Mercedes-Benz · Crankshaft Position SensorHigh

Mercedes-Benz SPN 636 FMI 2: Crankshaft Position Sensor Erratic or Intermittent

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

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Quick Answer

SPN 636 FMI 2 on Mercedes-Benz means the crank sensor signal is erratic or internally inconsistent. Check the harness for abrasion, the tone wheel for damage, and scope the signal before swapping the sensor.

Overview: Mercedes-Benz SPN 636 FMI 2

An erratic crank signal is one of the more frustrating intermittent faults in the shop because it comes and goes with heat, vibration, and load. SPN 636 FMI 2 on a Mercedes-Benz heavy-duty is the ECM telling you the signal is there — it just doesn't trust it. ProfessionalDieselRepair.com systematically works from harness to tone wheel to sensor, in that order.

Repair Snapshot

Time

1 - 3 Hours

Difficulty

Intermediate

Parts Cost

$60 - $350

Dealer Labor

$300+

Tools Required

  • Mercedes-Benz Xentry
  • Texa
  • Jaltest
  • Multimeter

What SPN 636 FMI 2 Means

HIGH: Mercedes-Benz SPN 636 FMI 2 means the ECM is receiving a crankshaft position signal that is present but erratic, intermittent, or internally inconsistent. This is not a clean open-circuit fault — the sensor is producing something, but the ECM does not trust it. Expect rough running, intermittent stall, or no-start complaints depending on when the signal degrades.

Symptoms Drivers Notice

  • Intermittent rough running or sudden misfires
  • Engine stall that may restart after a cool-down
  • Hard no-start if the signal becomes completely unreliable during cranking
  • MIL illuminated with possible torque reduction
  • Erratic RPM reading on the instrument cluster

Most Likely Root Causes

Chafed or intermittently open signal harness (electrical)

Very Common

The crank sensor harness runs near chassis members and heat shields where vibration-induced abrasion creates an intermittent open that produces exactly the FMI 2 erratic pattern.

Damaged or debris-packed tone wheel / trigger ring gear (mechanical)

Common

Missing teeth or packed metallic debris on the ring gear distorts the pulse pattern, causing the ECM to reject the signal as erratic even though the sensor itself is fine.

Failing crankshaft position sensor (internal weak magnet or cracked housing)

Occasional

A sensor that is on its way out produces variable amplitude output — enough signal for FMI 2, not consistent enough for FMI 12 or FMI 3.

Loose ECM connector terminal on the crank signal pin (electrical)

Less Common

A pin that has backed out slightly in the ECM connector causes a vibration-sensitive intermittent open that only triggers under load or rough road conditions.

Common Misdiagnoses (Don't Waste Parts)

Replacing the crankshaft position sensor as a parts-cannon first move without inspecting the harness

Why it happens: FMI 2 names the crank sensor parameter and techs assume a sensor fault, but the intermittent pattern is far more often a wiring issue than a dead sensor.

Do this instead: Scope the signal first and walk the harness from sensor to ECM before buying a sensor. A $12 harness repair often fixes what a $200 sensor swap won't.

Chasing camshaft sensor codes instead of crank sensor codes when both set together

Why it happens: SPN 636 FMI 2 and SPN 637 FMI 2 can set at the same time if the ECM loses confidence in timing correlation, leading techs to replace both sensors instead of finding the shared root cause.

Do this instead: Identify which signal degraded first using freeze-frame and fix that one. If it's a harness issue near the crankshaft, that typically explains both codes.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com finds that SPN 636 FMI 2 on Mercedes-Benz heavy-duty trucks is more likely a harness or connector issue than a failed sensor. The FMI 2 erratic pattern almost always points to an intermittent open in the signal circuit. Scope the signal under flex and vibration before condemning the sensor.

