Eaton Fuller Hybrid Transmission Fault Code 104 – High Voltage Battery 2 Control Unit Communication
Fault code for Eaton Hybrid Drive Systems models (MY08) EH-8E306A-U, EH-8E306A-UPG, EH-8E306A-UP, EH-8E306A-CD, EH-8E306A-T and EH-6E606B-CD. Read below to learn how to test, clear and fix fault code errors for your Freightliner, Mack, Peterbilt, Kenworth, Volvo, International, and heavy-duty truck and equipment. The high-voltage batteries each contain a cell controller that communicates over a twisted pair data link to the Battery Control Unit (BCU). The Battery Control Unit located in the Relay Box communicates with each battery to determine temperature, State Of Charge (SOC), and voltage. This information is then sent to the Inverter.
Fault Code 104 – High Voltage Battery 2 Control Unit Communication Detection
The following preconditions must be met before the system detects the fault:
- Inverter ignition voltage is greater than 7 volts and less than 16 volts.
- Battery Control Unit ignition voltage is greater than 7 volts and less than 16 volts.
Note: When troubleshooting an Inactive code refer to the Product Diagnostic Mode (PDM)
Conditions to Set Fault Code Active
- FMI 22 is set when the BCU fails to communicate over the CAN Data Link for 30 MS.
- FMI 23 is set when the BCU fails to receive HCM data over the CAN Data Link for 2.5 seconds or greater after communication is established.
- FMI 24 is set when the BCU fails to receive Inverter data over the CAN Data Link for 2.5 seconds or greater after communication is established.
- FMI 25 is set when the BCU detects an error on initial check.
- FMI 26 is set when the BCU fails to transmit anything over the data link for 2 seconds.
Fallback When Fault Code 104 is set the following conditions occur:
- Amber “Check Hybrid” light illuminates.
- Fault is stored in Hybrid Control Module (HCM) memory.
- Inverter shuts high-voltage system off.
- HCM continues to control the hybrid vehicle in a diesel-only mode.
- Transmission defaults start gear to 1st.
Conditions to Set Fault Code Inactive
Only Inactive faults can be cleared from the Transmission Electronic Control Unit (TECU) or HCM history using ServiceRanger. The TECU automatically clears the faults from history after 200 hours and the HCM automatically clears the faults from history after the fault has been Inactive for 200 hours.
This fault code can be caused by any of the following:
- FMI 26
- PEC
- FMI 22, 25
- PEC
- CAN Data Link between PEC and Inverter
- FMI 23
- PEC
- CAN Data Link between PEC and HCM
- FMI 24
- PEC
- Inverter
- CAN Data Link between PEC and Inverter
Fault Code 104 – High Voltage Battery 2 Control Unit Communication Test
Component Identification:

Harness Front View
(HCM – Hybrid System Interface Connector)

Inverter 56-Way Mating Connector View
(Inverter – Low Voltage Connector)

19 – Way Mating Connector View
(PEC – Low Voltage Connector)
NOTE: Refer to the Eaton Hybrid Component and Connector Location for Connector Locations in MY08 Systems

