John Deere Engines SPN637FMI2 Fault Code: Crankshaft Position Signal Invalid
Also called Crank Position Input Noise, Crankshaft Position Input Noise
Crankshaft Position Signal Invalid · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 637 FMI 2 means the engine ECU is seeing excessive noise or extra pulses on the crankshaft position sensor signal. The engine may hesitate, die, or need prolonged cranking to restart, and if a camshaft or pump position sensor code sets at the same time the engine will not restart until one of the two faults is fixed.
High severity. The engine can hesitate, stall, or fail to restart, and cranking time can increase. If a camshaft or pump position sensor fault sets at the same time, the engine will not run at all until one of the two codes is repaired. Engine power can also be derated up to 50 percent. Treat this as a stop-soon issue, not a run-it-and-see problem.
What does John Deere Engines error code SPN637FMI2 mean?
SPN 637 FMI 2 points to noise, meaning extra electrical pulses, on the crankshaft position sensor circuit. This sensor is an inductive pickup that reads notches or teeth on the crank timing wheel (some descriptions reference 46 evenly spaced teeth with 2 missing teeth used to find Top Dead Center). The ECU uses this signal along with the camshaft or pump position sensor to know exact piston position and engine speed, which it needs to time fuel injection correctly.
When the ECU sees noise instead of a clean signal, it can no longer trust the crank position data. It falls back to using only the camshaft or pump position sensor input to figure out piston position. This works as a backup, but if that secondary sensor also faults, the engine cannot start or will stall while running until one of the two problems is fixed.
Even without a second fault present, drivers may notice the engine hesitate or die the moment the code sets, though it will typically restart. Cranking time to start the engine may be longer than normal, and engine power output can be reduced by up to 50 percent as a protective measure.
Common causes of SPN637FMI2
- Bad or corroded terminals or connector at the crankshaft position sensor
- Bad or corroded terminals or connector at the ECU (including the 60-way ECU connector)
- Open or short circuit in the wiring harness between the crank sensor and ECU, including shorts to other signal wires
- Loose ground connections anywhere on the machine, including chassis and battery ground connections
- Loose alternator connections or excessive alternator electrical noise
- Electromagnetic interference (EMI) from an incorrectly installed 2-way radio
- Interference from a radar source
- Crank sensor wiring routed near strong electric or magnetic field sources
- Broken teeth, burrs, or damage on the crankshaft timing wheel or oil pump drive gear teeth
- High resistance in the crank sensor circuit
- Intermittent voltage on the ECU battery supply or ignition supply to ECU ground
- Bad crankshaft position sensor itself
- Bad ECU software or a bad ECU
How to troubleshoot John Deere Engines SPN637FMI2: first checks
- Visually inspect the ECU connectors and the crankshaft position sensor connector for contamination, corrosion, damage, or terminals that are not fully seated
- Check all harness connectors along the crank sensor circuit for water intrusion, dirt, or paint that could cause a poor or noisy connection
- Inspect wiring for intermittent open circuits or shorts, paying particular attention to the crank sensor wiring and its routing near alternators, 2-way radios, or other strong electrical or magnetic field sources
- Check alternator connections and all ground connections, including chassis ground and battery ground, for looseness or corrosion
- Verify correct pin location and secure seating of terminals in both the crank sensor connector and the ECU connector
- Look for physical damage such as broken teeth or burrs on the crankshaft timing wheel, which can generate false or extra pulses
- Check for any accompanying camshaft or pump position sensor trouble code, since that combination will prevent the engine from restarting until one fault is resolved
How the code clears
No separate clearing step is listed. Repair procedure centers on inspecting and correcting the wiring, connectors, grounds, and crankshaft timing wheel condition described above. Once the underlying noise source or connection problem is fixed, the code should not reset on its own; if a paired camshaft or pump position sensor code was present, both issues need to be addressed before the engine will run normally again.
Frequently asked questions
What does SPN 637 FMI 2 mean on a John Deere engine?
It means the ECU is detecting excessive noise, or extra electrical pulses, on the crankshaft position sensor input. The ECU can no longer fully trust the crank signal for timing fuel injection.
Can I keep driving or running the machine with this code?
It is risky. The engine may hesitate or die when the code sets, though it usually restarts. Power can be derated up to 50 percent, and if a camshaft or pump position sensor code sets at the same time, the engine will not restart at all until one of the two faults is repaired. Get it diagnosed as soon as possible.
Why does my engine take longer to crank and start with this code active?
With the crank signal unreliable, the ECU relies only on the camshaft or pump position sensor to figure out piston position. This backup method can require prolonged cranking time before the engine catches and starts.
What usually causes this crankshaft position noise code?
Most cases trace back to electrical noise or poor connections: corroded or loose terminals at the crank sensor or ECU, loose grounds or alternator connections, shorts or opens in the harness, or interference from an improperly installed 2-way radio or radar source. Physical damage such as broken teeth on the crankshaft timing wheel can also cause it.
Is this the same as a camshaft or pump position sensor code?
No. SPN 637 FMI 2 is specific to the crankshaft position sensor circuit. However, if a related camshaft or pump position sensor trouble code sets at the same time, the two faults interact and the engine will not restart until at least one is repaired.
Where is the crankshaft position sensor located?
It is an inductive pickup sensor mounted at the front of the crankshaft, behind the pressed-on crank gear, or in some engine descriptions it reads notches on the oil pump drive gear. It detects the crank timing wheel teeth to determine engine speed and piston position.
Will simply clearing the code fix the problem?
No separate clearing step is listed for this code. The real fix is inspecting and repairing the wiring, connectors, grounds, or timing wheel damage causing the electrical noise. Once that root cause is corrected, the code should not return.