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Cummins SPN 636 FMI 2: Crankshaft Position Sensor Erratic

SPN 636 FMI 2 (FMI 2) — Engine Position Sensor - Erratic, Intermittent, or Incorrect

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Quick Answer

SPN 636 FMI 2 means the crankshaft position sensor signal is erratic or intermittent rather than a clean steady pulse. It's a High severity fault that can cause random stalling or hard starting, and in rare cases a complete signal loss can shut the engine down while driving.

Overview: Cummins SPN 636 FMI 2

The crankshaft position sensor is what tells the Cummins ECM exactly where each piston is in its stroke, which is critical for firing injectors at the right instant. SPN 636 FMI 2 specifically flags an erratic or noisy signal, distinct from a total signal loss, which is why the symptoms are often intermittent rather than a hard no-start. This guide from ProfessionalDieselRepair.com walks through separating a genuinely bad sensor from the far more common culprits: a widened air gap after service work or a chafed harness near the timing case.

Repair Snapshot

Time

1 - 2.5 Hours

Difficulty

Intermediate

Parts Cost

$80 - $220

Dealer Labor

$350+

Tools Required

  • Cummins INSITE
  • Multimeter
  • Feeler Gauge
  • Snap-on NEXIQ USB Link

What SPN 636 FMI 2 Means

SPN 636 FMI 2 means the ECM is receiving an erratic or noisy signal from the crankshaft position sensor rather than a clean, steady pulse pattern. On Cummins engines this can cause hard starting, random misfires, or a stall at idle since the ECM depends on this signal for injection timing. It sets the MIL and, if the signal drops out completely while running, can cause an unexpected shutdown.

Symptoms Drivers Notice

  • Intermittent hard starting or extended cranking
  • Random stalling at idle or under load
  • MIL illuminated, sometimes with an accompanying camshaft sensor code
  • Engine may run rough as if missing on multiple cylinders randomly

Most Likely Root Causes

Chafed crank sensor harness near the timing cover

Common

Engine vibration wears the harness against the timing case or accessory bracket, introducing intermittent noise into the signal.

Incorrect sensor air gap after service

Common

If the sensor wasn't properly shimmed or torqued after a timing cover or gear train repair, the air gap can be too wide, producing a weak, erratic signal.

Failing crank sensor

Occasional

Internal degradation of the sensor element itself, often heat-related, causes intermittent dropouts especially once the engine reaches operating temperature.

Damaged or missing teeth on the reluctor/tone wheel

Rare

Physical damage to the toothed wheel the sensor reads creates a genuinely erratic mechanical signal that no amount of sensor or wiring repair will fix.

Common Misdiagnoses (Don't Waste Parts)

Replacing the crank sensor without checking the air gap

Why it happens: It's a fast parts-cannon fix that seems logical since the sensor is the named component, but a brand new sensor installed with the wrong gap will throw the same code.

Do this instead: Always verify the air gap with a feeler gauge against Cummins spec before and after sensor replacement.

Assuming it's a fuel delivery problem because the engine stalls

Why it happens: Random stalling looks like fuel starvation to techs who haven't pulled codes first.

Do this instead: Always scan for stored codes before chasing fuel system components - an erratic crank signal will stall an engine just as effectively as a fuel problem.

Overlooking the camshaft sensor correlation

Why it happens: Techs focus only on the crank sensor and miss that the ECM cross-checks crank and cam signals for correlation faults.

Do this instead: Check for accompanying SPN 723 (camshaft/engine speed sensor #2) codes, since a correlation mismatch can point to either sensor or a timing issue.

Master Tech Commentary

Mechanic's Notes

Field tip: an erratic crank signal is one of the most common causes of a truck that occasionally dies at a stoplight but restarts fine minutes later. ProfessionalDieselRepair.com always checks the sensor air gap and connector before condemning the sensor itself - a gap set too wide from a sloppy reinstall after timing cover work reads exactly like a failing sensor on a scope.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect Cummins INSITE and check the freeze-frame data for when the fault set - a hot-engine intermittent points toward heat-related sensor failure or connector expansion.
  2. Step 2: Inspect the crank sensor connector for corrosion, and check that the sensor is seated fully against its mounting boss with the correct air gap per the service manual.
  3. Step 3: With the connector unplugged, measure sensor resistance across its terminals with a multimeter and compare to the Cummins spec (most Hall-effect crank sensors read in the 500-900 ohm range, magnetic pickup types vary more).
  4. Step 4: Inspect the wiring harness from the sensor back to the ECM for chafing near the front gear cover or timing case, a common wear point from engine vibration.
  5. Step 5: Clear the code, road test while monitoring live crank signal quality on INSITE, and confirm the fault does not return under varying RPM and load.

Live Data & Freeze-Frame Tips

On INSITE, watch the engine speed signal quality parameter if your ECM calibration supports it, and note any RPM signal dropouts correlated with engine temperature or vibration. A scope on the sensor signal wire, if available through Diesel Laptops hardware, will show a clean sine or square wave pattern when the sensor and harness are healthy, versus a noisy or intermittent trace when there's a wiring or air gap problem.

Safety Warnings

  • Never work near the crankshaft pulley or timing cover with the engine running.
  • Allow the engine to cool fully before accessing sensors near the timing case to avoid burns.
  • Disconnect the batteries before removing or adjusting the crank sensor.
  • Use proper lifting equipment if front engine covers must be removed for access.

When to Limp In vs Tow

If the engine stalls repeatedly and will not consistently restart, or if the fault is accompanied by a complete loss of crank signal (a different FMI), towing is the safer option rather than risking a stall in traffic.

Prevention Tips for Fleets & Owner-Operators

  • Always verify crank sensor air gap with a feeler gauge any time the sensor or timing cover is disturbed.
  • Route the crank sensor harness away from vibration-prone brackets during any front engine service.
  • Inspect the sensor connector for corrosion at regular PM intervals.
  • Replace aging sensors proactively on high-mileage engines showing early signs of intermittent hesitation.

Related Fault Codes

SPN 636 FMI 8

Abnormal frequency or pulse width on the same crank position sensor circuit.

SPN 723 FMI 2

Camshaft/engine speed sensor #2 erratic signal, often checked together since the ECM cross-correlates both signals.

SPN 637 FMI 2

Camshaft timing sensor erratic signal - a related timing reference sensor on the same engine.

Frequently Asked Questions

Can I drive with SPN 636 FMI 2 active?

You can drive, but expect the possibility of a random stall since the ECM relies on this signal continuously for injection timing. It's safer to get it diagnosed promptly rather than risk a stall in traffic.

How do I reset this code without a scanner?

Disconnecting the batteries can clear the stored code temporarily, but if the underlying wiring or air gap issue isn't fixed, the fault will set again as soon as the erratic signal reoccurs.

What triggers the derate on this code?

This particular fault doesn't typically trigger an emissions-style derate countdown, but severe signal loss can cause the engine to stall or refuse to start rather than gradually limiting power.

Is a crank sensor expensive to replace?

The part itself is usually inexpensive, often under $150, but labor varies depending on how accessible the sensor is on your specific engine platform.

Can a bad ground cause this code?

Yes, a poor sensor ground connection can introduce the same kind of noisy, erratic signal as a failing sensor or chafed signal wire.

Why does the engine only stall when it's warm?

Heat causes both wiring insulation and sensor internals to expand slightly, which can turn a marginal connection or air gap into an intermittent open circuit only once the engine reaches operating temperature.