Mack · Crankshaft Position SensorHigh

Mack SPN 636 FMI 2 – Engine Position Sensor Data Erratic or Incorrect

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 a Mack engine means the crankshaft position sensor is sending an erratic or intermittent signal—not missing entirely, but inconsistent. This causes stall events, rough running, and hard start conditions on MP7 and MP8 platforms. Check the connector, air gap, and reluctor ring before touching the sensor.

Overview: Mack SPN 636 FMI 2

Mack SPN 636 FMI 2 is the ECM telling you it can't trust the crankshaft position signal—it's getting something, but what it's getting doesn't consistently match the expected reluctor ring pattern. On MP7 and MP8 engines, the crankshaft position sensor is a passive variable reluctance (VR) pickup reading a 60-2 tooth reluctor ring. FMI 2 ('erratic, intermittent, or incorrect') means the signal is either dropping in and out of range, producing tooth-count mismatches, or showing noise spikes that corrupt the injection timing calculation. The connector, air gap, and reluctor ring integrity are the first three things to check—ProfessionalDieselRepair.com has the MP7/MP8 crank sensor air gap procedure with feeler gauge specs.

Repair Snapshot

Time

1–3 hours (sensor and wiring inspection; reluctor ring requires pan-off access if damaged)

Difficulty

Moderate

Parts Cost

$40–$600 (sensor $40–$120; wiring repair $50–$200; reluctor ring $200–$600 plus pan gasket)

Dealer Labor

$200–$1,200

Tools Required

  • Mack Premium Tech Tool
  • Jaltest
  • Snap-on NEXIQ
  • Multimeter

What SPN 636 FMI 2 Means

SPN 636 FMI 2 on a Mack engine means the ECM is receiving a crankshaft position signal that is inconsistent, dropping in and out, or generating tooth-count errors against the expected pattern. On MP7 and MP8 platforms this code can cause stall events, rough running, hard start, or reduced power. FMI 2 ('erratic/intermittent/incorrect') is distinct from FMI 8 (abnormal signal frequency) and typically points to a marginal wiring connection, air gap issue, damaged reluctor ring, or ferrous debris contamination on the sensor tip.

Symptoms Drivers Notice

  • Yellow or red check engine lamp with SPN 636 FMI 2 logged
  • Intermittent stall or stumble, especially at low RPM transitions
  • Hard start—engine cranks but may not fire if crank signal is absent at key-on
  • Rough running or erratic idle with RPM fluctuating on the tachometer
  • Fault may be active only under vibration or specific temperature conditions (wiring intermittent)

Most Likely Root Causes

Backed-out or corroded sensor connector terminal (electrical/sensor)

High

The crankshaft position sensor connector on MP7/MP8 engines is exposed to vibration, heat cycles, and oil splash. A terminal that has partially backed out of the connector housing creates an intermittent contact that drops the signal under vibration or heat expansion—exactly the FMI 2 erratic pattern. This is the most common cause and requires only connector inspection and terminal reseating.

Incorrect air gap between sensor tip and reluctor ring (mechanical)

Medium–High

The passive VR sensor generates signal voltage from reluctor ring tooth proximity—a gap wider than the 0.060 in. maximum produces a signal amplitude too low for the ECM to reliably decode, causing erratic tooth-count readings. Gap drift occurs after sensor replacement if the correct spacer or shimming procedure was not followed.

Ferrous debris on sensor tip (mechanical)

Medium

Metal filings from normal engine wear accumulate on the magnetic sensor tip, filling the gap between the sensor and reluctor ring. This changes the effective sensor-to-ring distance and creates signal irregularity—most common in engines that have had recent bearing or ring wear generating metallic debris.

Damaged or missing reluctor ring tooth (mechanical)

Medium

A missing or bent tooth on the crankshaft reluctor ring produces a repeating dropout at a specific crank angle—the ECM receives no signal for that angular window and flags the timing pattern as erratic. The 60-2 tooth pattern relies on the two-tooth gap for TDC reference; any additional missing tooth creates a false reference that corrupts injection timing.

Chafed or damaged sensor wiring harness (electrical/sensor)

Medium

The wiring harness routing from the sensor to the ECM passes near the block, frame rail, and heat sources. Chafed insulation that grounds intermittently under vibration produces noise spikes on the signal line that the ECM interprets as extra teeth—an 'incorrect' tooth count that triggers FMI 2.

