3126B Engine for Caterpillar Built Machines Electrical Power Supply Circuit - Test Caterpillar

System Operation Description:

Engine shutdowns that are accompanied by a loss of other electrical accessories indicate a problem in the keyswitch or the relay circuit. This does not indicate a problem in the wiring at the ECM. Verify that there are no problems with the starting system or the charging system before you use this test.

The cause of an intermittent power supply to the ECM can occur on either the positive side (unswitched + battery) or the negative side. Both sides are routed from the ECM to the battery.

Usually, battery power to the service tool connector is independent of the keyswitch. Therefore, you will be able to power up ET, but you may not be able to communicate with the engine ECM. The engine ECM requires the keyswitch to be in the ON position in order to establish communications.

For intermittent problems such as intermittent shutdowns that could be caused by machine wiring, temporarily bypassing the machine wiring may be an effective means of determining the root cause. If the symptoms disappear with the bypass wiring, the machine wiring is the cause of the problem. A means of bypassing machine wiring is explained in this test procedure.

Illustration 1	g00809336
Schematic

Note: The 307-OR wire that is connected to P1:64 (Keyswitch (Start)) is not currently utilized by the ECM.

Test Step 1. Inspect Electrical Connectors and Wiring

Illustration 2	g00809337
ECM connector P1 (wire side)

1. Thoroughly inspect the J1/P1 ECM connector, the battery connections, and the connections to the keyswitch. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.

2. Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the electrical power supply. Refer to Illustration 2.

3. Check the ECM connector (allen head screw) for the proper torque of 6.0 N·m (55 lb in).

4. Check the harness and wiring for abrasions and for pinch points from the battery to the ECM. Also, check the harness and wiring for abrasions and for pinch points from the keyswitch to the ECM.

Expected Result:

All connectors, pins, and sockets should be completely coupled and/or inserted. The harness and wiring should be free of corrosion, abrasion, and/or pinch points.

Results:

• OK - Proceed to Test Step 2.

• Not OK - There is a problem with the connectors or wiring.

  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.

  STOP

Test Step 2. Check the Battery Voltage at the Batteries

1. Measure the no-load battery voltage and load test the batteries. Use the 4C-4911 Battery Load Tester. Refer to Special Instruction, SEHS9249, "Use of 4C-4911 Battery Load Tester for 6, 8 and 12 Volt Lead Acid Batteries" and Special Instruction, SEHS7633, "Battery Test Procedure".

Expected Result:

The battery voltage is OK and the batteries pass the load test.

Results:

• OK - Proceed to Test Step 3.

• Not OK -

  Repair: Recharge or replace the faulty batteries. Verify that the repair eliminates the problem.

  STOP

Test Step 3. Check the Unswitched Battery Voltage at the ECM

Illustration 3	g00809340
ECM connector P1 (ECM side)

1. Ensure that the keyswitch is in the OFF position.

2. Disconnect the J1/P1 ECM connector.

3. Measure the voltage between all of the +Battery terminals and all of the −Battery terminals in the P1 ECM connector. Refer to Illustration 3.

Expected Result:

The measured voltage is a constant voltage that is within ± 1 VDC of battery voltage.

Results:

• OK - The ECM is receiving the correct unswitched battery voltage. Proceed to Test Step 4.

• Not OK -

  Repair: If an intermittent condition is suspected, refer to Troubleshooting, "Electrical Connectors - Inspect".Isolate the circuit that is not providing the correct voltage. Repair the circuit. Verify that the repair eliminates the problem.

  STOP

Test Step 4. Check the Keyswitch Voltage at the ECM

Illustration 4	g00809340
ECM connector P1 (ECM side)

1. Ensure that the J1/P1 ECM connector is disconnected.

2. Turn the keyswitch to the ON position.

3. Measure the voltage between P1:70 (Keyswitch (Run)) and P1:63 (−Battery).

4. Turn the keyswitch to the OFF position and reconnect the J1/P1 ECM connector.

Expected Result:

The measured voltage is a constant voltage that is within ± 1 VDC of battery voltage.

Results:

• OK - The ECM is receiving the correct voltage at this time. Proceed to Test Step 5.

• Not OK -

  Repair: Repair the keyswitch circuit. Verify that the repair eliminates the problem.

  STOP

Test Step 5. Bypass the Machine Wiring

Show/hide table

Batteries give off flammable fumes which can explode. To avoid injury or death, do not strike a match, cause a spark, or smoke in the vicinity of a battery.

Show/hide table

NOTICE
Do Not connect the bypass harness to the battery until the 20 Amp in-line fuse has been removed from the +Battery line. If the fuse is not removed before connection to the battery, a spark may result.

Show/hide table



Illustration 5	g00781203
Bypass harness

Note: This bypass harness is only for test applications. This bypass harness may be left only temporarily on the vehicle. The bypass harness can be used in order to determine if the cause of the intermittent problem is interruptions in battery power to the ECM or the keyswitch circuit.

1. Turn the keyswitch to the OFF position.

2. Disconnect the J1/P1 ECM connector.

3. Connect a bypass harness to the J1/P1 ECM connector.

4. Remove the 20 Amp in-line fuse from the positive battery circuit of the bypass harness and connect the alligator clamps directly to the battery posts.

   Note: This bypass directly connects the circuit for the keyswitch to the ECM. The ECM will remain powered until the positive battery circuit is interrupted. Remove the 20 Amp fuse from the in-line fuse holder in order to power down the ECM. Do not connect the bypass to the battery posts without first removing the 20 Amp in-line fuse. Do not remove the bypass from the battery posts without first removing the 20 Amp in-line fuse.

5. Connect an ET to the service tool connector of the bypass harness and verify that communication is established.

6. Restore all wiring to the original condition after testing.

Expected Result:

Installing the bypass eliminates the problem.

Note: The status of the keyswitch will always indicate "On" while the bypass harness is installed.

Results:

• OK - The symptoms disappear when the bypass harness is installed. Also, the symptoms return when the bypass harness is removed. The problem is in the machine wiring that supplies power to the ECM. Repair the wiring that supplies power to the ECM.STOP

• Not OK -

  Repair: Connect the bypass to another battery and verify if the problem is resolved. If the problem is resolved, the problem is with the machine batteries.If the problem still exists, temporarily connect a test ECM. Remove all jumpers and replace all connectors. Recheck the system for active diagnostic codes and repeat the Test Step. If the problem is resolved with the test ECM, reconnect the suspect ECM. If the problem returns with the suspect ECM, replace the ECM.Verify that the repair eliminates the problem.

  STOP