Case IH 3883 (DTC 797D) - ECU – DCU (Dosing Control Unit): Supply UB3 short circuit to ground failure

The system has detected a low voltage / short-to-ground condition on Voltage Supply 3 (UB3). The UB3 circuit is responsible for powering the Coolant Control Valve and Reverting Valve, which controls the flow of engine coolant used to heat and thaw the Diesel Exhaust Fluid (DEF/AdBlue) system.

• MAXXUM T4E
• 240 Series (7240, 8240, 9240)
• 130/140/150/160 Series
• Magnum 235-340 (Series 4)
• Steiger 350-600
• Puma
• Maxxum
• Optum
• Farmall series (115U, 120A-140A)
• Patriot Series
• Farmlift series
• CX220E

1. 1

   Check DCU Voltage: Verify the main voltage supply to the Dosing Control Unit is within specification.

2. 2

   Wiring Harness Inspection (Wiring Short Cause): Inspect the wiring harness connecting the DCU to the Coolant Control Valve (UB3 circuit). Look for chafed wires, pinched lines, or exposed copper grounding against the chassis.

3. 3

   Component Isolation Test (Valve Short Cause): Disconnect the electrical connector at the Coolant Control Valve. If the active fault code changes from a short to ground (P20A2 / 19084) to an open circuit (P20A0 / 19083), the valve is internally shorted.

4. 4

   Check for Leaks: Ensure the Coolant Control Valve is not leaking engine coolant into its electrical connector, causing a short.

5. 5

   Valve Operation Check: Check if the heating valve is stuck open by feeling if the coolant lines are constantly warm leading to the DEF tank level unit, even when heating is not commanded.

6. 6

   Pump Module Internal Failure (Module Short Cause): If wiring and external valve test fine, repeated failures on this circuit can indicate an internal failure of the DEF pump module or the DCU itself.

1. 1

   Power Down

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

2. 2

   Locate the Valve

   Locate the DEF Coolant Control Valve / Reverting Valve. It is typically mounted near the DEF storage tank or integrated near the DEF pump module plumbing.

3. 3

   Repair Wiring (Wiring Short Cause)

   If the diagnostic phase revealed a chafed wire on the UB3 circuit, cut out the damaged section, splice in new wire using weather-sealed heat-shrink connectors, and re-loom the harness to prevent future rubbing.

4. 4

   Replace the Valve (Valve Short Cause)

   Pinch off the coolant lines leading to the valve to minimize fluid loss. Remove the retaining bolts/brackets securing the valve (e.g., on some Farmall models, remove the two M14 retaining bolts securing the support bracket). Disconnect the coolant/DEF lines and remove the faulty valve. Install the new Coolant Control Valve and torque the mounting bolts to OEM specifications (e.g., 73 - 80 lb-ft for Farmall support brackets).

5. 5

   Replace DEF Pump Module (Module Short Cause)

   If the short is internal to the pump module, drain the DEF tank, disconnect the supply/return lines and electrical harnesses, unbolt the flange assembly, and install a replacement DEF supply module.

6. 6

   Reassembly and Clearing

   Reconnect the battery, top off any lost engine coolant, and use a diagnostic scan tool to erase the fault code. Run the machine to verify the code does not return.