ROD 1246913 - Caterpillar

Which Generate This Code:This diagnostic code is recorded when the ECM reads an abnormal frequency from the engine speed sensor.Note: The engine must be running in order to show that this diagnostic code is present. The possible causes of this diagnostic code are listed below:
The machine harness is shorted or the harness is open.
The sensor has failed.
The ECM has failed. This is unlikely.System Response:The results of this diagnostic code are listed below:
A warning (Category 1) is shown on the monitor.
The underspeed function will not operate. The engine may lug excessively during a normal loading cycle. The engine may stall during a normal loading cycle. Test Step 1. CHECK THE CASE OF THE SENSOR .
Turn the key start switch to the OFF position.
Disconnect the machine harness from the speed sensor.
At the sensor, measure the resistance from contact 1 (wire 451-BR) and from contact 2 (wire 450-YL) to the case of the sensor.Expected Result:The resistance should be greater than 10000 ohms.Results:
OK - The resistance of the case is correct. Proceed to test step 2.
NOT OK - The resistance of the case is not correct.Repair: The speed sensor has failed. Replace the sensor. Adjust the speed sensor properly when you install the sensor. See Testing and Adjusting, "Speed Sensor - Adjust". Verify that the new sensor corrects the problem. Stop.Test Step 2. CHECK THE SENSOR.
The key start switch remains in the OFF position. The sensor remains disconnected from the machine harness.
At the sensor, measure the resistance between contact 1 (wire 451-BR) and contact 2 (wire 450-YL).Expected Result:The resistance should be between 110 and 200 ohms.Results:
OK - The sensor resistance is correct. Proceed to test step 3.
NOT OK - The sensor resistance is not correct.Repair: The speed sensor is failed. Replace the sensor. Adjust the speed sensor properly when you are installing the sensor. See Testing and Adjusting, "Speed Sensor - Adjust". Verify that the new sensor corrects the problem. Stop.Test Step 3. CHECK THE HARNESS FOR AN OPEN.
The key start switch remains in the OFF position.
Reconnect the machine harness to the sensor.
Disconnect the machine harness connectors J1 and J2 from the ECM.
At the machine harness connector J1, measure the resistance from contact J1-40 (wire 450-YL) to contact J1-41 (451-BR). Contact J1-40 corresponds to +signal, while contact J1-41 corresponds to -signal.Expected Result:The resistance should be approximately between 110 and 200 ohms.Results:
OK - The measurement is within an acceptable range. Proceed to test step 4.
NOT OK - The measurement is not within an acceptable range. Repair: The sensor signal circuits have failed. Wire 450-YL or wire 451-BR is open in the machine harness. Repair the machine harness or replace the machine harness. Stop.Test Step 4. CHECK THE HARNESS FOR A SHORT TO THE RETURN.
The key start switch remains in the OFF position. The ECM remains disconnected from the machine harness.
Disconnect the machine harness from the sensor.
At the machine harness connectors J1 and J2, measure the resistance from contact J1-40 (wire 450-YL) to contact J1-12 (wire J764-BR). Also, measure the resistance from contact J1-41