The engine speed control is used to provide a throttle signal via the throttle position sensor 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 Electronic Control Module (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.
Some applications may choose not to use the speed control. Some applications may use a load sharing unit. The unit is configured to provide the engine control with a rated speed signal. Some of these units can not provide a signal until the engine is running. In these cases, troubleshooting should be performed while the engine is running.
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| Illustration 1 | g00894806 |
Typical example | |
Test Step 1. Inspecting Electrical Connectors
- Turn the engine control to the OFF position.
- Thoroughly inspect the ECM connectors J1/P1 and J2/P2. Inspect all of the other connectors. Refer to the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors" for details.
- Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the circuit.
- Check the ECM connector (allen head screw) for the proper torque of 6.0 N·m (55 lb in).
- Check the customer connector (allen head screw) for the proper torque of 2.25 ± 0.25 N·m (20 ± 2 lb in).
- Check the harness and the wiring for abrasion and pinch points.
Expected Result:
The components and wiring should be free of the following problems: damage, corrosion, abrasion and incorrect attachment.
Results:
- OK - The wiring and connectors are okay. 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.
- Connect the red probe of the digital multimeter to the + Battery terminal of the Speed Control. Connect the black probe of the digital multimeter to the − Battery terminal of the Speed Control.
Expected Result:
The digital multimeter should read 24 ± 5 VDC.
Results:
- OK - The voltage supply to the Speed Control is correct. Proceed to test step 3.
- Not OK - The digital multimeter did not read 24 ± 5 VDC.
Repair: There is a problem with the supply voltage to the Speed Control.
STOP
Test Step 3. Check the Throttle Position on the Caterpillar Electronic Technician (ET).
- Observe the position of the throttle on the 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 position of the throttle on the Cat ET.
Expected Result:
The Throttle Position on the Cat ET should read 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 the problem is intermittent 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 Speed Control.
- Remove the wire from the S terminal 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 − Battery 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 the Cat ET will NOT match the percent duty cycle on the multimeter.
Expected Result:
The Duty Cycle should be 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: 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 Input to the ECM.
- Turn the engine control to the ON position.
- Set the multimeter to "VDC". Press the "Hz" button twice so that the % symbol is displayed.
- Turn the circuit breaker at the battery to the ON position.
- Measure the PWM signal between ECM connector P2-24 and ECM connector P1-29.
- Monitor the output of the percent duty cycle from the Speed Control on the multimeter. 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 Duty Cycle should be between 4 and 10 percent at the low setting and between 90 and 95 percent at the high setting. The percent Throttle Position should be between 0 and 100 percent for the above settings of the Speed Adjust Potentiometer.
Results:
- OK - The Duty Cycle is correct.
Repair: There may be an intermittent problem. If the problem is intermittent perform the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors".
STOP
- Duty Cycle Not OK - The Duty Cycle is not correct.
Repair: There is a problem in the wiring between P2-24 and the S terminal of the Speed Control. Verify that the low idle switch is functioning properly.
STOP
- Throttle Position Not OK - The Duty Cycle is correct but the Throttle Position is not correct. The ECM is not processing the correct signal.
Repair: Replace the ECM. Refer to the diagnostic procedure Troubleshooting, "Replacing the ECM".
STOP