C13, C15, and C18 Engines Troubleshooting – Speed Control – Test

System Operation Description:

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 one of the diagnostic codes in Table 2 is active or easily repeated.

Note: Some of the following codes may not be applicable for certain applications.

Note: Performing steps within this procedure requires the use of a multimeter capable of measuring a PWM duty cycle and frequency. Refer to Table 1 for the tools recommended for the procedure.

Table 1
Recommended Tools
146-4080 Digital Multimeter Gp
7X-1710 Multimeter Probe
326-4904 Adapter Cable As
Table 2
Diagnostic Trouble Codes
J1939 Code CDL Code Code Description
(code descriptions may vary)
Comments
29-2 774-2 Accelerator Pedal Position 2 : Erratic, Intermittent, or Incorrect The Electronic Control Module (ECM) has detected an invalid combination of positions for the multi-position switches.
The code is logged. If equipped, the warning light will come on.
Engine RPM defaults to a predetermined safe speed. The throttle switch is ignored by the ECM until the fault is cleared and the keyswitch is cycled.
29-3 774-3 Accelerator Pedal Position 2 : Voltage Above Normal The ECM detects signal voltage that is not in the acceptable range.
The code is logged. The ECM flags the throttle position as invalid data and a default value is used. The engine speed is limited to high idle.
29-4 774-4 Accelerator Pedal Position 2 : Voltage Below Normal The ECM detects signal voltage that is not in the acceptable range.
The code is logged. The ECM flags the throttle position as invalid data and a default value is used. The engine speed is limited to high idle.
29-8 774-8 Accelerator Pedal Position 2 : Abnormal Frequency, Pulse Width, or Period The 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 and a default value is used. The engine speed is limited to high idle.
91-2 91-2 Accelerator Pedal Position 1 : Erratic, Intermittent, or Incorrect The Electronic Control Module (ECM) has detected an invalid combination of positions for the multi-position switches.
The code is logged. If equipped, the warning light will come on.
Engine RPM defaults to a predetermined safe speed. The throttle switch is ignored by the ECM until the fault is cleared and the keyswitch is cycled.
91-3 91-3 Accelerator Pedal Position 1 : Voltage Above Normal The ECM detects signal voltage that is not in the acceptable range.
The code is logged. The ECM flags the throttle position as invalid data and a default value is used. The engine speed is limited to high idle.
91-4 91-4 Accelerator Pedal Position 1 : Voltage Below Normal The ECM detects signal voltage that is not in the acceptable range.
The code is logged. The ECM flags the throttle position as invalid data and a default value is used. The engine speed is limited to high idle.
91-8 91-8 Accelerator Pedal Position 1 : Abnormal Frequency, Pulse Width, or Period The 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 and a default value is used. The engine speed is limited to high idle.

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 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.


Illustration 1 g02075215
Schematic for throttle position sensor

Illustration 2 g01214474
P1 ECM connector

(P1-4) 8 VDC supply

(P1-5) Digital return

(P1-66) Throttle position

Table 3
Troubleshooting Test Steps Values Results
1. Check the Signal Frequency and the Duty Cycle at the Sensor

A. Turn the keyswitch to the OFF position.

B. Remove the wire from terminal location C (throttle position) on the harness side of the connector for the throttle position sensor.

C. Install a 326-4904 Adapter Cable As (3-PIN BREAKOUT) at the connector for the throttle position sensor.

D.Connect the connector for the throttle position sensor.

E. Turn the keyswitch to the ON position.

F. Measure the frequency and the duty cycle between terminal B (digital return) and terminal C (throttle position) at the 326-4904 Adapter Cable As (3-PIN BREAKOUT). Operate the throttle pedal while making the measurements.

Turn the keyswitch to the OFF position.

Remove the breakout harness. Restore the wiring for the harness connector to the original configuration.

Frequency

Duty cycle

Result: The frequency and the duty cycle are within specifications for the entire range of the pedal.

Repair the harness or replace the harness. Verify that the problem is resolved.

Result: The 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 is above 1000 Hz. The duty cycle is less than 5 percent or the duty cycle is greater than 95 percent.

Proceed to Test Step 2.

2. Check the Supply Voltage to the Sensor

A. Turn the keyswitch to the OFF position.

B. Disconnect the harness connector for the throttle position sensor.

C. Measure the voltage across terminal A (8 V supply) and terminal B (digital return) on the harness side of the connector.

7.5 VDC Result: The supply voltage is above 7.5 VDC at the sensor connector.

The wiring is OK. Replace the sensor. Verify that the problem is resolved.

Result: The supply voltage is not above 7.5 VDC at the sensor connector.

The supply voltage is not reaching the speed control.
Repair the harness wiring or replace the harness. Verify that the problem is resolved.

Test Step 1.

Complete the procedure in the order in which the steps are listed.

Results:

 

  • The procedure corrected the issue. – STOP 
  • The procedure did not correct the issue. –Repair: Contact your Cat dealer Technical Communicator (TC). For further assistance, your TC can confer with the Dealer Solutions Network (DSN).

    STOP

 

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