Use this procedure under the following conditions:
Use this procedure if the following diagnostic code is indicated:
- 91-8 Throttle Position Signal Abnormal
Also, use this procedure if the digital speed control device is suspected of improper operation.
Speed Control Signal
The digital speed control device is used to provide a digital signal to the Electronic Control Module (ECM). The output from the device is a constant frequency signal with a fixed pulse width that corresponds to the required engine speed. This output signal is referred to as a pulse width modulated (PWM) signal and this output signal is expressed as a percentage or as a duty cycle.
The digital signal is generated by the system that controls the speed of the engine.
In other applications, the control system is referred to as an accelerator or a throttle. These terms are still displayed on the electronic service tool for this application.
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| Illustration 1 | g01810534 |
Typical schematic of the digital speed demand device | |
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| Illustration 2 | g01810793 |
Typical example of the pin locations on the P1 connector (33) Digital speed demand device return (53) Digital speed demand device input | |
Test Step 1. Inspect Electrical Connectors and Wiring
- Inspect the P1/J1 connector, OEM harness and the associated connectors. Thoroughly inspect the connector on the digital speed control device. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
- Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the digital speed control device:
- P1:33
- P1:53
- P1:33
- Check the screw for the ECM connector for the correct torque of 5.0 N·m (44 lb in).
- Check the harness for abrasion and pinch points from the digital throttle position sensor to the ECM.
Expected Result:
All connectors, pins and sockets are completely coupled and/or inserted and the harness is free of corrosion, abrasion or pinch points.
Results:
- OK - Proceed to Test Step 2.
- Not OK -
Repair: Perform the following repair:Repair the connectors or the harness and/or replace the connectors or the harness. Ensure that all of the seals are correctly in place and ensure that the connectors are completely coupled.Verify that the repair eliminates the fault.
STOP
Test Step 2. Check for Active Diagnostic Codes
- Connect the electronic service tool to the diagnostic connector.
- Turn the keyswitch to the ON position.
- Monitor the active diagnostic code screen on the electronic service tool. Check and record active diagnostic codes.
Expected Result:
Result 1 Diagnostic code 91-8 is active.
Result 2 There are no active diagnostic codes that are associated with the speed control circuit at this time. However, a fault is suspected.
Results:
- Result 1 - Proceed to Test Step 3.
- Result 2 - There may be an intermittent connection in the wiring. Proceed to Test Step 3.
Test Step 3. Check the Control Signal
- Refer to the OEM supplied manual for the control system.
- Check that the output signal from the control system is correct.
Expected Result:
The output signal from the control system should be within the specification in the OEM supplied manual.
Results:
- OK - The output signal from the control system is correct. Proceed to Test Step 4.
- Not OK - The output signal from the control system is not correct or the control system is incorrectly configured.
Repair: Repair the control system or configure the control system. Refer to the OEM supplied manual.Verify that the repair eliminates the fault.
STOP
Test Step 4. Check the Signal at the ECM
- Turn the keyswitch to the OFF position.
- Remove the P1 connector.
- Temporarily remove the pin from P1:53.
- Reconnect the P1 connector.
- Connect the red probe of a multimeter to the removed pin and the black probe of the multimeter to P1:33.
- Turn the keyswitch to the ON position.
- Check that the signal at the ECM is the same value as the output signal from the control system.
- Turn the keyswitch to the OFF position.
- Remove the P1 connector and reinstall P1:53.
- Reconnect P1 connector.
Expected Result:
The signal at the ECM is the same value as the output signal from the control system.
Results:
- OK - The terminals on the ECM have the correct voltage for the control system.
Repair: Perform the following repair:
- Make sure that the latest flash file for the application is installed in the ECM. Refer to Troubleshooting, "Flash Programming".
- Contact the Technical Communicator.
Note: This consultation can greatly reduce the repair time.
- If the Technical Communicator recommends the use of a test ECM, install a test ECM. Refer to Troubleshooting, "Replacing the ECM".
- Use the electronic service tool to recheck the system for active diagnostic codes.
- If the fault is resolved with the test ECM, reconnect the suspect ECM.
- If the fault returns with the suspect ECM, replace the ECM.
- Use the electronic service tool in order to clear all logged diagnostic codes and then verify that the repair eliminates the fault.
STOP
- Not OK - There is a fault in the harness or the connectors between the sensor and the ECM. Check all of the connections between the ECM and the control system. Repair the damaged cables or replace the damaged cables. Check that the repairs have eliminated the fault.STOP