794 AC Off-Highway Truck Systems Caterpillar

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The Power Train Electric Drive System will contain hazardous voltage levels during machine operation and for a short period of time after engine shutdown. Do not remove any covers that will expose energized high voltage electrical components while the engine is operating. Any type of maintenance on the following components can only be performed after the Power Train Electrical System Service Shutdown procedure has been followed: High voltage compartments in the inverter cabinet The rear axle housing that contains the electric drive traction motors The generator The retarding resistor grid, the grid blower motor and the grid system cabling The excitation field regulator The high voltage cables and connection enclosures Failure to follow these instructions could result in personal injury or death.

Introduction

The "Chopper/Power Transistor Signal Test" requires the use of Cat^® Electronic Technician (Cat^® ET). The test is used to help troubleshoot power transistor driver circuits, feedback circuits, and specific power transistors. The transistors are inside both the "Motor Phase and Chopper" modules.

This test is required when a "Phase Module Signal Mismatch Event" (E1279 through E1284) is activated by either the "Motor Control 1 ECM" or "Motor Control 2 ECM". The test helps isolate problem causes, and anytime a problem is suspected in a Phase or Chopper module .

Before replacing a phase module for a suspected transistor problem, use this test, to confirm that the power transistor has failed. Verify that the problem is not in the external driver and feedback circuits.

Test Preparation

Before performing this procedure, the user should be familiar with the traction motor control system components and cabinet locations for each motor control ECM.

The following illustrations and text will explain the setup of each phase module and the power transistors that are in the phase module.

Illustration 1	g06293472

Power transistor configuration and control in each motor control phase module. For the Retarder Chopper Module, only the negative side (2) switching power transistors are used.

Power transistor configuration facts:

Inverter Cabinet and Phase Module Configurations

• A Phase Module has two pairs of power transistors - four total power transistors.

• Each pair of parallel-connected power transistors is controlled by one ECM driver circuit. Two ECM driver circuits per Phase Module, except for the Retarder Chopper module.

• Each pair of parallel connected power transistors provides one diagnostic feedback circuit to the ECM. Two feedback circuits per Phase Module, except for the Retarder Chopper module.

• For each Phase Module, each pair of transistors is used to switch both the positive (high) and negative (low) DC Power Bus sides.

• For each Phase Module, each pair of transistors produces one complete AC signal output.

• Driver and feedback circuits between the Motor Control ECMs and the Interface Modules are copper wire circuits. Driver and feedback circuits between the Interface Modules and the Phase or Chopper modules are fiber optic circuits.

• The Retarder Chopper Module is configured the same as the motor phase modules. However, the Motor 1 ECM only uses the negative-side power transistors (2) for switching. The positive-side power transistors are not used. Therefore, the Chopper Module has one transistor driver circuit and one diagnostic feedback circuit connected to the "Motor 1 ECM".

Illustration 2	g03788910
Interface Module fiber optic circuit connections

For power transistor circuit connections to the ECM and the phase module, refer to the "System Schematics" section at the end of this manual. The full machine electrical schematics in the Service Manual can also be referenced for the circuit connections.

Reference Illustration 3 for the locations of the motor control and chopper control components.

Illustration 3	g03788916
"Inverter Cabinet Traction Motor Control" Components: (1) Interface Module 2 (2) Motor Control 2 ECM (3) Interface Module 1 (4) Motor Control 1 ECM (5) PMA2 (6) PMB2 (7) PMA1 (8) PMB1 (9) PMC2 (10) Retarder Chopper Module (11) PMC1

Note: Phase Modules all have one AC output terminal. The Chopper Module has 1 AC output terminal. All seven modules have 2 DC terminals, one Positive (P), and one Negative (N). During any test measurement, make sure to connect the test meter leads to the correct terminal on the modules.

Test Procedure

When work must be done in high-voltage areas or components: A Power Train Electrical System Service Shutdown procedure must be performed, before Service or Maintenance work is performed, or high-voltage area covers are removed.

