Case IH 3690 – Engine ECU (Electronic Diesel Control / EDC): DFC for glow module error in GCU-T

The Engine Control Unit detects an internal fault, communication error, or operational failure within the Glow Control Unit (GCU-T), which manages the low-voltage glow plug system for cold starting.

• Farmall 105U
• Farmall 115U
• Maxxum series
• Magnum series
• Crawler Dozers 650M LT Tier 4B

1. 1

   Check for related fault codes (3691, 3692, 3693, 3694) to rule out specific circuit failures rather than a module failure

2. 2

   Inspect the wiring harness between the Engine ECU and the GCU-T for chafing, corrosion, or loose connector pins

3. 3

   Test the individual glow plugs and the glow plug relay for proper resistance to ensure a shorted glow plug hasn't overloaded the GCU-T

4. 4

   Clear the diagnostic code from the fault log, cycle the key (turn ECS to OFF/RESET, then to STOP), and restart the tractor to observe if the error returns

1. 1

   Disconnect Battery Power

   Always disconnect the battery before servicing engine management modules to prevent voltage spikes.

2. 2

   Locate the GCU-T

   Locate the Glow Plug Control Unit (GCU-T) in the engine bay (often mounted near the engine bulkhead or directly on the engine harness bracket).

3. 3

   Disconnect the Harness

   Carefully unplug the multi-pin connector from the glow plug module. Inspect the terminals for signs of arcing, melting, or water intrusion.

4. 4

   Replace the Module

   If the wiring tests good and no short circuits are found in the glow plugs (which would cause codes 3693/3694), the GCU-T has failed internally. Unbolt the old module and install the replacement.

5. 5

   Reconnect and Clear Codes

   Reconnect the wiring harness, reconnect the battery, and use the EASy tool (or equivalent diagnostic scanner) to clear the inactive 3690 code from the Engine ECU.