Step-by-Step Diagnostic Procedure

  1. Step 1: 1. Connect Mercedes-Benz Xentry or Jaltest and read freeze-frame data — note RPM, coolant temp, and load at the moment FMI 2 set.
  2. Step 2: 2. Inspect the crankshaft position sensor and its trigger wheel: pull the sensor, check the tip for cracks or physical damage, and look at the ring gear teeth for chips or packed debris.
  3. Step 3: 3. Measure sensor resistance across the signal terminals with a multimeter and compare to spec; then check for intermittent opens by wiggling the harness while monitoring resistance.
  4. Step 4: 4. Inspect harness routing from the sensor back to the ECM connector for abrasion points on chassis members or heat shield edges — FMI 2 is notorious for harness-chafe faults that only show up on the road.
  5. Step 5: 5. Use Texa or Jaltest to scope the crank signal pattern at cranking speed and verify the waveform amplitude and tooth-gap pattern are consistent — a dropout or distorted pulse confirms a failing sensor or damaged tone wheel.
  6. Step 6: 6. Confirm connector terminal tension at the ECM end; a loose pin on the crank signal circuit can produce exactly this erratic FMI 2 pattern.

Professional service / OEM-level scan tools are recommended before clearing this fault for good.

Live Data & Freeze-Frame Tips

In Xentry or Jaltest, use the oscilloscope function to capture the crank signal waveform at cranking speed. A healthy signal shows consistent amplitude and clean, evenly-spaced pulses with one missing-tooth gap per revolution. Any dropout, amplitude variation, or extra noise indicates harness integrity or tone wheel issues.

Safety Warnings

  • An erratic crank signal can cause an unexpected stall — do not diagnose on a hoist with the engine running if driveshaft rotation is possible.
  • Verify the engine is fully cooled before probing near the crankshaft sensor — thermal burns from the block are common during rushed diagnosis.
  • Clear codes and confirm no-reoccurrence before returning a truck to highway service — intermittent stall on a loaded vehicle is a road safety issue.

When to Limp In vs Tow

Tow if the engine has stalled and will not restart, or if the fault is recurring frequently enough to create a stall risk on a public road.

Prevention Tips for Fleets & Owner-Operators

  • Inspect crankshaft sensor harness routing at every major service and add protective loom where abrasion points exist.
  • Include tone wheel visual inspection when performing lower engine work — a bent or chipped tooth sets up future FMI 2 faults.
  • Verify ECM connector terminal tension during any harness repair in the front of the engine area.

Related Fault Codes

SPN 636 FMI 3

Voltage above normal — complete signal loss or shorted-high, a more severe version of the same sensor circuit.

SPN 637 FMI 2

Camshaft (timing) sensor erratic — often sets alongside 636 FMI 2 when timing correlation fails.

SPN 190 FMI 8

Engine speed abnormal frequency — can set when the crank signal disrupts RPM calculation.

Frequently Asked Questions

Will SPN 636 FMI 2 cause a no-start?

It can, especially if the erratic signal degrades to the point where the ECM cannot determine crank position reliably during cranking. Intermittent no-start or stall-then-restart is a classic symptom.

Can the engine run with SPN 636 FMI 2 active?

Often yes, but unreliably. The engine may run fine at idle and stall under load or vibration when the harness flexes and the signal drops out.

How do I know if it is the sensor or the wiring?

Oscilloscope the signal at the sensor connector and again at the ECM connector. If the waveform is clean at the sensor but corrupt at the ECM end, the harness is the problem.

Is this the same sensor as on the camshaft?

No. SPN 636 is the crankshaft position sensor. SPN 637 is the camshaft (timing) sensor. They are separate sensors, usually on opposite ends of the engine.

Does FMI 2 trigger a derate?

Typically yes, at least a moderate torque reduction, since timing accuracy is compromised. Severity depends on how degraded the signal is.

How do I clear SPN 636 FMI 2 on a Mercedes-Benz truck?

Clear with Xentry or Jaltest after repairing the root cause. An intermittent fault must be reproduced and verified absent before the repair can be confirmed — clear and road-test under the conditions that originally triggered it.