John Deere 523666.01 – ECU (Engine Control Unit) / TCU (Transmission Control Unit): Supply Voltage (BAT) Too Low / ECU Power Circuit #1 Open

The supply voltage to the control unit has dropped below 9 volts. Adequate battery voltage is essential for the proper operation of the control software and the vehicle's electrical systems; when it drops below this threshold, the control software may experience limited or no functionality.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 9470RT
• 9520RT
• 9570RT
• 8R Series
• 8RT Series
• 8RX Series
• 8245R
• 8400R
• 5080G
• 5090G
• 5090GH
• 5080GV
• 5090GV
• 5100GV
• 5080GF
• 5090GF
• 5100GF

1. 1

   Test Battery Voltage: Use a multimeter to measure the battery voltage directly at the terminals. Ensure it is reading well above 9 volts (ideally ~12.6V for a fully charged 12V battery).

2. 2

   Inspect Battery and Connections: Check the battery for adequate charge. Ensure that all battery terminal connections and ground straps are secure, tight, and free of corrosion.

3. 3

   Check the Charging System: Inspect the alternator and charging system to ensure it is properly charging the battery while the engine is running.

4. 4

   Check Fuses: Inspect the ECU power circuit fuses (often located near the battery compartment, such as F5005, F5006, and F5007 depending on the machine model) to ensure none are blown.

5. 5

   Monitor Battery Usage (Parasitic Draw): Ensure that no electrical components are draining the battery excessively when the vehicle is turned off.

1. 1

   Clean/Tighten Connections

   If corrosion or loose cables are found, clean the battery terminals and cable ends thoroughly with a wire brush, then tighten them securely.

2. 2

   Recharge or Replace Battery

   If the battery voltage is low and fails a load test, recharge the battery fully. If it cannot hold a charge, replace the battery.

3. 3

   Repair/Replace Charging Components

   If the alternator is failing to charge the battery, repair or replace the faulty alternator.

4. 4

   Replace Fuses

   If an ECU power circuit fuse is blown, replace it with a fuse of the exact same amperage rating.

5. 5

   Reset ECU and Monitor

   After resolving the voltage issue, reset the ECU (often done by cycling the key switch or using a diagnostic tool like Service ADVISOR) to clear the fault code. Monitor the system to ensure normal operation and verify the code does not return.