CONE-SPECIAL TAPERED 1396871 - Caterpillar

Illustration 1 g01310588
Schematic for the Service Brake SwitchThe Service Brake Switch has two inputs to the ECM. The two inputs are normally open and normally closed. During normal operation, one of the inputs is open. The other input is grounded. When a circuit is measured with a voltmeter, an open circuit will measure approximately 24 DCV to ground and a closed circuit will measure approximately 0 DCV to ground. An FMI 04 diagnostic code is recorded when both of the inputs are grounded.The possible causes of this diagnostic code are listed below:
The "normally open" signal circuit (wire C992-PU) is always shorted to ground.
The "normally closed" signal circuit (wire F931-PK) is always shorted to ground.
The switch has failed.
The ECM has failed. This is unlikely. Before performing this procedure, inspect the harness connectors that are involved in the 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 this diagnostic code is active before performing this procedure.Note: The following test procedure may create other diagnostic codes. Ignore these created diagnostic codes and clear these diagnostic codes when the original diagnostic code has been corrected. Ensure that the diagnostic code of CID 0291 FMI 06 is active before performing this procedure.
Table 1
Status of the Switch    
MODE    Switch Position    J2-28 (N/O)
C992-PU Circuit    J2-23 (N/C)
F931-PK Circuit    
Released     Up     Open     Closed    
Pressed     Down     Closed     Open    System Response:A level 1 warning is generated. Component functionality is lost. The ECM assumes switch is always released.Test Step 1. CHECK THE SWITCH
Check for continuity between contact 1 and contact 2 of the switch.
Press the switch and then release the switch while checking for continuity.
Check for continuity between contact 2 and contact 3 of the switch.
Press the switch and then release the switch while checking for continuity.Expected Result:The continuity check agrees with the preceding Table.Results:
OK - The continuity check agrees with the preceding Table. Proceed to Test Step 2.
NOT OK - The continuity check does not agree with the preceding Table. The switch has failed.Repair: Replace the switch. Verify that the new switch corrects the diagnostic code.STOPTest Step 2. CHECK THE SIGNAL CIRCUITS FOR A SHORT TO GROUND
Reconnect the switch to the machine harness.
The machine harness connectors remain disconnected from the ECM.
At the machine harness connector for the ECM, check for continuity between contact J2-28 (wire C992-PU) and frame ground.
Press the switch and then release the switch while checking for continuity.
At the machine harness connector for the ECM, check for continuity between contact J2-23 (wire F931-PK) and frame ground.
Press the switch and then release the switch while checking for continuity.Expected Result:The continuity check agrees with the preceding Table.Results:
OK - The continuity check agrees with the preceding Table.Repair: If the cause of the problem is not found, contact the Technical Communicator for your dealership