SPN 5746 FMI 5: Meaning and Fix
SPN 5746 FMI 5 indicates the ECM detected current below normal or an open circuit in the relay controlling the Aftertreatment 1 DEF Dosing Unit 1 Heater. This relay supplies battery voltage to heat the dosing unit, preventing DEF crystallization in cold climates. Technicians frequently encounter this fault after a forced DPF regeneration when the heater relay fails to close, or after replacing the ECM without proper relay pin-out verification, leading to no heater operation and potential dosing unit damage.
Common Symptoms
- No DEF Heating: Dosing unit heater remains cold; DEF may freeze below -11°C, preventing dosing.
- Active MIL Lamp: Malfunction Indicator Lamp illuminates on dash, often with amber stop engine lamp.
- Dosing Unit Fault: ECM logs additional SPN 3361 FMI 5 for DEF pump control circuit.
- Torque Derate: Engine power reduced by up to 25% after 30 minutes of continuous fault active.
Probable Causes
- Open Relay Coil: Relay coil resistance >100 ohms; internal winding open due to thermal stress.
- Corroded Connector: Pin 30 or 87 at relay socket corroded from DEF spillage, causing intermittent open.
- Fused Harness Wire: Wire from ECM pin J2-45 to relay coil melted against exhaust shield.
- ECM Driver Failure: Internal MOSFET open circuit; output voltage at ECM pin remains at 0V.
Advanced Technical Analysis
The ECM microcontroller monitors relay current via a low-side driver with a 1.5-amp fuse. When the relay is commanded on, the driver expects a current draw between 0.15 and 0.35 amps. If current stays below 0.05 amps for more than 200 ms, the fault sets. This debouncing timer prevents false triggers from transient vibration. Real-world: After a DEF spill, dried crystals on relay pins cause intermittent open, setting the code only during cold starts.
Electrical analysis shows the relay coil inductance creates a back-EMF spike. The ECM uses an internal flyback diode to clamp voltage to 36V. If the diode fails short, the driver sees continuous current below threshold, mimicking an open circuit. This rare failure mode is often misdiagnosed as a bad relay. Technicians must measure voltage at relay pin 86 during commanded on state to distinguish.
When this fault is active, the ECM enters a safety fallback: it disables the DEF dosing unit heater and sets a torque derate of 25% after 30 minutes of continuous operation. The derate ramps linearly to protect the SCR system from DEF freezing. If the heater remains off in sub-zero temperatures, DEF crystallizes in the dosing unit, leading to physical blockage and requiring unit replacement.
Long-term strategy: verify relay coil resistance at 20°C (70-90 ohms). Common workshop scenario: after replacing the ECM, technicians forget to transfer the relay ground wire from old harness, causing an open circuit. Prevention includes dielectric grease on relay pins and routing harness away from exhaust. In Deutz TCD 2.9 engines, this fault often appears after DPF ash cleaning when the relay connector is disturbed.
Step-by-Step Troubleshooting Guide
- Measure Relay Coil: Disconnect relay; measure resistance between pins 85 and 86; should be 70-90 ohms.
- Check Supply Voltage: Ignition on; measure 12V at relay pin 30 (battery) and pin 86 (ECM driver).
- Inspect Connector Pins: Look for corrosion, bent pins, or DEF crystals at relay socket and ECM J2 connector.
- Test ECM Driver: Command relay on via diagnostic tool; measure voltage at ECM pin J2-45; should drop to 0V.