Case IH 3879 (DTC 687D) - ECU – DCU (Dosing Control Unit) or Engine ECU (EDC): Supply UB3 short circuit to battery failure

The UB3 voltage supply circuit has detected a short to battery voltage (often when the key is in the "off" position). The UB3 circuit is responsible for supplying electrical power to the Coolant Control Valve and the Reverting Valve (also known as the Reductant Purge Control Valve) within the DEF/AdBlue (SCR) system.

• MAXXUM T4E
• Axial-Flow Combines 240 Series (7240, 8240, 9240)
• Steiger series
• Magnum series (235-340 Series 4)
• Farmall
• Maxxum
• Austoft 9900 series

1. 1

   Inspect the wiring harness running between the DCU and the DEF Coolant Control Valve / Reverting Valve. Look for chafed wires that may be shorting to a 12V power source.

2. 2

   Disconnect the connectors at the DCU, the Coolant Control Valve, and the Reverting Valve. Inspect for water ingress, corrosion, or bent pins which can bridge circuits and cause a short to battery.

3. 3

   Disconnect the Coolant Control Valve and the Reverting Valve one at a time. Use a multimeter to check if the short to battery on the UB3 circuit disappears. If the short goes away when a valve is unplugged, the valve itself is internally shorted.

4. 4

   With the key in the OFF position (Key 15 off), probe the UB3 supply circuit. If voltage is present, the short to battery is active.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the main battery switch to prevent electrical spikes.

2. 2

   Locate Components

   Locate the DEF/AdBlue supply module. Identify the Coolant Control Valve (which regulates engine coolant flow to thaw the DEF tank) and the Reverting Valve (which purges DEF back into the tank when the machine shuts down).

3. 3

   Repair Wiring

   If the diagnostic checks revealed a chafed wire or melted harness shorting to battery voltage, repair the damaged section of the wiring harness using OEM-approved waterproof splices, or replace the harness section.

4. 4

   Replace Faulty Valve

   If the component isolation test proved that the Coolant Control Valve or Reverting Valve is internally shorted, unbolt the faulty valve, disconnect the fluid lines (be prepared to catch minor coolant or DEF spillage), and install the replacement valve.

5. 5

   Reassemble and Test

   Reconnect all electrical connectors and battery power.

6. 6

   Clear Codes and Verify

   Connect a diagnostic tool (such as the Case IH EST) to clear the fault codes. Run the engine and command a DEF system purge/dosing test to verify that the UB3 circuit operates normally without throwing the 3879 / 687D code.