SEAL 4H6520 - Caterpillar

This procedure covers the following diagnostic codes:
Table 1
Diagnostic Codes for the Speed/Timing Sensors
J1939 Code Code Description Comments
4201-2 Engine Speed Sensor #1 : Erratic, Intermittent, or Incorrect The Electronic Control Module (ECM) detects the following conditions:
An intermittent loss of signal or a complete loss of signal from the primary speed/timing sensor for 2 seconds
Engine speed at least 300 rpm for more than 0.5 seconds
The warning light will come on and the diagnostic code will be logged.
The ECM will use the signal from the secondary speed/timing sensor.
The engine will be derated. If the signal from the secondary speed/timing sensor is also lost, the engine will stop.
4201-8 Engine Speed Sensor #1 : Abnormal Frequency, Pulse Width, or Period The Electronic Control Module (ECM) detects the following conditions:
An abnormal signal from the primary speed/timing sensor
Engine speed at least 300 rpm for more than 0.5 seconds
The warning light will come on and the diagnostic code will be logged.
The ECM will use the signal from the secondary speed/timing sensor.
The engine will be derated. If the signal from the secondary speed/timing sensor is also lost, the engine will stop.
723-2 Engine Speed Sensor #2 : Erratic, Intermittent, or Incorrect The Electronic Control Module (ECM) detects the following conditions:
An intermittent loss of signal or a complete loss of signal from the secondary speed/timing sensor for 2 seconds
Engine speed at least 300 rpm for more than 0.5 seconds
The warning light will come on and the diagnostic code will be logged.
The ECM will use the signal from the primary speed/timing sensor.
The engine will be derated. If the signal from the primary speed/timing sensor is also lost, the engine will stop.
723-8 Engine Speed Sensor #2 : Abnormal Frequency, Pulse Width, or Period The Electronic Control Module (ECM) detects the following conditions:
An abnormal signal from the secondary speed/timing sensor
Engine speed at least 300 rpm for more than 0.5 seconds
The warning light will come on and the diagnostic code will be logged.
The ECM will use the signal from the primary speed/timing sensor.
The engine will be derated. If the signal from the primary speed/timing sensor is also lost, the engine will stop.
Follow the troubleshooting procedure to identify the root cause of the fault.
Illustration 1 g03838252
Speed/timing sensor locations
(1) Primary speed/timing sensor
(2) Secondary speed/timing sensor
Illustration 2 g03838253
Speed/timing sensors
(1) Primary speed/timing sensor
(2) Secondary speed/timing sensorThe engine uses two engine speed/timing sensors. The primary speed/timing sensor is on the right-hand side of the flywheel housing. The primary speed/timing sensor generates a signal by detecting the movement of the holes around the rim of the flywheel. The signal