SPN 1639 FMI 2 is the Detroit DD-series fan speed error code, raised when the Motor Control Module cannot verify fan speed, and this sheet walks the mechanical checks, the fan-speed-sensor wiring and the final fan-drive test. Use it before condemning the fan clutch.
Official document details
| Brand | Detroit |
| Model coverage | DD13, DD15, DD16 (EPA 2007-current) |
| System / group | Engine cooling — cooling fan drive |
| Fault code | SPN 1639 / FMI 2 |
| Publication no. | Not printed on sheet |
| Year / revision | Reference dated 2017 |
| Pages | 5 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault header: SPN 1639 / FMI 2 — fan speed error
- Step 1 — check for companion codes SPN 1071/5 and 647/5
- Steps 2-8 — cooling, thermostat, belt, shroud and fan mechanical checks
- Steps 9-15 — fan speed sensor supply voltage at MCM2.1 pins 85, 114 and 47
- Steps 16-18 — live fan-drive test with manual override
SPN 1639 FMI 2 troubleshooting steps
Step 1 first checks for SPN 1071/FMI 5 or 647/FMI 5, which point to an open to pin 91 of the Motor Control Module (MCM2.1) 120-pin connector. Steps 2-8 are mechanical: coolant loss, thermostat, radiator and charge-air-cooler blockage, belt, shroud, hose and fan checks. Steps 9-15 disconnect the engine harness 10-pin connector and read the fan-speed-sensor supply — 4.5-5.5 volts is good; a high or shorted reading is traced to MCM2.1 pins 85 and 114, and resistance over 5 ohms to pin 47 is an open. Finally the tool is connected and the fan is engaged with the manual override; if fan speed does not follow engine speed, the fan drive is replaced.
Applicable truck models
This procedure covers Detroit DD-series engines — DD13, DD15 and DD16 on the EPA 2007-current platform — and shares its SPN/FMI structure with related Series 60 DDEC VI applications. Confirm the engine build and emissions calibration (EPA07, EPA10 or GHG14) first. For engine serial-number and calibration confirmation, use the Detroit Diesel official support site.
Who needs the SPN 1639 FMI 2 manual
Cooling and driveability technicians use this sheet to separate a mechanical fan-drive fault from a sensor-supply wiring problem, read the exact MCM2.1 pins, and verify the repair with a live override test.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download. Source: Detroit SPN/FMI fault-code reference (© 2017); fan speed is monitored live with the Detroit Diesel Diagnostic Link (DDDL).
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