors the crankshaft gear. Both of the engine speed/timing sensors detect the reference for engine speed and timing from a unique pattern on the respective gear. The ECM counts the time between pulses that is created by the sensor as the gear rotates in order to determine rpm.Under normal operation, the secondary engine speed/timing sensor is used to determine timing for starting purposes. The secondary engine speed/timing sensor is used to determine when the piston in the No. 1 cylinder is at the top of the compression stroke. When the timing has been established, the primary engine speed/timing sensor is then used to determine engine speed and the signal from the secondary camshaft sensor is ignored.After locating the No. 1 cylinder, the ECM triggers each injector in the correct firing order and at the correct unit injector timing. The actual timing and duration of each injection is based on engine rpm and load. If the engine is running and the signal from the primary engine speed/timing sensor is lost, a slight change in engine performance will be noticed when the ECM performs the changeover to the secondary engine speed/timing sensor. Loss of the signal from the secondary engine speed/timing sensor during engine operation will not result in any noticeable change in engine performance. However, if the signal from the secondary engine speed/timing sensor is not present during start-up the following conditions may exist:
The engine may require a slightly longer period of time to start up.
The engine may run rough for a few seconds until the ECM determines the proper firing order by