Use this procedure to troubleshoot the electrical system if a problem is suspected with the throttle position sensor or if the diagnostic code in Table 1 is active or easily repeated.
| Diagnostic Codes Table | ||
|---|---|---|
| Code and Description | Conditions which Generate this 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 frequency is below 150 Hz or the frequency in above 1000 Hz. The duty cycle is less than 5 percent or the duty cycle is greater than 95 percent. | The code is logged. The ECM flags the throttle position as invalid data and a default value of zero percent is used. The engine speed is limited to low idle. The ECM will disable the automatic control of engine speed through the throttle lock feature. |
The throttle position sensor provides the throttle signal to the ECM. The throttle position sensor is mounted to a pedal assembly that allows the operator to control the speed of the engine. The geometry of the pedal assembly controls both the travel of the sensor and the range of the output signal. The throttle position sensor cannot be adjusted.
The output from the throttle position sensor is a PWM signal that varies with the position of the throttle pedal. The throttle position sensor produces a minimum duty cycle in the released position and a maximum duty cycle in the fully depressed position. When the pedal is released, the ECM translates the sensor's output signal to a throttle position of 0 percent. When the pedal is fully depressed, the ECM translates the output signal to a throttle position of 100 percent.
The throttle position sensor is powered by 8.0 ± 0.4 VDC that is provided by the ECM. A digital return for the sensor is also provided by the ECM.
The ECM uses an automatic calibration function that allows the ECM to maximize the usable range of the sensor. The automatic calibration function assumes a range of PWM duty cycle at powerup. If the duty cycle of the sensor exceeds this initial range, the automatic calibration function adjusts the limits of the duty cycle in order to adjust for the new range.
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| Illustration 1 | g01339599 |
Schematic for the throttle position sensor | |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Turn the keyswitch to the OFF position.
- Thoroughly inspect connectors (1), (2), and (5). Also, thoroughly inspect the mechanical condition of pedal assembly (3) and throttle position sensor (5). Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
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| Illustration 2 | g01339611 |
Locations of the connectors for the speed control (left side engine view) (1) J1/P1 ECM connectors (2) Machine interface connector (20-Pin) | |
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| Illustration 3 | g01339633 |
Location of the components for the throttle assembly (3) Throttle pedal assembly (4) Throttle position sensor (5) Sensor connector | |
- Perform a 45 N (10 lb) pull test on each of the wires in the circuit.
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| Illustration 4 | g01339623 |
P1 ECM connector (P1-4) 8 VDC supply (P1-5) Digital return (P1-66) Throttle position | |
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| Illustration 5 | g01339654 |
Terminal locations at the machine interface connector (20-Pin) for the throttle position sensor (Terminal 6) Throttle position (Terminal 9) 8 V supply (Terminal 12) Digital return | |
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| Illustration 6 | g01159881 |
Terminal locations at the connector for the throttle position sensor (A) 8 V supply (B) Digital return (C) Throttle position | |
- Check the allen head screw for each of the ECM connectors for the proper torque. Refer to Troubleshooting, "Electrical Connectors - Inspect" for the correct torque value.
- Check the harness and the wiring for abrasion and for pinch points.
Expected Result:
All of the connectors, pins and sockets are completely coupled and/or inserted and the harness and wiring are free of corrosion, of abrasion and of pinch points.
Results:
- OK - The harness and connectors 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. Ensure that all of the seals are properly in place and ensure that the connectors are completely coupled.Verify that the repair eliminates the problem.
STOP
Test Step 2. Inspect the Throttle Pedal Assembly
- Inspect the throttle pedal assembly for components that are loose, bent, missing, or worn.
- Slowly depress the pedal and release the pedal.
Expected Result:
The throttle pedal assembly operates smoothly without excessive drag. The pedal springs back smoothly to the low idle position without assistance.
Results:
- OK - The throttle pedal assembly functions properly. Proceed to Test Step 3.
- Not OK - The throttle pedal assembly is in poor repair.
Repair: Repair the throttle pedal assembly or replace the throttle pedal assembly, as required.
STOP
Test Step 3. Check the Status for "Throttle Position"
- Connect Caterpillar Electronic Technician (ET) to the service tool connector.
- Turn the keyswitch to the ON position.
- Observe the status for "Throttle Position" on Cat ET while you operate the throttle pedal through the entire range of travel.
Expected Result:
The status is zero percent when the throttle pedal is released. The status progressively increases to 100 percent when the throttle pedal is fully depressed.
Results:
- OK - The throttle position sensor is working properly.
Repair: There may be an intermittent electrical problem with the harness or the connectors. Refer to Troubleshooting, "Electrical Connectors - Inspect" if the problem is intermittent.
STOP
- Not OK - The throttle position that is displayed on Cat ET does not correspond to the movement of the throttle pedal. Proceed to Test Step 4.
Test Step 4. Check the Supply Voltage at the Sensor
- Disconnect the connector for the throttle position sensor.
- Turn the keyswitch to the ON position.
