KNOB AS 2039991 - Caterpillar

Machine ECM
The following is a list of Diagnostic Trouble Codes (DTCs) that are associated with the speed sensors of the machine.
Table 1
Machine ECM (MID 39)
DTC Code Description System Response
2776-8 Drive Motor Speed Sensor #2: Abnormal frequency, pulse width, or period Hystat downshift performance is reduced. This can affect deceleration and rimpull.
3271-8 Drive Motor Speed Sensor #1 Signal Line #1: Abnormal frequency, pulse width, or period Machine speed is greatly limited.
3272-8 Drive Motor Speed Sensor #1 Signal Line #2: Abnormal frequency, pulse width, or period Machine speed is greatly limited. Possible causes for an FMI 8 diagnostic code are:
The sensor has failed.
The spacing of the sensor is incorrect.
Intermittent connections or poor connections.
Mechanical devices are loose.
Incorrect software is loaded to the ECM.
The ECM has failed. ECM failure is unlikely.
Illustration 1 g03470396
Schematic of the Speed Sensors
The preceding diagram is a simplified schematic of the Machine ECM and the Speed Sensor connections. The schematic is electrically correct. However, not all the possible harness connectors are shown. Refer to the latest revision of the Electrical Schematic to view the entire circuitry of the machine.Diagnostic Trouble Code Procedure
Prior to beginning this procedure, inspect the harness connectors that are involved in this circuit. Poor connections can often be the cause of a problem in an electrical circuit. Verify that all connections in the circuit are clean, secure, and in good condition. If a problem with a connection is found, correct the problem and verify that the diagnostic code is active before performing a troubleshooting procedure.
Table 2
FMI 8 Troubleshooting
1. Check The Speed Sensor Circuits For An Open
A. Turn the key switch and the disconnect switch to the OFF position.
B. Disconnect the sensor from the ECM.
C. Disconnect the J1 and J2 wire harness from the ECM.
D. At the harness connector for the sensor, place a jumper wire between the signal contact and the supply contact.
E. At machine harness connector for ECM, measure the resistance from the signal contact the supply contact.
There should be continuity (less than 5.0 Ω resistance) OK -The resistance measurement is less than 5 ohms. The resistance measurement for the speed sensor circuits is not indicating that there is an open circuit or a poor connection in the speed sensor circuits between the ECM and the speed sensor.
Proceed to Test Step 2.
NOT OK -The resistance measurement is greater than 5 Ω at all times or the resistance measurement is greater than 5 Ω part of the time during the manipulation of the circuit wiring. There is an open circuit or a poor connection in one of the speed sensor circuits
Repair:Repair or replace the machine harness
Proceed to Test Step 3.
2. Check The Speed Sensor Signal Circuit For A Short
Remove the jumper between contact 1 and contact 2 for the machine harness connector for the speed sensor
B. Reconnect the speed sensor to the machine harness
C. At the J1 and J2 harness connectors, measure the resistance between speed sensor power and all possible sources of positive voltage and ground
D. Repeat the previous Test Step for the speed sensor