Common Misdiagnoses (Don't Waste Parts)

Replacing the crankshaft position sensor as a parts-cannon first move without checking the connector and air gap

Why it happens: SPN 636 FMI 2 sounds like a sensor fault, and the sensor is a relatively cheap part. Shops often swap it without inspecting the harness connector or measuring the air gap, missing the most common actual causes.

Do this instead: Start with the connector—remove, clean, inspect for backed-out pins, and reseat all terminals before ordering a sensor. Then measure the air gap with a feeler gauge. Only if both checks are normal is the sensor itself a likely replacement candidate.

Replacing the reluctor ring without first inspecting it through the sensor bore while slowly cranking the engine

Why it happens: A missing tooth pattern is a recognized failure mode, and shops sometimes assume the ring is damaged without doing the visual check. The reluctor ring requires oil pan removal to replace—an expensive misdiagnosis if the ring is actually fine.

Do this instead: Rotate the engine slowly by hand with the sensor removed and inspect each tooth of the reluctor ring with a flashlight through the sensor bore. A damaged or missing tooth is visually obvious. If the ring is intact, the problem is upstream in the sensor or wiring.

Ignoring the possibility of ferrous debris on the sensor tip and reinstalling the original sensor after clearing the fault

Why it happens: When the fault clears after the sensor connector is cleaned, technicians assume the problem is solved. If ferrous debris was the cause and the sensor tip is not cleaned, the fault returns.

Do this instead: Every time the crank sensor is removed, inspect and clean the magnetic tip with a clean rag. Note whether any metallic debris is present—significant debris suggests a bearing wear condition that should be investigated further.

Master Tech Commentary

Mechanic's Notes

FMI 2 on the crank sensor is the connector/wiring fault until proven otherwise on MP7/MP8 platforms. The single most common cause is a connector with a backed-out terminal or a harness that has chafed against the block near the sensor boss—check the connector before condemning the sensor. The reluctor ring on the MP8 is pressed onto the crankshaft; a single missing tooth produces a characteristic 'crank sync error' that FMI 2 captures before FMI 8 (abnormal frequency) kicks in. The Snap-on NEXIQ is particularly useful for graphing the raw crank signal waveform to spot the intermittent dropout. ProfessionalDieselRepair.com covers the MP8 crank sensor replacement and air gap procedure with torque specs.

Step-by-Step Diagnostic Procedure

  1. Step 1: 1. Connect Mack Premium Tech Tool (PTT) or Snap-on NEXIQ and pull all active and inactive codes. Note whether SPN 636 FMI 2 is active constantly or only under specific conditions (hot, cold, high vibration).
  2. Step 2: 2. With the engine off and key off, visually inspect the crankshaft position sensor connector and wiring harness at the sensor mounting boss. Look for chafed insulation, oil-soaked connectors, or a backed-out terminal pin.
  3. Step 3: 3. Measure the air gap between the sensor tip and the reluctor ring tooth with a feeler gauge (access through the sensor bore). Mack MP7/MP8 spec: 0.030–0.060 in. (0.76–1.52 mm). A gap outside this range produces erratic signal amplitude.
  4. Step 4: 4. Inspect the reluctor ring (crankshaft tone ring) through the sensor bore with a flashlight while rotating the engine by hand. Look for a missing tooth, bent tooth, or surface rust that could cause a drop-out in the signal pattern.
  5. Step 5: 5. Check for ferrous debris on the sensor tip—metal filings attracted to the magnetic sensor body fill the gap and corrupt the reluctor signal. Clean the tip and recheck the air gap.
  6. Step 6: 6. With a multimeter, perform an oscilloscope or AC millivolt test on the sensor signal wire while cranking. A healthy magnetic pickup (passive VR sensor) on Mack MP7 produces a clean sinusoidal signal at 0.5–5V AC amplitude at cranking RPM. A signal that drops out or shows noise spikes during cranking confirms a wiring or sensor issue.
  7. Step 7: 7. Use Jaltest or Mack PTT to monitor the crankshaft signal quality parameter if available—some MP8 calibrations report a crank signal 'tooth error count' that pinpoints a specific tooth gap position as the source.

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

Live Data & Freeze-Frame Tips

In Mack PTT or Jaltest, monitor 'Engine Speed' (SPN 190) and 'Crank Sensor Signal Status' if the platform exposes it. An intermittent FMI 2 that appears only at specific RPMs (e.g., during deceleration or low-load transitions) points to a signal amplitude problem related to tooth speed—either a marginal air gap or a wiring resistance issue that only manifests at low-signal frequencies. The Snap-on NEXIQ with its scope mode can graph the raw sensor voltage waveform at cranking RPM to show the sinusoidal pattern—a healthy sensor produces consistent amplitude across all 60 teeth with a clean two-tooth gap at TDC.