Reference: Refer to Troubleshooting, UENR3890, "Cat 794 AC Off-Highway Truck/Tractor".

Perform the "Power Train Electrical System Service Shutdown" procedure, to verify that the voltage in the drive train system has been discharged. Once the machine DC Power Bus voltage is verified as discharged, and a jumper wire is in place, return to this point in the procedure.

Phase Module Preparation and Static Diode Test

1. Remove the compartment cover for the phase module that will be tested.
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   Illustration 4	g01961471
   Phase Module

2. Use a digital multimeter to measure voltage between the DC positive (DCP) and the DC negative (DCN) bus bars in the phase module. Verify that there is less than 50.0V DC between the bus bars. Once the voltage level is verified, continue to the next step.

3. At the front of the phase module that will be tested, remove the “L” shaped bus bar connections that are marked "N", "AC", and "P".
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   Illustration 5	g06294324
   View of the bolt hole threads and outer contact area for the diode check location.

   Note: Place the meter probes on the outer contact area when taking a measurement. The threads in the bolt holes will NOT provide a good reading.

4. The digital multimeter will charge the internal capacitor in the phase module. Wait 10 seconds for the reading to stabilize before reading the final measurement.
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   Illustration 6	g06294376
   Measuring from the DCN lead (positive meter test probe) to the AC lead (negative meter test probe).

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   Illustration 7	g06294344
   Measuring from the DCN lead (negative meter test probe) to the AC lead (positive meter test probe).

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   Illustration 8	g06294349
   Measuring from the DCP lead (positive meter test probe) to the AC lead (negative meter test probe).

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   Illustration 9	g06294355
   Measuring from the DCP lead (negative meter test probe) to the AC lead (positive meter test probe).

5. Check the voltage across the diodes that are connected in parallel with the transistors using the "Diode" setting (scale) on the multimeter. Review the illustrations above for the correct probable locations for the voltage checks. Refer to the "Power Transistor Voltage Checks" table for the expected results when checking across the phase module contacts.

Note: Capacitance can be measured from post P to post N if the meter can measure up to 1700 uF. The measured capacitance should exceed 1500 uF.

If any voltage drop check listed in the table does not agree with the values in the "Expected Meter Reading" column, a problem exists with the phase module.

Repeat this test to confirm the results. If the results again indicate that there is a problem in the phase module, call the Dealer Solutions Network (DSN). Consult with company personnel concerning the results of this test. The DSN representative will provide instructions on the action that must be taken to resolve the problem.

If the results of the diode voltage check test match the values in the "Expected Meter Reading" column, continue with the procedure in the next "Chopper/Power Transistor Signal Test" section.

Chopper/Power Transistor Signal Test

Fiber Optic wires and connectors require special handling, to ensure proper operation. Mishandling of the wires or connectors will result in poor circuit operation, or a complete circuit failure.

Fiber Optic components are not serviceable.

If a Fiber Optic harness circuit or connector is damaged, or fails, replace the complete harness for the involved phase module.

If a Fiber Optic connector on an Interface Module or Phase Module is damaged, replace the Interface Module or Phase Module.

Refer to Testing and Adjusting, "Fiber Optic Connector Handling" for the proper disconnection and connection procedures.

The following test procedure helps to select a specific power transistor driver circuit, and to command the motor control ECM to energize the circuit. The result should be to gate (close) the power transistor and energize the feedback circuit for the transistor.

Once the close command is activated, fiber optic circuits can be visually checked for proper operation. The user can also check the copper wire circuit between the "Motor ECM" and the "Interface Module" for proper operation. In addition, when the transistor is gated ON, the voltage drop across the gated transistor is measured, to determine if the transistor operation is correct.

Note: When Cat^®ET indicates that the test has "Successfully Completed", this message only indicates that the test is done. The message does not indicate that the circuits or transistors are correct. This Cat^® ET "Power Transistor Circuit Test" is intended to provide an aid to troubleshooting the circuits and the transistor when a problem is present. Follow the listed instructions for visually checking the correct operation of the fiber optic section of a circuit. The user must perform voltage checks to verify proper operation of the transistor.

