Eaton Fuller Hybrid Transmission Fault Code 34 – Weak Battery Voltage Fault (TECU)

Fault code for Eaton Hybrid Drive Systems models (MY08) EH-8E306A-U, EH-8E306A-UPG, EH-8E306A-UP, EH-8E306A-CD, EH-8E306A-T, EH-6E606B-CD, (MY09) EH-8E406A-U/P, EH-8E406A-UP, EH-8E406A-UPG, EH-8E406A-CD, EH-8E406A-CDG, EH-8E406A-CDR, EH-8E406A-T, EH-6E706B-CD, EH-6E706B-P and EH-6E706B-UPG. The Transmission Electronic Control Unit (TECU) is mounted to the transmission and contains the software that controls transmission operation. The Hybrid Control Module (HCM) controls the operation of the TECU during normal hybrid mode operation. The HCM main battery power is a fused 12-volt feed that runs through the HCM 38-Way Vehicle Connector with the main ground wire returning to the battery negative terminal.

Fault Code 34 – Weak Battery Voltage Fault (TECU) Detection

The following preconditions must be met before the system detects the fault:

  • TECU ignition voltage is greater than 7 volts and less than 16 volts.
  • TECU System Battery Voltage Low Fault is inactive.

Conditions to Set Fault Code Active

  • FMI 14 is set when the TECU battery voltage drops below 9 volts for 10 seconds.

Fallback When Fault Code 34 is set the following conditions occur:

  • Fault is stored in TECU memory.
  • TECU switches to AutoShift Fallback mode.
  • If this fault occurs while moving, it causes a 1- speed fallback.
  • If the vehicle is stopped, the starting gear and reverse can be engaged.

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 14
    • Power supply to TECU
    • Low batteries or bad main power connection
    • Charging system
    • TECU

Fault Code 34 – Weak Battery Voltage Fault (TECU) Test

Component Identification:

Fuller Harness Front View TECU - Vehicle Interface Connector

Front View
(TECU – Vehicle Interface Connector)

NOTE: Refer to the Eaton Hybrid Component and Connector Location for Connector Locations in MY08 Systems

NOTE: Refer to the Eaton Hybrid Component and Connector Location for Connector Locations in MY09 Systems.

Eaton Fuller TECU to battery connector

STEP 1. Check for Active or Inactive fault code status and perform Electrical Pretest.

  1. Retrieve Active fault codes and FMIs with ServiceRanger using the 9-Way Diagnostic Connector.
  2. Perform the Electrical Pretest:
    • If Fault Code 34 is Active after performing the Electrical Pretest, replace Transmission Electronic Control Unit (TECU) (only if Fault Code is Active), then go to Step 2.
    • If Fault Code 34 is Inactive after performing the Electrical Pretest, clear codes. Go to Step 2.

STEP 4. Verify repair.

  1. Key off.
  2.  Reconnect all connectors and negative battery cable.
  3. Key on.
  4. Clear codes, see  Clear Inactive Faults.
  5. Drive the vehicle and attempt to reset the code.
  6. Check for codes, see View Active and Inactive Faults.
    • If no codes, test is complete.
    • If fault Code 34 appears, find error in testing, go to Step 1.
    • If a code other than 34 appears, see Fault Code Index List.

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