SPN 168 FMI 7 is the Detroit DD Series diagnostic trouble code for opt idle detected charging system or battery failure, and this page documents its official engine charging and battery power circuit troubleshooting sheet. The three-page sheet is used when the Optimized Idle feature reports a charging-system or battery fault, and it works from the alternator through the module power and ground wiring.
SPN 168 FMI 7 document details
| Brand | Detroit |
| Model coverage | DD Series — DD13, DD15, DD16 |
| System / group | DD Series (EPA 2007-Current) / DTCs |
| SPN | 168 |
| FMI | 7 |
| Fault | Opt Idle Detected Charging System or Battery Failure |
| Emissions / revision | GHG14 — Copyright 2017 |
| Pages | 3 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault description: Opt Idle charging or battery failure
- Alternator drive belt, mounting and operation
- Battery charge-holding test
- Ground-circuit resistance (MCM2.1, CPC4)
- Power-circuit resistance and current-drop tests
Troubleshooting steps summarised
The technician checks the alternator drive belt, mounting and operation, then the battery charge. If the fault persists, resistance is measured on the ground-circuit wires at MCM2.1 21/5, 21/6, 21/8, 21/9 and CPC4 2/2 and the power-circuit wires at 21/11, 21/12, 21/14, 21/15 and CPC4 2/1 with the engine running above 12.5 V; current-drop tests follow, and any wire over 0.5 ohms or 0.2 volts of drop is repaired or replaced.
Applicable truck models covered
The sheet applies to Detroit DD Series engines (DD13, DD15 and DD16) with GHG14 hardware and the MCM2.1 and CPC4 charging architecture. For engine specifications and warranty coverage, refer to the official Demand Detroit service resources. These troubleshooting steps are cross-checked with the Detroit Diesel Diagnostic Link (DDDL) and the official Demand Detroit service resources.
Who needs this SPN 168 FMI 7 guide
Fleet technicians investigating an Optimized Idle no-charge event, a failing battery bank, or high-resistance module feeds use this document to confirm the root cause.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download the complete step-by-step document.
Related manuals library
