C27 and C32 Engines for Caterpillar Built Machines Caterpillar

Sensor Signal (PWM) - Test

Usage:

990K A9P

Use this procedure to troubleshoot the electrical system if a problem is suspected with the decelerator position sensor. Also, use this procedure if one of the diagnostic codes in Table 1 is active.

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Table 1
Diagnostic Codes Table
CDL Code and Description	Conditions which Generate the Code	System Response
91-3 Throttle Position Sensor : Voltage Above Normal	The Electronic Control Module (ECM) detects a voltage that is above normal in the circuit for the decelerator position sensor.	The engine speed is limited to low idle.
91-4 Throttle Position Sensor : Voltage Below Normal	The ECM detects a voltage that is below normal in the circuit for the decelerator position sensor.	The engine speed is limited to low idle.
91-8 Throttle Position Sensor : Abnormal Frequency, Pulse Width, or Period	The ECM detects an incorrect frequency or an incorrect duty cycle in the circuit for the decel signal.	The engine speed is limited to low idle.

The decelerator position sensor provides the throttle signal to the ECM.

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Illustration 1	g02305153

Mounting location of the decelerator position sensor on the decelerator pedal (typical example)

The decelerator 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 decelerator position sensor cannot be adjusted.

The output from the decelerator position sensor is a PWM signal that varies with the position of the decel pedal. The decelerator 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 output signal of the sensor to a position of 0 percent deceleration. When the pedal is fully depressed, the ECM translates the output signal to a position of 100 percent deceleration.

The decelerator 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 minimize the deadband 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. The ECM input is then calibrated for the new range.

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Illustration 2	g02305733
Schematic for decelerator position sensor

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Table 2
Troubleshooting Test Steps	Values	Results
1. Inspect the Electrical Connectors and the Wiring A. Turn the main disconnect switch to the OFF position. B. Thoroughly inspect the connectors. Refer to Troubleshooting, "Electrical Connectors-Inspect" for more information. C. Perform a 45 N (10 lb) pull test on the connector wires. D. Check the harness for abrasions and pinch points.	Electrical Connectors and Wiring	Result: The connectors and wiring appear to be OK. Proceed to Test Step 2. Result: There is a problem with the connectors and/or wiring. Repair: Repair or replace the connectors or wiring. Ensure that all of the seals are properly in place and ensure that the connectors are coupled. If the problem is not resolved, proceed to Test Step 2.
2. Inspect the Decelerator Pedal Assembly A. Inspect the throttle pedal assembly for components that are loose, bent, missing, or worn. B. Slowly depress the pedal and release the pedal.	Throttle Pedal Assembly	Result: The throttle pedal assembly functions properly. The mechanical condition of the throttle pedal assembly is OK. Proceed to Test Step 3. Result: The decelerator pedal assembly is in poor condition. Repair: There is excessive drag or improper operation of the decelerator pedal assembly. Repair the pedal assembly or replace the throttle pedal assembly, as required. If the repair does not eliminate the problem, proceed to Test Step 3.
3. Check the Status for "Decelerator Position" on Cat^® Electronic Technician (ET) A. Connect Cat ET to the service tool connector. B. Turn the keyswitch to the ON position. C. Observe the status for "Decelerator Position" on Cat ET while you operate the throttle pedal through the entire range of travel.	Decelerator Position Status	Result: The decelerator position sensor is working properly. Repair: The original problem seems to be resolved. There may be an intermittent electrical problem with the harness or the connectors. Refer to Troubleshooting, "Electrical Connectors - Inspect" if the problem is intermittent. If the repair does not resolve the problem, proceed to Test Step 4. Result: The decelerator position that is displayed on Cat ET is questionable. Proceed to Test Step 4.
4. Check the Supply Voltage at the Sensor A. Turn the keyswitch to the OFF position. B. Disconnect the harness connector for the decelerator position sensor. C. Turn the keyswitch to the ON position. D. Measure the voltage across terminal 1 (8 V supply) and terminal 2 (digital return) on the harness side of the connector. E. Turn the keyswitch to the OFF position.	8.0 ± 0.4 VDC	Result: The correct voltage is measured at the sensor connector. The sensor is receiving the correct supply voltage. Proceed to Test Step 5. Result: The voltage is not correct at the sensor connector. Repair: There may be a problem with the sensor supply. Refer to Troubleshooting, "Sensor Supply - Test" for information that is related to troubleshooting the supply circuit. If the repair does not resolve the problem, proceed to Test Step 5.
5. Check the Signal Frequency and the Duty Cycle at the Sensor A. Remove the wire from terminal location C (decelerator position) on the harness side of the connector for the decelerator position sensor. B. Install a 3-pin breakout at the connector for the decelerator position sensor. C. Connect the connector for the decelerator position sensor. D. Turn the keyswitch to the ON position. E. Measure the frequency and the duty cycle between terminal 2 (digital return) and terminal 3 (decelerator position) at the 3-pin breakout. Operate the decelerator pedal during the measurements. F. Turn the keyswitch to the OFF position. G. Remove the breakout harness. Restore the wiring for the harness connector to the original configuration.	500 Hz	Result: 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. Result: The frequency and the duty cycle are not within specifications. Repair: The sensor is not producing a valid signal. Replace the decelerator position sensor. Verify that the repair eliminates the problem. If the repair does not resolve the problem, proceed to Test Step 6.

6. Check the Signal Frequency and the Duty Cycle at the ECM A. Disconnect the J1/P1 ECM connectors. B. Remove the wire from terminal location P1-66 (throttle position) at the ECM connector. C. Install a 208-0059 Adapter Cable As (70-PIN BREAKOUT) between the J1 and the P1 ECM connectors. D. Connect the J1/P1 ECM connectors. E. Turn the keyswitch to the ON position. F. Measure the frequency between the loose signal wire (throttle position) and P1-5 (digital return) on the breakout harness while you operate the throttle pedal. G. Set the multimeter to read the duty cycle of the sensor. Measure the duty cycle between the loose signal wire (throttle position) and P1-5 (digital return) on the breakout harness while you operate the throttle pedal. H. Turn the keyswitch to the OFF position.	500 Hz	Result: 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 does not appear to be reading the signal properly. Before you consider replacing the ECM, consult with the Dealer Solutions Network (DSN) for assistance. Verify that the repair eliminates the problem. Result: The frequency and the duty cycle are not within specifications. Repair: There is a valid signal in the isolated circuit between the sensor and the sensor connector. There does not appear to be a valid signal at the ECM connector. The problem is in the harness wiring or in a connector between the ECM and the harness connector for the decelerator position sensor. Repair the harness and/or the connectors. Replace parts, if necessary. Verify that the repair eliminates the problem. Verify that the repair eliminates the problem.