3126B Machine Engines Caterpillar

The throttle switch and the decelerator pedal provide the operator with the ability to select the desired engine speed.

The throttle switch is a two-position switch. When the engine is at low idle, pressing the throttle switch to the high idle position will cause the engine speed to increase to high idle. When the engine is above low idle, pressing the throttle switch in the low idle position will cause the engine speed to return to low idle.

The decelerator pedal can be used to decrease the engine speed once the engine is at high idle. Holding the throttle switch in the high idle position will cause the ECM to control the engine speed at the current speed.

The throttle switch is connected to four throttle inputs at the Electronic Control Module (ECM). Each position of the throttle switch generates a specific on/off pattern on the throttle inputs. A diagnostic code is generated if the pattern does not correspond with any of the valid patterns.

Once a diagnostic code is generated, the ECM ignores the throttle input signals and desired engine speed is set to the last valid pattern that was recognized by the ECM.

Voltage at the throttle inputs to the ECM is 7.5 ± 0.5 VDC when the throttle inputs are open. The voltage is less than 0.5 VDC when the throttle inputs are closed.

Illustration 1	g02065558
Schematic

Illustration 2	g01013030
P1 ECM connector

Test Step 1. Inspect Electrical Connectors and Wiring

1. Place the engine control in the OFF/RESET mode.

2. Thoroughly inspect the J1/P1 ECM connector, J61/P61 machine connector, and the J702/P702 throttle switch connector. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.

3. Check the harness and wiring for corrosion, for abrasion, and for pinch points from the throttle switch to the ECM.

Expected Result:

All connectors, pins, and sockets are completely inserted and coupled. The harness and wiring are free of corrosion, of abrasion, and of pinch points.

Results:

• OK - Proceed to Test Step 2.

• Not OK -

  Repair: Repair the circuit.

  Verify that the repair eliminates the problem.

  STOP

Test Step 2. Check the Status of the Throttle Switch

1. Connect the Caterpillar Electronic Technician (ET) to the service tool connector.

2. Place the engine control in the ON mode.

3. Observe the status of the throttle switch and the throttle inputs on Cat ET while you operate the throttle switch in each position.
   Show/hide table

   Table 1

   Status for the Throttle Switch
   	Released	Low Idle	High Idle
   Input 1	Off	On	Off
   Input 2	On	On	Off
   Input 3	Off	Off	On
   Input 4	On	Off	On

Expected Result:

The status of the throttle switch and the status of the inputs changes according to Table 1.

Results:

• OK - The throttle switch is functioning properly at this time.

  Repair: Refer to Troubleshooting, "Electrical Connectors - Inspect" if the problem is intermittent.

  STOP

• Not OK - Note the suspect input. Proceed to Test Step 3.

Test Step 3. Jumper the Switch Input at the Throttle Switch Connector

1. Disconnect the J702/P702 throttle switch connector.

2. Observe the status of the suspect throttle input on Cat ET.

3. Use a suitable piece of wire to short P702:2 (Digital Return) to the terminal for the suspect throttle input at the P702 throttle switch connector.

4. Observe the status of the suspect throttle input on Cat ET.

5. Remove the jumper wire.

Expected Result:

The status of the suspect throttle input is "Off" when the J702/P702 throttle switch connector is disconnected. The status is "On" when the jumper wire is installed.

Results:

• OK - The harness and the ECM are OK.

  Repair: Replace the throttle switch.

  Verify that the repair eliminates the problem.

  STOP

• Not OK - Proceed to Test Step 4.

Test Step 4. Check for Shorts in the Harness

1. Place the engine control in the OFF/RESET mode.

2. Ensure that the J1/P1 ECM connector and the J702/P702 throttle switch connector are disconnected.

3. Measure the resistance between engine ground and the terminal for the suspect throttle input at the P702 throttle switch connector.

4. Measure the resistance between the terminal for the suspect throttle input at the P702 throttle switch connector and the remaining terminals at the connector.

Expected Result:

The resistance is greater than 20,000 Ohms for each reading.

Results:

• OK - Proceed to Test Step 5.

• Not OK - There is a short in the harness between the ECM and the throttle switch.

  Repair: Repair the circuit.

  Verify that the repair eliminates the problem.

  STOP

Test Step 5. Check the Resistance through the Harness

1. Use a suitable piece of wire to short P702:2 (Digital Return) to the terminal of the suspect throttle input at the P702 throttle switch connector.

2. Measure the resistance between P1:5 (Digital Return) and the terminal for the suspect throttle input at the P1 ECM connector.

Expected Result:

The resistance is less than 10 Ohms.

Results:

• OK - Proceed to Test Step 6.

• Not OK - There is an open circuit or excessive resistance in the harness.

  Repair: Repair the circuit.

  Verify that the repair eliminates the problem.

  STOP

Test Step 6. Check the ECM

1. Place the engine control in the OFF/RESET mode.

2. Install a 140-2266 Cable (Seventy-Pin Breakout) between the J1/P1 ECM connectors.

3. Place the engine control in the ON mode.

4. Observe the status of the suspect throttle input on Cat ET.

5. Place the engine control in the OFF/RESET mode.

6. Fabricate a jumper wire with pins at both ends. Insert the jumper wire betweenP1:5 (Digital Return) of the breakout and the suspect throttle input at the breakout.

7. Place the engine control in the ON mode.

   Note: Additional diagnostic codes will be generated because P1:5 (Digital Return) will no longer be connected to other sensors and switches. Ignore the codes and clear the codes when you complete this test.

8. Observe the status of the suspect throttle input on Cat ET.

9. Place the engine control in the OFF/RESET mode.

10. Remove the jumper wire from the breakout. Remove the breakout and reconnect the J1/P1 ECM connector.

Expected Result:

The status of the suspect throttle input is "Off" when the breakout is installed. The status is "On" when the jumper wire (short) is installed in the breakout.

Results:

• OK - There is a problem in the harness between the ECM and the throttle switch.

  Repair: Repair the circuit.

  Verify that the repair eliminates the problem.

  STOP

• Not OK - The switch signal appears at the ECM. The ECM is not reading the switch properly.

  Repair: Verify your results. You may need to replace the ECM. Refer to Troubleshooting, "Replacing the ECM" before replacing the ECM.

  STOP