Glow Plug Circuit Fault Detected at Startup

Common Symptoms

• COMMANDER controller display shows fault code 7 during the pre-heat phase before crank
• Engine cranks but struggles to fire in cold weather (below 40 degrees F)
• White or gray smoke pours from exhaust on startup and takes longer than normal to clear
• Tower starts normally when ambient temps are warm but fault appears consistently in cold mornings
• Pre-heat indicator on the COMMANDER panel does not illuminate or stays on too briefly before crank attempt
• Engine misfires or runs rough for the first 30-60 seconds after a hard cold start
• Fault code 7 clears once the engine warms up but returns on the next cold start cycle

Probable Causes (Ranked by Likelihood)

• One or more Kubota glow plugs have failed open circuit or burned out Very Likely
• Glow plug relay has failed and is not supplying voltage to the glow plug bus Very Likely
• Corroded or broken wiring between the glow plug relay and individual glow plug terminals Likely
• COMMANDER controller not sending the preheat trigger signal to the relay due to a control wire fault Possible
• Weak or sulfated battery causing low voltage drop across the glow plug circuit during preheat draw Possible
• Glow plug timer circuit inside the COMMANDER controller has failed internally Less Likely
• High-resistance ground connection on the engine block affecting glow plug return path Less Likely

Step-by-Step Diagnostic Procedure

1. Step 1. Set the COMMANDER controller to OFF. Open the engine compartment and locate the glow plug relay, usually a single black cube relay mounted near the intake manifold or firewall on Kubota D902/D1105/D1305 engines. Visually inspect it for heat damage, cracked housing, or burnt terminals.

2. Step 2. With the battery connected and the unit in OFF, use a multimeter set to DC volts. Probe battery positive and negative terminals. You need at least 12.4V (12V systems) for the glow plug circuit to function properly. A reading below 12.0V means the battery is suspect and must be charged or replaced before further testing.

3. Step 3. Locate the glow plugs. On the Kubota D902 (3-cylinder) there are three glow plugs, one per cylinder, threaded into the prechamber on the intake side of the head. On the D1105/D1305 there are also three. Each has a single small wire nut on top. Disconnect each wire one at a time.

4. Step 4. Set your multimeter to resistance (ohms). Probe from each glow plug terminal to the engine block (ground). A good Kubota glow plug reads approximately 0.5 to 2.0 ohms. Any reading of OL (open) or above 5 ohms means that plug is failed and needs replacement. Replace all three as a set if one is bad. Individual plugs run $8-$15 each.

5. Step 5. Test the glow plug relay. With the ignition key or COMMANDER controller switched to the RUN position (pre-crank), use your multimeter on DC volts and probe the relay output terminal to ground. You should see battery voltage (12V) for approximately 6-15 seconds during the preheat cycle. No voltage at the output terminal with voltage confirmed at the input terminal means the relay is bad. Relay replacement is straightforward and costs under $20.

6. Step 6. Inspect the wiring harness from the relay down to each glow plug terminal. Look for melted insulation, corroded ring terminals, or wires that have chafed against the block or exhaust manifold. A single broken wire can cause code 7 even with all plugs intact. Repair or replace any damaged sections with appropriately rated wire (10-12 AWG for glow plug circuits).

7. Step 7. Check the engine block ground strap. Find the braided ground strap connecting the block to the frame or battery negative. Clean both ends with a wire brush and re-torque the fasteners. A high-resistance ground causes voltage drop across the entire glow plug circuit and can trigger code 7 without any actual plug failure.

8. Step 8. If all plugs, the relay, wiring, and battery test good and code 7 still appears, the fault likely lies in the COMMANDER controller preheat output circuit or internal timer logic. At this point the diagnosis requires a factory wiring diagram and COMMANDER controller-level testing. Contact your Allmand dealer or a qualified equipment technician for controller diagnostics.

Frequently Asked Questions