This Detroit turbo speed low reference documents fault code SPN 103/FMI 4 (Turbocharger Speed Sensor Circuit Failed Low) for Detroit DD-series engines with the TSS on the MCM harness. It reproduces the original TruckFaultCodes.com fault tree, from the first check through the final repair, so the code can be confirmed and worked in the exact order the factory intended. Each step lists the measured value or check and the pass or fail branch that follows, matching the layout used in the source document.
Detroit turbo speed low troubleshooting overview
The tree isolates a turbocharger speed sensor circuit that has failed low. After disconnecting the TSS and the 120-pin MCM connector it measures resistance across pins 1 and 2 and from each pin to ground, then directs repair of the short in MCM wires 50 and 51 or at the harness pins. Follow the checks in sequence and verify the repair before clearing the code and releasing the vehicle.
Official document details
| Brand | Detroit |
| Model coverage | Detroit DD-series engines with the TSS on the MCM harness |
| System / group | DTCs — turbocharger speed sensor (TSS) circuit |
| Fault code | SPN 103/FMI 4 |
| Source publication | TruckFaultCodes.com (© 2017) |
| Pages | 2 |
| File size | 0.2 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification — SPN 103/FMI 4
- Disconnect the Turbocharger Speed Sensor (TSS)
- Disconnect the 120-pin Motor Control Module connector
- Resistance across TSS pins 1 and 2 (below 5 Ω)
- Short-to-ground checks on TSS pins; repair MCM wires 50 and 51
Applicable truck models
It applies to Detroit DD-series engines using the TSS on the 120-pin MCM harness. Always cross-check the reading against any companion codes before condemning a component. For the official product and service literature, visit the Detroit Diesel website.
Who needs this Detroit turbo speed low manual
Mechanics who need the resistance limits and wire numbers to clear a TSS circuit failed-low code. It is written for shop use as a quick, printable reference to keep beside a diesel diagnostic tool during the job.
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