After using this section to activate the test for specific circuits and check the voltage drop across the transistors, refer to Troubleshooting, UENR3890, "Cat 794 AC Off-Highway Truck/Tractor" for tips on how to troubleshoot the circuits when energized.

1. The Battery Disconnect Switch should be in the ON position and the "Engine Lockout Control" should be in the ON (lock out) position. The low voltage control system will energize at the +battery level.

2. Connect and start Cat^® ET. When the "Select ECM" screen appears, select the "Motor Control #1 ECM" that has control over the phase module that will be tested. Either "Motor Control 1 ECM" or "Motor Control 2 ECM".
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   Illustration 10	g06356207

3. In the main screen for the ECM that is selected, select "Diagnostics" from the menu bar. From the pull-down menu, select "Diagnostic Tests", then select "Chopper/Power Transistor Signal Test".
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   Illustration 11	g06356210
   Phase/Chopper Module Power Transistor Signal Test screen

4. Follow the instructions that are presented on the screen to satisfy the preconditions for the test. As the conditions are satisfied, the condition will be “grayed out”. The ECM will not allow the test to proceed until all preconditions are satisfied.

   Note: Select Chopper/Power Transistor Signal Test in Cat^® ET.

5. In the main screen for the ECM that is selected, select "Diagnostics" from the menu bar. From the pull-down menu, select "Diagnostic Tests", then select "Chopper/Power Transistor Signal Test".

6. Follow the instructions presented on the screen to satisfy the preconditions for the test. As the conditions are satisfied, the condition will be "grayed out". The ECM will not allow the test to proceed until all preconditions are satisfied.

7. After the preconditions are satisfied, the list of available power transistors for testing is activated. Select any of the listed power transistor circuits to test. A check mark will appear to the left of the selected transistor line of the line may be highlighted.

   Note: In the next step, some software versions may use the "Next" button to activate or de-activate a transistor circuit. The ON button and the OFF button may not be available. Follow the displayed instructions.

8. Once the specific power transistor is selected, use the buttons in the "Transistor Control" box to turn the transistor circuits ON or OFF. The on button will cause the ECM to energize the selected transistor circuits. The circuit can be left in the ON state for as long as desired. Only one transistor circuit can be ON at a time.

   Note: In the next step, software versions may use a "Next" button to activate or de-activate a transistor circuit. Follow the displayed instructions.

9. With a selected power transistor circuit energized, determine where the mating fiber optic feedback circuit is connected at the appropriate interface module. Carefully disconnect the mating fiber optic feedback connector from that interface module. Use the following two tables to help determine the locations of the mating fiber optic feedback circuit.
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   Table 2

   INV 1
   ET Designation	Phase Module	Simpson R1	A4:P1-1	Circuit	IM-1	PM Pin	IM Color
   Phase A Power Transistor 1	PMA1	P + AC -	J2-9	OU1	58	B	R
   			J2-4	OU1F	40	C	G
   Phase A Power Transistor 2	PMA1	AC + N -	J2-28	OX1	60	R	B
   			J2-35	OX1F	42	S	RG
   Phase B Power Transistor 1	PMB1	P + AC -	J2-10	OV1	64	B	GB
   			J2-17	OV1F	62	C	BB
   Phase B Power Transistor 2	PMB1	AC + N -	J2-36	OY1	68	R	RR
   			J2-43	OY1F	44	S	GG
   Phase C Power Transistor 1	PMC1	P + AC -	J2-20	OW1	1	B	R
   			J2-27	OW1F	24	C	G
   Phase C Power Transistor 2	PMC1	AC + N -	J2-37	OZ1	3	R	B
   			J2-51	OZ1F	26	S	RG
   Electric Retarding Grid 1	CHOPPER	AC + N -	J2-38	OCM	7	R	BG
   			J2-61	OCMF	5	S	BB