Warnings & Cautions
Danger: Danger indicates you will be severely injured or killed if do not follow the indicated procedure.
Warning: Warning indicates an immediate hazard, which could result in severe personal injury if you do not follow the indicated procedure.
Caution: Caution indicates vehicle or property damage could occur if you do not follow the indicated procedure.
Note: Note indicates additional detail that will aid in the diagnosis or repair of a component/system.
Follow the specified procedures in the indicated order to avoid personal injury:
1. If the high-voltage cones are around the vehicle and the lockout is installed on the PEC, the only person that should be allowed to start the vehicle is the person who signed the lockout tag.
2. Before working on a vehicle or leaving the cab while the engine is running, you should place the shift lever in “N” set the parking brake, and block the wheels.
3. For safety reasons, always engage the service brakes prior to selecting gear positions from “N.”
4. Before starting a vehicle always be seated in the driver’s seat, select “N” on the shift control, and set the parking brakes.
5. In vehicles with ePTO, the engine and/or
Motor/Generator can start in ePTO mode. Never perform any maintenance or work on vehicle, while in this mode.
6. 12-Volt Battery positive (+) and negative (-) must be disconnected prior to any welding on any Hybrid equipped vehicle.
Caution: Follow the specified procedures in the indicated order to avoid equipment malfunction or damage.
Caution: Do not release the parking brake or attempt to select a gear until the air pressure is at the correct level.
Caution: To avoid damage to the transmission during towing place the shift lever in “N” and lift the drive wheels off the ground or disconnect the driveline.
High-Voltage Warnings & Cautions
- Use CO2 or Dry Chemical Fire Extinguishers.
- The high-voltage wiring is covered in orange insulation or convoluted tubing and marked with warning labels at the connectors.
- All Eaton® Hybrid Diesel/Electric vehicles will be marked ‘Hybrid’ on the outside of the vehicle, along with the shift label on the dash.
- Refer to OEM for specific location of chassis mounted hybrid components.
- Do NOT cut into the orange high-voltage cables.
- Do NOT cut into or open the PEC.
- Do NOT cut into or open the DC/DC Converter.
- Do NOT cut into or open the Inverter.
A buffer zone must be set up and high-voltage insulated rubber gloves (class 0 with leather protectors) are required prior to working on high-voltage. Failure to follow these instructions may result in severe personal injury or death.
The rubber-insulated gloves that must be worn while working on the high-voltage system are class 0 with leather protectors. The rubber gloves should be tested before EVERY use following the rubber insulation gloves testing procedure found in “Diagnostic Tools/Service Publications” on page 6. Failure to follow these instructions may result in severe personal injury or death.
Before inspecting or working on any high-voltage cables or components the “High-Voltage Service Shutdown and Power-Up Procedure” on page 4 should be followed. Failure to follow these instructions may result in severe personal injury or death.
The Lockout and Tagout devices should only be removed by the technician that placed the Lockout and Tagout devices on the vehicle. Failure to follow these instructions may result in severe personal injury or death.
High-voltage rubber insulated gloves (class 0 with leather protectors) must be worn when working on any
high-voltage cables. The “High-Voltage Service Shutdown and Power-Up Procedure” on page 4 must be followed prior to removing any high-voltage cables. Failure to follow these instructions may result in severe personal injury or death.
High-voltage cables and wiring are orange and contain a warning label at the connectors. High-voltage components are marked with a label. High-voltage rubber insulated gloves (class 0 with leather protectors) must be used when working on any of these components. Failure to follow these instructions may result in severe personal injury or death.
High-Voltage Service Shutdown Procedures
1. Follow “High-Voltage Work Area” procedure.
2. Locate the red PEC Service Switch on the front of the PEC and push switch off.
3. Remove the Service Switch Cover and install the Lockout Bracket (J48506).
4. Fasten tag to the Lockout Bracket.
5. Ensure the PEC Service Switch can not move from the off position.
6. Allow the system to set for a minimum of 5 minutes to discharge high voltage.
7. Connect ServiceRanger and view the Data Monitor PID 116 called “High-Voltage Battery Potential.” Use SPN 520323 for J1939 connection “Battery Voltage RB” (Relay Box).
8. The voltage should be 30 volts or less. If the voltage is above 30 volts, do not work on the vehicle and contact Eaton® at 1-800-826-HELP (4357).
9. Turn ignition key off and proceed to repair or troubleshooting step.
Note: The voltage drops to zero when the key is off.
High-Voltage Service Power-Up Procedure
1. Install all high-voltage connectors into their locked positions.
2. If you are the person working on the vehicle, remove the Lockout Bracket and tag.
3. Reinstall the Lockout Bracket over the Service Switch.
4. Pull the Service Switch out and let vehicle set for 2 minutes.
5. Start vehicle when appropriate.
Eaton Hybrid Component and Connector Location

Eaton Hybrid Component and Connector Location MY09 Systems

STEP 1.Verify Active or Inactive fault code status and perform Electrical Pretest.
- Review and follow the Warnings & Cautions.
- Key off.
- Key on.
- Retrieve Active fault codes and FMIs with ServiceRanger using the 9-Way Diagnostic Connector or manually.
- Perform the Electrical Pretest and Hybrid Electrical Pretest.
- If no issues are found during either Electrical Test, go to Step 2.
- If issue was repaired during either Electrical Test, go to Step 4.
STEP 2. Verify faults and FMIs present.
- Key off.
Danger: See High-Voltage Service Shutdown and Power-Up Procedures. Follow the procedures to avoid shock, burn or death from improperly handled high-voltage. - Which fault codes are present?
- If Fault Code 104 FMI 26 is listed, replace Power Electronics Carrier (PEC), then go to Step 4.
- If Fault Code 104 FMI 22, 23, 24, or 25 are listed with an Active Fault Code 111, go to procedures for Fault Code 111 and other appropriate FMIs see Fault Code 111 -Inverter Communication.
- If Fault Code 104 FMI 22, 23, 24, or 25 are listed with no Active Fault Code 111, go to Step 3.
- If Fault Code 88 is listed, see Fault Code 88 -CAN Inverter Message (HCM).
- If Fault Code 89 is listed, see Fault Code 89 -CAN BCU 1 Message (HCM).
STEP 3. Verify resistance on PEC CAN Harness.
- Key off.
- Disconnect the PEC 19-Way Connector.
- Inspect the connector pins for damage, corrosion, and loose wires.
- Measure the resistance between the PEC 19-Way Connector at the following pins:
- Pin 1 and 2
- Pin 1 and ground
- Pin 2 and ground
- If resistance between Pin 1 and Pin 2 is 50–70 ohms and resistance from Pin 1 and Pin 2 to ground are each 10K ohms or greater, replace Power Electronics Carrier (PEC). Go to Step 4.
- If resistance is outside of range, repair the CAN data link between the PEC and Inverter, then go to Step 4.
STEP 4. Verify repair.
- Key off.
- Reconnect all connectors and the negative battery cable.
- Key on.
- Clear codes, see Clear Inactive Faults.
- Drive the vehicle and attempt to reset the code.
- Check for codes, see View Active and Inactive Faults.
- If no codes, test is complete.
- If fault Code 104 appears, find error in testing, go to Step 1.
- If a code other than 104 appears, see Fault Code Index List.
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