New Holland 3691 – ECU: Glow Plug Circuit Malfunction

Indicates a 'Glow Relay Output Open Load/Short to Battery' or a 'Glow Plug Circuit - No Load Error For Low Voltage System'. The Engine Control Unit (ECU) has detected an issue within the glow plug pre-heating circuit. The ECU commanded the glow plug relay to activate but did not detect the expected electrical load, indicating an open circuit (a break in the circuit) or a short to the battery/power source.

• T4.110F
• L220
• C238
• TC35
• TC35D
• TC40
• TC40D
• TC45
• TC45D
• F5C engines
• ISM-DI engines

1. 1

   Check for other active or stored fault codes using a diagnostic tool (codes 3694, 3695-3698 can indicate specific shorts to ground or individual glow plug failures)

2. 2

   Inspect all related fuses for the glow plug circuit to ensure none are blown

3. 3

   Visually inspect the glow plug relay for any signs of damage or corrosion

4. 4

   Examine the wiring harness and connectors between the ECU, glow plug relay, and glow plugs for chafing, corrosion, abrasion, or insecure connections

5. 5

   Test each glow plug for continuity using a multimeter (expected resistance is typically between 0.5 and 2 ohms; infinite resistance indicates a failed, open-circuit plug)

6. 6

   Test the glow plug relay/controller by verifying it 'clicks' when activated and sends power to the output terminal leading to the glow plugs

1. 1

   Disconnect battery

   Disconnect the negative battery terminal to prevent short circuits before beginning any electrical repair.

2. 2

   Replace blown fuse

   If a fuse is blown, replace it with a new one of the identical amperage rating. If the new fuse blows immediately, locate and repair the persistent short circuit in the system.

3. 3

   Replace faulty glow plugs

   If one or more glow plugs test faulty (infinite resistance), replace them as a set. Apply penetrating oil before removal to avoid breakage in the cylinder head. Clean the threads and torque to the manufacturer's specification.

4. 4

   Replace glow plug relay

   If the relay fails testing, disconnect the electrical connectors and remove the mounting hardware (typically 10mm nuts). Mount the new relay securely and reconnect the wiring harnesses, ensuring they click into place.

5. 5

   Clear codes and verify repair

   Reconnect the battery. Use a diagnostic tool to clear the 3691 fault code from the ECU's memory. Start the engine or initiate the pre-heat cycle, re-scan for fault codes to ensure the issue has been resolved, and monitor the glow plug indicator light on the dashboard to confirm it operates as expected.