Use this procedure to troubleshoot the electrical system if a problem is suspected with the circuit that is for the throttle position sensor. Also use this procedure if the diagnostic code in Table 1 is active or easily repeated.
| Diagnostic Codes Table | ||
|---|---|---|
| Code and Description | Conditions that Generate the Code | System Response |
| 91-8 Throttle Position Sensor abnormal frequency, pulse width, or period | The Electronic Control Module (ECM) detects an incorrect frequency on the throttle signal or an incorrect duty cycle on the throttle signal. | The code is logged. The ECM flags the throttle position as invalid data. |
The load sharing module provides a throttle signal to the engine ECM. The output for rated speed is a pulse width modulated signal (PWM) at a constant frequency. The speed signal varies with the position of the speed adjust potentiometer. The output signal is referred to as a duty cycle or as a PWM signal. The output signal is expressed as a percentage between 0 and 100 percent for the speed adjust.
Note: Desired speed can be adjusted from 1141 to 1621 rpm over the range of the PWM signal for a 50 Hz application. For a 60 Hz application, the desired speed can be adjusted from 1369 to 1945 rpm over the range of the PWM signal.
The ECM calculates the desired engine rpm from the throttle signal and the droop signal. The throttle signal is valid when the duty cycle is in the range of 5 to 95 percent. If the ECM determines that the throttle signal is invalid, the engine rpm is set to rated speed and the droop percentage will be applied.
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| Illustration 1 | g02173667 |
|
Schematic of the circuit for the speed control | |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Remove electrical power from the ECM.
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Illustration 2 g02164341 Location of the J1/P1 ECM connectors (typical engine view)
(1) J1/P1 ECM connectors - 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, "Electrical Connectors - Inspect" for details.
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Illustration 3 g01207582 Terminal locations at the P1 ECM connector
(P1-5) Digital return
(P1-66) Throttle signal - Perform a
45 N (10 lb) pull test on each of the wires in the ECM connector and the engine interface connector that are associated with the circuit. - Check the allen head screw on each ECM connector and the engine interface connector for the proper torque. refer to the diagnostic functional test Troubleshooting, "Electrical Connectors - Inspect" for the correct torque values.
- Check the harness and the wiring for abrasion and for pinch points.
Expected Result:
All connectors, pins, and sockets are coupled and/or inserted, and the harness and wiring are free of corrosion, of abrasion or of pinch points.
Results:
- OK - The connectors and wiring appear to be OK. Proceed to Test Step 2.
- Not OK - The connectors and/or wiring are not OK.
Repair: Repair the connectors or wiring and/or replace the connectors or wiring.
STOP
Test Step 2. Check the Supply Voltage to the Speed Control
- Restore electrical power to the ECM.
- Measure the voltage between terminal 15 (+Battery) and terminal 16 (−Battery) on the speed control.
- Remove electrical power from the ECM.
Expected Result:
The voltage is 24 ± 3 V.
Results:
- OK - The voltage is 24 ± 3 V. The voltage supply to the speed control is correct. Proceed to Test Step 3.
- Not OK - The voltage is not 24 ± 3 V.
Repair: There is a problem in a harness or a connector between the batteries and the speed control. The problem may be in the circuit breaker or the batteries.
Perform the following procedure:
- Check the voltage between engine ground and the following test points. If voltage is not present at a test point, the problem is in the device, wire, or connector that precedes the point in the circuit. Refer to Illustration 1 as a reference.
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Table 2 Device Test location Battery +Battery terminal Auxiliary breaker Input terminal Output terminal Engine interface connector Terminal 4 - Measure the voltage between the +Battery terminal of the battery and the following test points. If voltage is not present at a test point, the problem is in the device or the circuit that precedes the point in the circuit. Refer to Illustration 1 as a reference.
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Table 3 Device Test location Battery disconnect switch Input terminal Output terminal Engine interface connector Terminal 39
If a problem with the batteries is suspected, refer to the diagnostic functional test Operating Manual, SEHS9249 and Special Instruction, SEHS7633 for information that is related to testing the batteries.
