Case IH 3691 – Engine Control Unit (ECU): Glow Plug Circuit. - No Load Error For Low Voltage System

The Engine Control Unit (ECU) is commanding the glow plug circuit to activate, but it is not detecting the expected electrical load (current draw). This is a classic open circuit condition, which is most commonly caused by a burnt-out glow plug, a severed wire, a disconnected harness, or a failed glow plug control unit/relay.

• Farmall
• skid steer
• SR250
• SV250
• Farmall 95U
• Farmall 105U
• Farmall 115U
• Farmall 55C
• Steiger
• Puma
• Maxxum

1. 1

   Inspect the glow plug wiring harness, starting from the glow plugs back to the glow plug control unit and the ECU. Look for chafed wires, disconnected plugs, or heavy corrosion at the terminals.

2. 2

   Disconnect the wiring from each glow plug and use a multimeter to measure the resistance of each plug to ground. A healthy glow plug will have very low resistance. If a plug reads as an open circuit (infinite resistance/OL), it is burnt out.

3. 3

   Check the glow plug control unit (relay) to ensure it is outputting voltage to the glow plug rail when the ignition is first turned on (during the pre-heat cycle).

4. 4

   Verify continuity on the signal wire between the glow plug control unit and the Engine ECU.

1. 1

   Disconnect Power

   Always disconnect the negative battery cable before servicing engine electrical components.

2. 2

   Address Wiring Issues

   If the visual inspection or continuity test revealed broken wires or corroded connectors, repair the harness or replace the damaged pigtails.

3. 3

   Replace Faulty Glow Plugs

   If any glow plugs failed the resistance test, remove them using a deep socket. Apply a small amount of anti-seize to the threads of the new glow plugs, thread them in carefully to avoid cross-threading, and torque them to the OEM specification. Reconnect the glow plug harness.

4. 4

   Replace the Control Unit

   If the glow plugs and wiring test perfectly but the system still fails to draw a load (or the control unit fails to output voltage), replace the Glow Plug Control Unit.

5. 5

   Clear and Retest

   Reconnect the battery. Use the Case IH Electronic Service Tool (EST) or an equivalent diagnostic scanner to clear the 3691 fault code. Cycle the ignition to trigger a cold start pre-heat and verify the code does not return.