- Measure the voltage across terminal A (8 V supply) and terminal B (digital return) on the harness side of the connector.
- Turn the keyswitch to the OFF position.
Expected Result:
The supply voltage measures 8.0 ± 0.4 VDC at the sensor connector.
Results:
- OK - The correct voltage is measured at the sensor connector. The sensor is receiving the correct supply voltage. Proceed to Test Step 5.
- Not OK - The voltage is not correct at the sensor connector. The sensor is not receiving the correct supply voltage. Proceed to Test Step 7.
Test Step 5. Check the Signal Frequency and the Duty Cycle at the Sensor
- Disconnect the sensor connector for the throttle position sensor.
- Remove the signal wire from terminal C of the harness side of the connector for the throttle position sensor.
- Install a 7X-6370 Adapter Cable As (3-PIN BREAKOUT) at the sensor connector.
- Connect the sensor connectors.
- Turn the keyswitch to the ON position.
- Measure the frequency and the duty cycle between terminal B (digital return) and terminal C (throttle position) at the 7X-6370 Adapter Cable As (3-PIN BREAKOUT) while you operate the throttle pedal.
- Turn the keyswitch to the OFF position.
- Remove the breakout harness. Restore the wiring to the original configuration.
Expected Result:
The frequency is between 150 and 1000 Hz. The duty cycle is between 5 and 95 percent.
Results:
- OK - The frequency and the duty cycle are within specifications for the entire range of the pedal. The sensor is producing a valid signal. Proceed to Test Step 6.
- Not OK - The frequency and the duty cycle are not within specifications.
Repair: The sensor is not producing a valid signal. Replace the throttle position sensor. Verify that the repair eliminates the problem.
STOP
Test Step 6. Check the Signal Frequency and the Duty Cycle at the ECM
- Remove the throttle signal wire at terminal location P1-66 from the ECM connector in order to isolate the circuit.
- Turn the keyswitch to the ON position.
- Install a 7X-1709 Multimeter Probe (BLACK) into terminal location P1-5 (digital return) of the ECM connector.
Note: Ensure that the multimeter probe is in good repair. A bent probe may contact other terminals inside the connector.
- Measure the duty cycle of the throttle position sensor. Measure the frequency between the loose wire and the multimeter probe at the ECM connector while you operate the throttle pedal.
- Turn the keyswitch to the OFF position.
Expected Result:
The frequency is between 150 and 1000 Hz. The duty cycle is between 5 and 95 percent.
Results:
- OK - The frequency and the duty cycle are within specifications for the entire range of the pedal.
Repair: A valid signal appears at the ECM, but the ECM is not reading the signal properly. There is a problem with the ECM. Perform the following procedure:
- Temporarily connect a test ECM.
Refer to Troubleshooting, "ECM - Replace ".
- Recheck the circuit in order to ensure that the original problem has been resolved.
If the problem is resolved with the test ECM, install the suspect ECM. If the problem returns with the suspect ECM, replace the ECM. Verify that the problem is resolved.
If the problem is not resolved with the test ECM, install the original ECM. There is a problem in the wiring.
STOP
- Temporarily connect a test ECM.
- Not OK - The frequency and the duty cycle are not within specifications.
Repair: There is not a valid signal at the ECM connector. Repair the signal wire in the harness. Verify that the repair eliminates the problem.
STOP
Test Step 7. Check the Supply Voltage at the ECM
- Disconnect the J1/P1 ECM connector.
- Fabricate two jumper wires that are long enough to create a test circuit across the ECM connector. Crimp a connector socket to one end of each of the jumper wires.
- Remove the wires from terminal locations P1-4 (8 VDC supply) and P1-5 (digital return) at the ECM connector. Install a jumper wire into each of these terminal locations.
- Reconnect the J1/P1 ECM connector.
- Turn the keyswitch to the ON position.
- Measure the voltage between the wire jumpers in terminals P1-4 (8 VDC supply) and P1-5 (digital return).
- Turn the keyswitch to the OFF position.
Expected Result:
The supply voltage measures 8.0 ± 0.4 VDC at the ECM.
Results:
- OK - The correct voltage is measured at the ECM.
Repair: There is an open circuit or excessive resistance in the supply circuit between the ECM and the throttle position sensor. There may be a problem in a connector. Repair the wiring and/or the connector, when possible. Replace parts, if necessary. Verify that the repair eliminates the problem.
STOP
- Not OK - The voltage is not correct at the ECM.
Repair: There is a problem with the ECM. Perform the following procedure:
- Temporarily connect a test ECM.
Refer to Troubleshooting, "ECM - Replace".
- Recheck the circuit in order to ensure that the original problem has been resolved.
If the problem is resolved with the test ECM, install the suspect ECM. If the problem returns with the suspect ECM, replace the ECM. Verify that the problem is resolved.
If the problem is not resolved with the test ECM, install the original ECM. There is a problem in the wiring.
STOP
- Temporarily connect a test ECM.