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   Table 3

   INV 2
   ET Designation	Phase Module	Simpson R1	A4:P1-1	Circuit	IM-1	PM Pin	IM Color
   Phase A Power Transistor 1	PMA2	P + AC -	J2-9	OU2	58	B	R
   			J2-4	OU2F	40	C	G
   Phase A Power Transistor 2	PMA2	AC + N -	J2-28	OX2	60	R	B
   			J2-35	OX2F	42	S	RG
   Phase B Power Transistor 1	PMB2	P + AC -	J2-10	OV2	64	B	GB
   			J2-17	OV2F	62	C	BB
   Phase B Power Transistor 2	PMB2	AC + N -	J2-36	OY2	68	R	RR
   			J2-43	OY2F	44	S	GG
   Phase C Power Transistor 1	PMC2	P + AC -	J2-20	OW2	1	B	R
   			J2-27	OW2F	24	C	G
   Phase C Power Transistor 2	PMC2	AC + N -	J2-37	OZ2	3	R	B
   			J2-51	OZ2F	26	S	RG

   1. Observe the fiber optic harness connector. The energized driver circuit should feed into the transistor being tested. The energized driver will cause the electronics in the transistor to activate the mating feedback circuit disconnected from the interface module. Disconnect the command fiber optic wire from the interface module and check for a steady red tinted beam of light. If that light is not observed, verify that you have the correct feedback circuit, and correct interface module. If the correct feedback circuit is disconnected, there is a problem between the ECM driver circuit and the interface module. Refer to Troubleshooting, UENR3890, "Cat 794 AC Off-Highway Truck/Tractor" for tips on troubleshooting this problem.

   2. A beam of light present indicates the ECM driver circuit, power transistor electronics, and "Fiber Optic Feedback" circuit to the interface module are good. Carefully connect the harness connector back on the interface module connector.

10. The circuit remains energized. The copper wire feedback circuit that is connected between the interface module and the "Motor Control ECM" must be checked to verify proper operation.

    1. Using a meter with "spoon" type probes, find the correct feedback circuit contact on the "Motor Control ECM" J2 connector. Insert the negative probe along the ECM ground contact J2-5, and the positive probe along the selected feedback circuit contact. The feedback circuit DC voltage for this test is 6.3 ± 0.5V DC.

    2. If the voltage of the feedback circuit is within the expected range, the feedback circuit is operating properly between the interface module and the ECM.

11. When operation of the ECM, the interface module, the transistor electronics, and the feedback circuits are correct, a gate ON voltage check of the transistor is required. The voltage check verifies that the transistor operation is correct.

    1. With the selected power transistor circuit energized, check voltage drop across the parallel-connected transistors. Use the "Diode" scale on the meter. Expected results depend on whether a positive (switching) transistor is being gated ON or a negative switching transistor (negative transistor) is being gated ON. Refer the appropriate tables for the expected results when checking across the phase module contacts.

       Note: For accurate readings, place the meter probes on the bolt holes threads when taking this measurement. The outer contact area will not provide a good reading. The digital multimeter will charge the internal capacitor in the phase module. Wait for 20 seconds to 30 seconds for the reading to stabilize before determining the final measurement.

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       Table 4

       Positive Power Transistor Gate ON Voltage Drop Checks (Meter set to “Diode” scale)
       	AC	P (DCP)	Expected Meter Reading
       Meter Probe Position	NEG(1)	POS(2)	Less than 0.5 VDC
       	POS	NEG	Less than 0.5 VDC(3)

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       (1)	NEG = negative multimeter probe.
       (2)	POS = positive multimeter probe.
       (3)	This is a diode check. The meter value will not change to infinity when the energize command is removed.

