Case IH 19740 – DCU: Main Relay opens too early / too late

Fault code 19740 indicates a timing and circuit issue with the Main Relay that powers the emissions system. The control loop monitoring the internal feedback voltage of the main relay is detecting an improper voltage state, meaning the relay is either stuck, shorted to battery positive, or suffering from an open circuit.

• Tier 4a
• Case IH Magnum 235 - 340 (Series 4)
• Case STX Steiger (350 - 600 HP)
• Case IH Puma
• Case IH Farmall

1. 1

   Perform a relay swap test by swapping the DCU main relay with another known-good relay (such as the DEF heater main relay) to see if the fault changes or clears.

2. 2

   Check the main relay wiring harness for an open circuit or a short circuit to the battery positive (Battery+) on the supply line.

3. 3

   Monitor the internal feedback voltage of the main relays using a diagnostic scan tool.

4. 4

   Check the DCU dataset version. Application errors or outdated EEPROM software can cause the relay to shut off too early or too late.

1. 1

   Locate the Relay

   Identify the main power relay for the DCU/Denox module in the machine's power distribution box.

2. 2

   Inspect and Repair Wiring

   Thoroughly inspect the wiring harness connecting the relay to the DCU. Look for chafed wires, corrosion at the pins, or broken connections. Repair any open or shorted wiring.

3. 3

   Replace the Relay

   If the wiring is intact but the relay is sticking or failing the swap test, replace the main relay.

4. 4

   Update Software

   Connect the CNH Electronic Service Tool (EST) and verify if there is an EEPROM/dataset update available for the DCU. Flash the module if an update is required to resolve relay timing logic errors.

5. 5

   Replace the DCU

   If the wiring is good, the relay is new, the software is up to date, and the failure occurs repeatedly, the internal circuitry of the pump module is likely defective. Replace the Denox Pump Module (DCU). Note: CNH requires troubleshooting to be performed before issuing passwords for the new software installation.