Safety Warnings

  • Never use an impact wrench on the crankshaft position sensor—it is a fragile magnetic pickup. Torque by hand to spec (typically 8–10 ft-lb on Mack MP8).
  • Confirm the engine is fully stopped before inserting a feeler gauge into the sensor bore—contact with a rotating reluctor ring at cranking speed will eject the gauge and cause injury.
  • After any crank sensor or air gap work, perform a no-load idle verification in PTT before road testing—confirm crank RPM signal matches the actual idle before putting the truck under load.

When to Limp In vs Tow

Tow if the engine stalls and will not restart with SPN 636 FMI 2 active, or if the engine is running rough enough to risk a loss of vehicle control. An active crank signal loss at highway speed can cause unexpected deceleration.

Prevention Tips for Fleets & Owner-Operators

  • Inspect the crank sensor connector and harness routing at every major PM (150,000 miles or annually) for chafing against the block or nearby hot surfaces.
  • At each oil change, check the magnetic sensor tip for debris accumulation—ferrous buildup can be cleaned with a rag without removing the sensor.
  • During any engine repair that involves the oil pan, visually inspect the reluctor ring for tooth damage before closing.
  • When replacing the crank sensor, always measure and verify the air gap with a feeler gauge before final torque—do not assume the replacement sensor will self-set.

Related Fault Codes

SPN 636 FMI 8

Abnormal crank sensor signal frequency—typically points to reluctor ring tooth damage rather than the connector/air gap issues common with FMI 2

SPN 637 FMI 2

Camshaft position sensor erratic signal—often appears alongside SPN 636 FMI 2 when the engine cannot achieve crank-cam sync after a crank signal dropout

SPN 190 FMI 8

Engine speed signal abnormal—logged by the ECM when the SPN 636 signal dropout is severe enough to corrupt the RPM calculation

Frequently Asked Questions

What causes SPN 636 FMI 2 on a Mack MP8?

The most common causes are a backed-out or corroded connector terminal, an air gap outside the 0.030–0.060 in. specification, ferrous debris on the magnetic sensor tip, a damaged reluctor ring tooth, or a chafed wiring harness. FMI 2 (erratic/intermittent) is almost always a mechanical or wiring issue rather than a failed sensor element itself.

Will Mack SPN 636 FMI 2 cause the engine to stall?

Yes, it can. If the crank sensor signal drops out completely during operation, the ECM loses injection timing reference and cuts fuel. This causes an unexpected stall. FMI 2 with only intermittent signal loss typically produces rough running and RPM fluctuation before a full stall event.

What is the correct air gap for the crankshaft position sensor on a Mack MP7/MP8?

Mack specifies 0.030–0.060 in. (0.76–1.52 mm) between the sensor tip face and the top of the reluctor ring tooth. Measure with a feeler gauge through the sensor bore with the sensor removed. Gap below minimum can cause interference with the ring; gap above maximum drops signal amplitude to an unreliable level.

Can ferrous debris on the sensor tip cause SPN 636 FMI 2 without the sensor being damaged?

Yes. Metal filings from normal engine wear accumulate on the magnetic sensor tip and effectively reduce the active gap between the sensor and reluctor ring. Cleaning the tip can resolve SPN 636 FMI 2 without any other repair. If debris is heavy, investigate the source—significant metallic content in the oil can indicate bearing wear.

How do I distinguish SPN 636 FMI 2 (erratic signal) from SPN 636 FMI 8 (abnormal frequency) on a Mack engine?

FMI 2 (erratic/intermittent/incorrect) means the signal is present but inconsistent—dropping in and out, producing noise spikes, or generating tooth-count errors. FMI 8 (abnormal frequency) means the signal's pulse rate doesn't match what's expected for the known RPM. FMI 2 points toward connectors and air gap; FMI 8 more strongly suggests physical reluctor ring damage.

Does SPN 636 FMI 2 require oil pan removal to fix?

Only if the reluctor ring itself is damaged—replacing a pressed-on reluctor ring requires oil pan removal and significant disassembly. However, the majority of SPN 636 FMI 2 diagnoses resolve with a connector repair, air gap adjustment, or sensor replacement—all accessible without removing the pan. Always inspect the ring through the sensor bore before assuming pan removal is necessary.