SPN 168 FMI 1 is the Detroit DD Series diagnostic trouble code for battery voltage low, and this page documents its official engine charging and battery power circuit troubleshooting sheet. The three-page sheet takes the technician from alternator and battery checks into MCM2.1 21-pin ground and power measurements and a wiggle test with the diesel diagnostic tool.
SPN 168 FMI 1 document details
| Brand | Detroit |
| Model coverage | DD Series — DD13, DD15, DD16 |
| System / group | DD Series (EPA 2007-Current) / DTCs |
| SPN | 168 |
| FMI | 1 |
| Fault | Battery Voltage Low |
| Emissions / revision | GHG14 — Copyright 2017 |
| Pages | 3 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault description: battery voltage low
- Alternator and battery checks
- MCM2.1 21-pin ground-circuit resistance
- Power-circuit battery voltage (pins 11-15)
- Wiggle test and test-MCM verification
Troubleshooting steps summarised
After the alternator drive belt, mounting, operation and battery charge are verified, the MCM2.1 21-pin connector is disconnected and inspected, ground-circuit resistance is measured at pins 5, 6, 8 and 9, and battery voltage is read at power pins 11, 12, 14 and 15. A wiggle test then watches for abnormal voltage spikes, and clearing codes confirms whether a test MCM2.1 is required.
Applicable truck models covered
The document applies to Detroit DD Series engines (DD13, DD15 and DD16) with GHG14 hardware using the MCM2.1 21-pin power and ground circuits. 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 1 guide
Technicians diagnosing a low module supply voltage, an intermittent harness spike, or a repeat battery-voltage complaint use this sheet to decide between wiring repair and a controller swap.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download the complete step-by-step document.
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