SPN 677 FMI 18 is the Detroit DD Series diagnostic fault code that identifies Starter relay – low resistance. This DiagTechPro reference summarizes the official 1-page Detroit diagnostic sheet so a technician can confirm the condition before opening the truck, and it links to the full downloadable PDF.
Official document details
| Brand | Detroit |
| Model coverage | Detroit DD Series (EPA 2007-Current) diesel engines |
| System / Group | DD Series (EPA 2007-Current) / DTCs |
| Fault condition | Starter relay – low resistance |
| Published source | TruckFaultCodes.com, Copyright © 2017 |
| Pages | 1 |
| File size | 0.2 MB |
| Format | |
| Language | English |
SPN 677 FMI 18 diagnostic overview
This Detroit sheet covers a starter relay low-resistance fault. The ignition must be off for at least 30 seconds before testing. The technician disconnects the Motor Control Module (MCM) 21-pin connector, turns the ignition on, and measures the voltage between pin 21 of the MCM 21-pin connector and ground. If voltage is present, the repair is to fix a short to power on pin 21; if no voltage is present, the starter relay is replaced per OEM procedures.
Document table of contents
- Fault code identification
- Ignition-off notice (30 seconds)
- MCM 21-pin voltage check on pin 21
- Repair or starter relay replacement
Applicable truck models covered
This diagnostic applies to Detroit DD Series (EPA 2007-Current) diesel engines across all model years, and is also grouped under the Series 60 (DDEC VI – EPA 2007) DTCs. It covers the starter relay control circuit at the Motor Control Module 21-pin connector. For the OEM wiring schematics referenced in this procedure, see the official Detroit engine manufacturer site.
Who needs the SPN 677 FMI 18 manual
Workshop technicians and fleet owners using the diesel diagnostic tool to read Detroit fault codes and test the starter relay and Motor Control Module wiring will need this sheet. The full step-by-step PDF is available in the DiagTechPro manuals library, and a free account is required to download it.
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