Use this procedure to troubleshoot the electrical system 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 Sensor abnormal frequency, pulse width, or period | 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 output signal is expressed as a percentage between 0 and 100 percent.
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.
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| Illustration 1 | g01287955 |
Typical schematic | |
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| Illustration 2 | g01287957 |
P1 ECM connector (P1-10) Primary throttle | |
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| Illustration 3 | g01285750 |
Location of ECM connectors | |
Test Step 1. Inspecting Electrical Connectors and Wiring
- Turn the "main disconnect switch (MDS)" to the OFF position.
- Thoroughly inspect the ECM connector J1/P1. Inspect all of the other connectors. Refer to the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors" for details.
- Check the ECM connector (allen head screw) for the proper torque. Refer to 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 harness and the wiring for abrasion and 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 Throttle Position on Caterpillar Electronic Technician (ET)
- Turn the "MDS" to the ON position.
- Observe the position of the throttle on Cat ET.
- Adjust the throttle from the lowest setting to the highest setting.
- Monitor the position of the throttle on Cat ET.
Expected Result:
The throttle position on Cat ET reads 0 percent at the lowest setting and 0 to 100 percent as the throttle is adjusted.
Results:
- OK - The throttle signal is being received by the ECM.
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 3.
Test Step 3. Check the Throttle Signal at the Main Junction Box
- Turn the "MDS" to the OFF position.
- Remove wire F702-RD/BK to the ECM from terminal TBI-7 in the main junction box.
- Set the multimeter to "VDC". Press the "Hz" button twice so that the % symbol is displayed.
- Place one probe of the multimeter on terminal TBI-7 in the main junction box. Place the other probe on terminal TB-2 in the main junction box.
- Turn the "MDS" to the ON position.
- Monitor the output of the percent duty cycle of the throttle signal on the multimeter. Adjust the throttle from the low setting to the high setting.
- Replace all wires 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 throttle signal is correct. Proceed to Test Step 4.
- Not OK - The duty cycle is not correct on the multimeter.
Repair: The problem is in the customer supplied throttle signal. Refer to the original equipment manufacturer (OEM) for the correct procedure for troubleshooting the problem.
STOP
Test Step 4. Check the Throttle Signal at the ADEM Box
- Turn the "MDS" to the OFF position.
- Remove wire F702-RD/BK from P1-10 at the ECM connector P1.
- Turn the "MDS" to the ON position.
- Measure the duty cycle between wire F702-RD/BK and terminal 2 on the terminal strip in the ADEM box.
Note: Set the multimeter to "VDC". Press the "Hz" button twice so that the % symbol is displayed.
- Monitor the output of the percent duty cycle from the throttle signal on the multimeter. Adjust the throttle from the low setting to the high setting.
- Turn the "MDS" 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 5.
- Duty Cycle Not OK - The duty cycle is not correct.
Repair: There is a problem in the wiring between ECM connector P1-10 and terminal TBI-7 in the main junction box. Repair the wire and/or connectors.
STOP
Test Step 5. Check the ECM
- Turn the "MDS" to the ON position.
- Monitor the percent throttle position on Cat ET. Adjust the throttle from the low setting to the high setting.
Expected Result:
The percent throttle position is between 0 and 100 percent.
Results:
- OK - The throttle position on Cat ET is correct. There is an intermittent problem in the wiring and/or connectors between the ECM and the throttle position sensor.
Repair: If the problem is intermittent, perform the diagnostic functional test Troubleshooting, "Inspecting Electrical Connectors".
STOP
- Throttle Position Not OK - The throttle position 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