SPN 4374 FMI 4: Meaning and Fix
SPN 4374 FMI 4 indicates the ECM detected the DEF pump motor speed signal voltage below the normal operating range, typically below 0.5 V. This fault often appears after a DEF system fluid leak or freeze event damages the pump wiring. Technicians frequently encounter this code following a failed DEF pump replacement where the harness was pinched during reassembly, causing a direct short to ground on the speed sensor circuit.
Common Symptoms
- Engine Derate: ECM reduces engine torque by up to 40% to protect aftertreatment system from insufficient DEF dosing.
- MIL Illuminated: Amber malfunction indicator lamp and red stop lamp activate on dash within one drive cycle.
- No DEF Dosing: Aftertreatment 1 DEF pump remains stationary; no fluid delivered to the doser injector.
- Frequent Regen: DPF regeneration cycles occur more often due to elevated NOx levels from lack of DEF.
Probable Causes
- Shorted Wiring: Speed sensor signal wire shorted to ground inside harness, often at chassis rub points near frame rail.
- Failed Pump Motor: Internal motor winding short or Hall-effect sensor failure within the DEF pump assembly.
- Corroded Connector: Moisture ingress into the pump 6-pin connector causing pin-to-pin corrosion and low resistance to ground.
- ECM Fault: Internal ECM pull-up resistor failure or damaged input circuit after a voltage spike from jump-starting.
Advanced Technical Analysis
The ECM monitors DEF pump motor speed via a digital Hall-effect sensor outputting a 0-5 V square wave proportional to rpm. A voltage below 0.5 V for more than 10 consecutive samples triggers FMI 4. The ECM debounces the fault using a 2-second timer to avoid false triggers from transient noise, but a sustained low condition confirms a hard electrical short.
Electrical analysis shows that the sensor pull-up resistor inside the ECM (typically 2.2 kΩ to 5 V) creates a voltage divider with the sensor ground path. If the signal wire resistance to ground drops below 100 Ω, the measured voltage falls below the threshold. This is commonly caused by chafed insulation contacting the engine block or frame rail.
When FMI 4 is active, the ECM immediately stops PWM output to the pump motor and sets a fallback strategy: DEF dosing is disabled, causing NOx conversion efficiency to drop below 50%. The engine torque is derated linearly over 60 minutes to 60% of maximum, and a forced DPF regeneration is inhibited to prevent thermal damage.
Long-term diagnostic strategy involves first performing a resistance check between pin 3 (signal) and pin 1 (ground) at the pump connector, expecting >10 MΩ. A reading below 1 kΩ confirms a short. In workshop practice, this fault is often misdiagnosed as a pump failure when the actual cause is a corroded connector at the chassis harness bulkhead, common on MAN TGS trucks operating in humid environments.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect DEF pump harness from pump to ECM for chafing, pinched wires, or melted insulation near exhaust components.
- Measure Resistance: Disconnect pump and ECM; measure resistance between signal pin and ground. Expect >10 MΩ; replace harness if <1 kΩ.
- Check Connector: Unplug pump connector; inspect for green corrosion or bent pins. Clean with electrical contact cleaner and dielectric grease.
- Verify ECM Input: Reconnect pump; backprobe ECM signal pin with multimeter. Crank engine; expect 0-5 V square wave. Replace ECM if no signal.