Use this procedure to troubleshoot the electrical system if a problem is suspected with the speed control or if the diagnostic code in Table 1 is active or easily repeated.
| Diagnostic Codes Table | ||
|---|---|---|
| Description | Conditions which Generate this Code | System Response |
| 91-8 Throttle Position signal abnormal | The Engine's Electronic Control Module (ECM) detects a sensor frequency that is less than 150 Hz or a sensor frequency that is greater than 1000 Hz for two seconds.
or The ECM detects a duty cycle that is less than 5 percent or a duty cycle that is greater than 95 percent for one second. |
The code is logged.
The throttle signal defaults to a zero percent duty cycle (low idle). |
The speed control is used to provide a throttle signal to the engine control. The output for rated speed is a pulse width modulated signal. The signal varies with the position of the speed adjust potentiometer. The output signal is expressed as a percentage between 0 and 100 percent for the speed adjust potentiometer.
The ECM calculates the desired engine speed. The desired engine speed is produced by the signal from the rated speed setting. The rated speed signal is valid when the duty cycle is in the range of 5 to 95 percent. If the rated speed signal is invalid, the engine will be set to the low idle.
| |
| Illustration 1 | g01228453 |
|
Schematic for the speed control circuit | |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Turn the engine control to the OFF position.
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Illustration 2 g01228349 Location of the connectors for the speed control circuit(typical left side engine view)
(1) J1/P1 ECM connectors
(2) Customer connector - Thoroughly inspect connectors (1) and (2). Also, thoroughly inspect all of the other connectors that are in the circuit. Refer to the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors" for details.
- Check the allen head screw of each of the ECM connectors for the proper torque. Refer to Troubleshooting, "Inspecting Electrical Connectors" for the correct torque value.
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Illustration 3 g01228457 P1 ECM connector
(P1-05) Digital return
(P1-10) Primary throttleShow/hide table
Illustration 4 g01228458 Customer connector
(Terminal 09) Digital return
(Terminal 10) Primary throttle - Perform a
45 N (10 lb) pull test on each of the wires in the ECM connector and the customer connector that are associated with the circuit. - Check the harness and the wiring for abrasion and for pinch points.
Expected Result:
The components and wiring are free of the following problems: damage, corrosion, abrasion and incorrect attachment.
Results:
- OK - The wiring and connectors are OK. Proceed to Test Step 2.
- Not OK - The wiring and/or connectors need repair.
Repair: Repair the connectors or wiring and/or replace the connectors or wiring.
STOP
Test Step 2. Check the Voltage at the Speed Control
- Turn the engine control to the ON position.
- Measure the voltage between terminals +B and −B of the speed control.
Expected Result:
The voltage at the speed control is 24 ± 3 VDC.
Results:
- OK - The voltage supply to the speed control is correct. Proceed to Test Step 3.
- Not OK - The voltage at the speed control is not 24 ± 3 VDC.
Repair: There is a problem with the supply voltage to the speed control. Make the necessary repairs to the circuit for the voltage supply to the speed control.
STOP
Test Step 3. Check the Status of the Throttle Position on Caterpillar Electronic Technician (ET)
- Observe the status of the throttle position on Cat ET.
- Turn the speed adjust potentiometer to the lowest setting.
- Start at the low idle position. Slowly turn the speed adjust potentiometer in the other direction. Monitor the status of the throttle position on the Cat ET.
Expected Result:
The throttle position on the Cat ET reads 0 percent at the lowest setting and 0 to 100 percent as the speed adjust potentiometer is adjusted.
Results:
- OK - The speed control is operating correctly.
Repair: There may be an intermittent problem. If an intermittent problem is suspected perform the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors".
STOP
- Not OK - The throttle position on Cat ET does not change correctly. Proceed to Test Step 4.
Test Step 4. Check the Output of the Speed Control
- Remove the wire from terminal S at the speed control.
- Set the multimeter to "VDC". Press the "Hz" button twice so that the % symbol is displayed.
- Place one probe of the multimeter on the S terminal of the speed control. Place the other probe on the −B terminal of the speed control.
- Monitor the output of the percent duty cycle from the speed control on the multimeter. Turn the potentiometer from the low setting to the high setting.
Note: The throttle position on Cat ET will not exactly match the percent duty cycle on the multimeter.
Expected Result:
The duty cycle is between 4 and 10 percent at the low setting and between 90 and 95 percent at the high setting.
Results:
- OK - The speed control is supplying the correct signal. Proceed to Test Step 5.
- Not OK - The duty cycle is not correct on the multimeter.
Repair: Check the speed adjust potentiometer for proper operation. If the speed adjust potentiometer is operating correctly, temporarily install another speed control. Verify that the problem is no longer present. Reinstall the old speed control. If the problem returns replace the speed control.
STOP
Test Step 5. Check the Throttle Signal at the ECM
- Turn the engine control to the OFF position.
- Disconnect the J1/P1 ECM connectors.
- Remove the wire from terminal location P1-10 (primary throttle) at the ECM connector. Also, remove the wire from terminal location P1-5 (digital return) at the ECM connector.
- Connect the J1/P1 ECM connectors.
- Turn the engine control to the ON position.
- Measure the duty cycle between the wires for the primary throttle signal and the digital return.
Note: Set the multimeter to "VDC". Press the "Hz" button twice so that the % symbol is displayed.
- Turn the speed adjust potentiometer from the low setting to the high setting. Monitor the output of the percent duty cycle from the speed control on the multimeter.
- Turn the engine control to the OFF position. Reconnect all wires and connectors to the original configuration.
Expected Result:
The duty cycle is between 4 and 10 percent at the low setting and between 90 and 95 percent at the high setting.
Results:
- OK - The duty cycle is correct. Proceed to Test Step 6.
- Not OK - The duty cycle is not correct.
Repair: There is a problem in the wiring between the ECM connector and the speed control. Make the necessary repairs to the harness wiring.
STOP
Test Step 6. Check for Proper Operation of the ECM
- Turn the engine control to the ON position.
- Monitor the percent throttle position on the Cat ET. Turn the speed adjust potentiometer from the low setting to the high setting.
Expected Result:
The percent throttle position is between 0 and 100 percent for the above settings of the speed adjust potentiometer.
Results:
- OK - The status for the throttle position on Cat ET is correct.
Repair: The ECM is operating properly. There may be an intermittent problem in the harness or in a connector. If an intermittent problem is suspected, perform the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors".
STOP
- Not OK - The status for the throttle position on Cat ET is not correct. The ECM is not processing the signal correctly.
Repair: Replace the ECM. Refer to the diagnostic procedure Troubleshooting, "Replacing the ECM".
STOP