STOP
- Check the voltage between engine ground and the following test points. If voltage is not present at a test point, the problem is in the device, wire, or connector that precedes the point in the circuit. Refer to Illustration 1 as a reference.
Test Step 3. Check the Status for the Throttle Position on Caterpillar Electronic Technician (ET)
- Connect Cat ET to the service tool connector. Observe the status parameter for the throttle position on Cat ET.
- Restore electrical power to the ECM.
- Turn the speed adjust potentiometer to the lowest setting.
- Adjust the speed adjust potentiometer from the lowest setting to the highest setting. Monitor the status for "Throttle Position" on Cat ET while you perform the test.
- Remove electrical power from the ECM.
Expected Result:
The status of the "Throttle Position" on Cat ET changes proportionally as the speed adjust potentiometer is adjusted.
Results:
- OK - The speed control is operating correctly.
Repair: There may be an intermittent problem in a harness or a connector. If an intermittent problem is suspected, refer to Troubleshooting, "Inspecting Electrical Connectors" for troubleshooting information.
STOP
- Not OK - The status of "Throttle Position" on Cat ET does not vary as the speed adjust potentiometer is adjusted. There is a problem in the circuit between the speed control and the ECM. Proceed to Test Step 4.
Test Step 4. Check the PWM Signal at the Speed Control
- Remove the wires from terminal locations 19 (+PWM out) and 20 (−PWM out) at the speed control.
- Use a multimeter capable of reading a percent duty cycle to measure the output at terminal locations 19 and 20 at the speed control.
- Restore electrical power to the ECM.
- Monitor the output of the percent duty cycle from the terminal locations at the speed control on the multimeter. Turn the speed adjust potentiometer from the low setting to the high setting.
- Remove electrical power from the ECM.
Restore the wiring to the original configuration.
Expected Result:
The duty cycle changes as the speed adjust potentiometer is adjusted.
Results:
- OK - The throttle signal is present at the speed control. Proceed to Test Step 5.
- Not OK - The throttle signal is not present at the speed control.
Repair: There is a problem with the speed control. Replace the speed control. Verify that the problem is no longer present.
STOP
Test Step 5. Check the PWM Signal at the Engine Interface Connector
- Disconnect the engine interface connectors.
- Remove the wires from terminal locations 15 (digital return) and 21 (rated speed) on the speed control side of the engine interface connector.
- Connect the engine interface connectors.
- Measure the percent duty cycle output at the wires from terminal locations 15 and 21 at the engine interface connectors.
- Restore electrical power to the ECM.
- Monitor the output of the percent duty cycle from these wires on the multimeter. Turn the speed adjust potentiometer from the low setting to the high setting.
- Remove electrical power from the ECM.
Restore the wiring to the original configuration.
Expected Result:
The throttle signal is reaching the engine interface connectors.
Results:
- OK - The throttle signal is reaching the connector. Proceed to Test Step 6.
- Not OK - The throttle signal is not reaching the connector.
Repair: Repair the connectors or wiring and/or replace the connectors or wiring between the speed control and the engine interface connector.
STOP
Test Step 6. Check the PWM Signal at the ECM
- Disconnect the J1/P1 ECM connectors.
- Remove the wires from terminals P1-5 (digital return) and P1-66 (throttle signal) at the ECM connector.
- Connect the J1/P1 ECM connectors.
- Measure the percent duty cycle output at the wires from terminals P1-5 (digital return) and P1-66 (throttle signal) at the ECM connector.
- Restore electrical power to the ECM.
- Monitor the output of the percent duty cycle from the speed control on the multimeter. Turn the speed adjust potentiometer from the low setting to the high setting.
- Remove electrical power from the ECM.
Expected Result:
The throttle signal is present at the connector.
Results:
- OK - The throttle signal is present at the connector.
Repair: The ECM is not detecting the signal correctly. There may be a problem with the ECM. Before you consider replacing the ECM, consult the Dealer Solution Network (DSN).
STOP
- Not OK - The throttle signal is not present at the ECM connector.
Repair: The problem is in the harness between ECM connector P1 and the engine interface connectors. There may be a problem with a connector. Repair the connectors or wiring and/or replace the connectors or wiring.
STOP