SPN 677 FMI 15: Meaning and Fix
SPN 677 FMI 15 indicates the Engine Starter Motor Relay signal is valid but exceeds the normal operating threshold at its least severe level. The ECM monitors relay coil feedback voltage; when it reads above the calibrated upper boundary, this fault logs. This commonly appears in cold-climate heavy equipment after extended cranking cycles, where relay coil resistance drops cause elevated voltage feedback. Technicians frequently encounter this code on MAN TGX and Deutz TCD engines following starter harness modifications or relay substitutions with incorrect coil resistance specifications.
Common Symptoms
- Extended Cranking Duration: Engine cranks longer than normal before starting, indicating relay engagement timing is disrupted by abnormal voltage signal levels.
- Intermittent Start Failure: Vehicle occasionally fails to start despite battery charge being adequate, caused by ECM suppressing relay activation after threshold violations.
- Active Fault Lamp: Amber or red MIL illuminates on the dashboard cluster, logged alongside J1939 DTC broadcasts detected by fleet diagnostic tools.
- Relay Chatter Noise: Audible rapid clicking from the starter relay circuit occurs when ECM detects marginal over-range voltage and cycles relay output reactively.
Probable Causes
- Incorrect Relay Specification: Replacement relay with lower coil resistance than OEM specification generates elevated feedback voltage exceeding ECM-calibrated upper threshold limits.
- Wiring Harness Fault: Chafed or shorted wiring between ECM pin and relay coil creates unintended voltage paths, producing above-normal signal readings during crank.
- ECM Output Driver Fault: Internal ECM driver channel regulating relay coil current loses precision control, pushing feedback voltage beyond the FMI 15 detection boundary.
- Battery Voltage Spike: Unregulated voltage transients from failing alternators or jump-start events propagate through relay circuits, temporarily elevating monitored signal above threshold.
Advanced Technical Analysis
The ECM microcontroller continuously samples the SPN 677 relay control circuit using an analog-to-digital input channel. Per SAE J1939-71 parameter mapping, the relay feedback signal is compared against a manufacturer-programmed upper threshold. On Bosch EDC17 platforms used in MAN and Mercedes-Benz heavy trucks, this threshold typically sits near 32V equivalent signal representation. FMI 15 triggers when the sampled value persistently exceeds this boundary across a defined sample window without reaching hardware fault levels.
Electrical debouncing timers prevent false FMI 15 logging during normal cranking transients. Bosch factory calibration documents specify debounce periods between 200ms and 500ms for starter relay monitoring nodes. If the above-normal condition persists beyond this window, the fault is latched into non-volatile ECM memory. A single jump-start with a 24V booster on a 12V system can saturate the relay driver input momentarily, but the debounce filter typically prevents permanent fault registration unless the condition is sustained.
Upon confirming SPN 677 FMI 15, ECM safety logic on Deutz TCD and MAN D26 engines may restrict successive start attempts to protect the starter motor from thermal overload. While FMI 15 is classified least severe, repeated detections within a key-cycle can escalate the ECM response to blocking relay activation entirely as a protective fallback. Torque derate is not directly associated with this SPN; however, failed start attempts indirectly prevent engine torque delivery, causing operational downtime in time-critical construction applications.
Long-term diagnostic strategy requires verifying OEM relay part numbers against Bosch or manufacturer-approved cross-reference catalogs before installation. Technicians in workshops servicing municipal fleet trucks frequently find this fault after preventive maintenance where generic relays were installed. Using a calibrated oscilloscope to measure relay coil supply voltage during cranking, comparing waveform peaks against ECM datasheet specifications, and performing insulation resistance tests on harness pin 677 routing are essential prevention steps validated across Mercedes-Benz Actros and Deutz industrial engine overhaul procedures.
Step-by-Step Troubleshooting Guide
- Verify Relay Part Number: Confirm installed relay coil resistance matches OEM specification; replace generic parts with manufacturer-approved components to restore correct voltage feedback.
- Inspect Starter Harness: Perform visual and continuity inspection on wiring between ECM relay output pin and starter relay coil terminal, checking for shorts or chafing.
- Measure Coil Feedback Voltage: Use calibrated oscilloscope during cranking cycle to capture peak relay feedback voltage; compare readings against ECM datasheet upper threshold boundary.
- Clear and Retest ECM: After repairs, clear fault codes using J1939-compliant diagnostic software, perform three consecutive start cycles, and confirm SPN 677 FMI 15 does not recur.