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       Table 5

       Negative Power Transistor Gate ON Voltage Drop Checks (Meter set to “Diode” scale)
       	N	AC	Expected Meter Reading
       Meter Probe Position	POS(1)	NEG(2)	Less than 0.5 VDC(3)
       	NEG	POS	Less than 0.5 VDC

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       (1)	POS = positive multimeter probe.
       (2)	NEG = negative multimeter probe.
       (3)	This is a diode check. The meter value will not change to infinity when the energize command is removed.

    2. If any of the voltage drop checks listed in the table do not agree with the values in the "Expected Meter Reading" column, the indication is that there is a problem in the phase module. Repeat this test to confirm the results. If the results again indicate that there is a problem in the phase module, call the Dealer Solutions Network (DSN) for consultation with Caterpillar. The DSN representative will provide instructions on the action that must be taken to resolve this problem.

12. If the results of the voltage drop check match the values in the "Expected Meter Reading" column, the testing or the troubleshooting for the transistor is finished. Press the "NEXT" button to select another transistor to test. The check mark or the highlight will remain on the transistor line that has been tested.

13. Repeat the steps starting at Step 5 to check another power transistor and the circuits.

14. Press the "CANCEL" button to exit the test at any time.

Note: When a specific power transistor circuit has been energized, the user must verify the results of the testing. Disconnect the fiber optic connectors and observe the presence or absence of the light pulses. The light will appear as a steady red tinted light. Do not look directly into the fiber optic circuit when the circuit is energized. For the copper sections of the circuits, a voltage check will be used to verify correct operation.

The following troubleshooting tips will help to determine if there is a problem in the transistor circuits or a problem with a component.

Illustration 12	g01958551
Phase module low voltage connector (12) Fiber optic driver circuits are at contact "B" and "R". (13) Fiber optic feedback circuits are at contacts "C" and "S".

• When the transistor driver circuit is energized by the "Motor Control ECM", a steady red tinted light should be seen at any of the disconnected fiber optic connectors at the interface module or the phase module. If the light is not present, follow the circuit back to the optical source to isolate the cause of the problem.

• The fiber optic circuits and connectors are not serviceable. If a problem is found with a fiber optic conductor or connector, the phase module control harness must be replaced. If a problem is found with any of the phase module connector contacts, the phase module must be replaced.

• The driver circuit originates at the "Motor Control ECM" driver contact. The circuit voltage can be measured. Measure the voltage using "spoon" type probes at the ECM connector. A voltage of approximately 7.5 ± 0.5V DC to ECM ground at the ECM connector contact or the interface module connector contact is acceptable . This voltage is the result of 15 VDC driver pulses at a 50 percent duty cycle. The duty cycle can also be measured.

• The driver circuit should be seen as the visible light at the interface module optical connector. The light may also be seen at the low voltage harness connector for the phase module.

• A flashlight can be used to illuminate a fiber optic conductor at the disconnected connector. The light should be visible at the other end of the conductor.

• When the low voltage connector is connected at the phase module, the feedback signal should be seen at the disconnected optical feedback circuit connector at the interface module.

• The feedback signal can be measured as a voltage of approximately 6.3 ± 0.5V DC. Measure between a controller ground on the interface module connector and the ECM connector circuit contact using "spoon" type probes. This voltage is the result of interface module 12 VDC feedback pulses at a 50 percent duty cycle. The duty cycle can also be measured.

• When a problem is not found in any of the driver or the feedback circuits, the interface modules can be switched to determine the interface module causing the problem.

• When a problem is not found in any of the circuits or the interface module, the motor ECM's can be switched without flashing software. Switching the ECM's helps to determine if the ECM is causing the problem. If an ECM must be replaced, always flash the latest version of "Motor Control ECM" software before replacing the ECM. After flashing new software, determine if the software is causing the problem.

• If the indication of the testing is that a power transistor has failed, repeat the test to confirm the results. If the results again indicate that there is a problem in the phase module, call the Dealer Solutions Network (DSN). Indicate the results of this test. The DSN representative will provide instructions on the action that must be taken to resolve this problem.