New Holland 3693 – ECU: Glow Plug Circuit Short to Battery

The Engine Control Unit (ECU) has detected abnormally high voltage in the glow plug circuit when it should be off, indicating a short circuit to the battery. This means one or more components in the glow plug system are receiving continuous power, bypassing the intended control from the ECU. This condition can lead to difficulty starting in cold weather, excessive white smoke on startup, rough engine idle, and abnormal behavior of the glow plug indicator light.

• Boomer
• TC18
• TC33
• TC35
• TC40
• TC40DA
• TC45
• TC55DA
• T1510
• T2210
• L125
• LS125
• L140
• LS140
• L150
• LS150
• L160
• LS160
• L170
• LS170
• L175
• L465
• LX465
• LX565

1. 1

   Visually inspect the wiring harness leading to the glow plugs, relay, and timer for melted insulation, chafed wires, or loose/corroded connectors.

2. 2

   Check the battery to ensure it is fully charged and verify that main chassis and engine ground connections are secure and free of corrosion.

3. 3

   Test the glow plug relay/timer by listening for an audible click when the ignition is turned to ON; if there is no click, the relay may not be receiving a signal or may have failed internally.

4. 4

   Check for constant power at the glow plug bus bar with the key OFF using a voltmeter; battery voltage present indicates a short in the wiring or a stuck relay.

5. 5

   Test the glow plug relay output voltage during the pre-heat cycle (should be battery voltage) and after the cycle ends (should drop to zero).

6. 6

   Test individual glow plugs for resistance (typically 0.5 to 2.0 ohms) to identify any that have failed open.

1. 1

   Disconnect battery

   Disconnect the negative battery terminal to prevent electrical shorts during the repair procedure.

2. 2

   Access components

   Remove necessary side panels or covers to access the engine, glow plug timer, and relay (e.g., on a TC40DA, a side cover must be removed).

3. 3

   Repair wiring or replace relay

   If a short is found in the harness, repair the damaged section with appropriate connectors and heat shrink tubing. If the relay is stuck closed (failing the voltage drop test), disconnect and unbolt the faulty glow plug relay/timer and install a replacement.

4. 4

   Replace faulty glow plugs

   Remove the electrical bus bar. Use a deep socket to carefully unscrew faulty glow plugs. If stuck, apply penetrating oil and let soak to avoid breakage. Thread new glow plugs in by hand and torque to 12-15 ft-lbs (consult specific model manual for exact torque).

5. 5

   Reassemble and test

   Reconnect the glow plug wiring, covers, and the negative battery terminal. Turn ignition ON to verify the glow plug light illuminates for the pre-heat cycle and turns off. Start the engine and clear the fault codes using a diagnostic tool.