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 throttle position sensor provides the throttle signal to the ECM. The throttle position sensor is mounted to a mechanism that allows the operator to control the speed of the engine. The geometry of the mechanism 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 mechanism. The throttle position sensor produces a minimum duty cycle in the deactivated position and a maximum duty cycle in the fully activated position. When the throttle is deactivated, the ECM translates the sensor's output signal to a throttle position of 0 percent. When the throttle is fully activated, the ECM translates the output signal to a throttle position of 100 percent.
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| Illustration 1 | g01421694 |
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Schematic for the throttle position sensor | |
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The connection of any electrical equipment and the disconnection of any electrical equipment may cause an explosion hazard which may result in injury or death. Do not connect any electrical equipment or disconnect any electrical equipment in an explosive atmosphere. |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Remove the electrical power from the ECM.
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Illustration 2 g01388291 Locations of the connectors for the speed control (left side engine view)
(1) J1/P1 ECM connectorsShow/hide table
Illustration 3 g01123466 Electronic throttle mechanism Show/hide table
Illustration 4 g01122049 Throttle lever mechanism Show/hide table
Illustration 5 g01122050 Throttle knob mechanism - Thoroughly inspect connector (1). Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
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Illustration 6 g01382651 P1 ECM connector
(P1-5) Digital return
(P1-66) Throttle positionShow/hide table
Illustration 7 g01389255 Terminals at the customer connector that are associated with the throttle position sensor circuit
(9) Digital return
(10) Throttle positionShow/hide table
Illustration 8 g01159881 Terminal locations at the connector for the throttle position sensor
(A) Supply
(B) Digital return
(C) Throttle position - Perform a
45 N (10 lb) pull test on each of the wires in the circuit. - 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. Check the Status for "Throttle Position"
- Connect Caterpillar Electronic Technician (ET) to the service tool connector.
- Restore the electrical power to the ECM.
- Observe the status for "Throttle Position" on Cat ET while you operate the throttle mechanism through the entire range of travel.
Expected Result:
The status is zero percent when the throttle mechanism is fully deactivated. The status progressively increases to 100 percent when the throttle mechanism is fully activated.
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 mechanism. Proceed to Test Step 3.
Test Step 3. Check the Supply Voltage at the Sensor
- Disconnect the connector for the throttle position sensor.
- Restore the electrical power to the ECM.
- Measure the voltage across terminal A (Supply) and terminal B (digital return) on the harness side of the connector.
- Remove the electrical power from the ECM.
Expected Result:
The measured voltage is between 22.0 VDC and 27.0 VDC for a 24 volt system or the measured voltage is between 11.0 VDC and 13.5 VDC for a 12 volt system.
Results:
- OK - The correct voltage is measured at the sensor connector. The sensor is receiving the correct supply voltage. Proceed to Test Step 4.
- Not OK - The voltage is not correct at the sensor connector.
Repair: Repair the supply wire or replace the supply wire between the sensor connector and the +Battery.
Verify that the repair eliminates the problem.
STOP
Test Step 4. 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.
- Restore the electrical power to the ECM.
- 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 mechanism.
- Remove the electrical power from the ECM.
- 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 throttle mechanism. The sensor is producing a valid signal. Proceed to Test Step 5.
- 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 5. 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.
- Restore the electrical power to the ECM.
- Measure the frequency and the duty cycle between terminal P1-5 (digital return) and the loose wire while you operate the throttle mechanism.
- Remove the electrical power from the ECM.
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 throttle mechanism.
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