FMI 8 — Abnormal frequency, pulse width or period
The signal is there but the timing of it is wrong — wrong pulse rate, wrong duty cycle, wrong spacing.
Common causes
- Damaged tone/reluctor wheel or debris on it
- Incorrect sensor air gap
- Failing speed or position sensor
- Shielding or routing problem picking up noise
What to test, in order
- Scope the signal and compare waveform, amplitude and spacing to a known-good
- Inspect the tone ring for missing or damaged teeth and metal debris
- Set the sensor air gap to spec
- Verify the shield ground is intact and the harness is routed away from ignition/charging noise
Codes we document with FMI 8
SPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · CATSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · DetroitSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · InternationalSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · PACCARSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · MERCEDESSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · VolvoSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · MackSPN 636 FMI 8Engine position (camshaft) sensor abnormal signal · CumminsSPN 637 FMI 8Crankshaft position sensor abnormal signal · CATSPN 637 FMI 8Crankshaft position sensor abnormal signal · DetroitSPN 637 FMI 8Crankshaft position sensor abnormal signal · InternationalSPN 637 FMI 8Crankshaft position sensor abnormal signal · PACCARSPN 637 FMI 8Crankshaft position sensor abnormal signal · MERCEDESSPN 637 FMI 8Crankshaft position sensor abnormal signal · VolvoSPN 637 FMI 8Crankshaft position sensor abnormal signal · MackSPN 637 FMI 8Crankshaft position sensor abnormal signal · Cummins
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