Crank & Cam Position Sensors
Location, air gap specs, test procedure, and related fault codes for crankshaft and camshaft position sensors on Class 8 diesel trucks.
Last updated: 2026-09-20
Overview
The crankshaft position sensor (CKP) and camshaft position sensor (CMP) provide engine speed and timing signals to the ECM. The ECM uses these signals to calculate injection timing, fuel quantity, and cylinder identification. A failed crank or cam sensor causes no-start, stall, or severe derate conditions.
Sensor Locations by Platform
Cummins X15
Crankshaft sensor: driver side, lower front of engine block, near crankshaft damper. Camshaft sensor: driver side, front of engine near gear train cover.
X15 crank sensor is a magnetic pickup type. Air gap is critical — verify after any front seal work.
PACCAR MX-13
Crankshaft sensor: driver side, lower front of engine block. Camshaft sensor: front of engine, near gear train cover.
MX-13 uses Hall-effect sensors — check 5V reference and ground before condemning sensor.
Detroit DD15
Crankshaft sensor: driver side, lower front of engine block near flywheel housing. Camshaft sensor: front of engine near gear train.
DD15 crank sensor is accessible from under the truck. Inspect reluctor ring for damage.
Volvo/Mack MP8
Crankshaft sensor: driver side, lower front of engine block. Camshaft sensor: front of engine near gear train cover.
MP8 crank and cam sensors are magnetic pickup type. Verify air gap after any front cover work.
DieselVision™ identifies sensor labels and connector pin numbers — upload a close-up photo and the system maps pins automatically.
Connector Pinout
| Pin | Function | Specification |
|---|---|---|
| A | 5V Reference (Hall-effect) or Signal+ (Magnetic) | 4.75–5.25V (Hall-effect) or AC voltage proportional to RPM (magnetic) |
| B | Ground or Signal- (Magnetic) | <0.1V to chassis ground |
| C | Signal Output (Hall-effect only) | 0V/5V square wave — frequency proportional to engine speed |
Test Procedure
Read engine speed (RPM) in live data during cranking
Expected: RPM should register during cranking. If RPM = 0 during cranking = crank sensor fault
Tool: OEM diagnostic software
Check sensor air gap (magnetic pickup type)
Expected: Air gap: 0.020–0.060 inches (0.5–1.5mm) between sensor tip and reluctor ring. Verify with feeler gauge
Tool: Feeler gauge
Inspect reluctor ring for missing or damaged teeth
Expected: All teeth should be present and undamaged. Missing tooth causes erratic RPM signal
Tool: Visual inspection (borescope if needed)
Measure sensor resistance (magnetic pickup, sensor disconnected, key off)
Expected: 800–1,500 ohms between signal pins. Infinite resistance = open coil. Near 0 ohms = shorted coil
Tool: Multimeter (ohms)
Check sensor wiring for shorts and opens
Expected: No short to ground on signal wires. No open circuit in signal or ground wires
Tool: Multimeter
Inspect sensor connector and sensor body for damage or oil contamination
Expected: Clean, undamaged connector. Oil on sensor tip = front seal leak — address root cause
Tool: Visual inspection
Common Failure Modes
- •Sensor internal failure — no signal output
- •Incorrect air gap — signal too weak or intermittent
- •Damaged reluctor ring — missing or bent teeth
- •Open circuit in signal wire
- •Connector corrosion causing intermittent signal
- •Oil contamination on sensor tip from front seal leak
- •Sensor body cracked from heat cycling
Frequently Asked Questions
Where is the crankshaft position sensor located on a diesel engine?
The crankshaft position sensor is located on the driver side of the lower front engine block, near the crankshaft damper or flywheel housing, on most Class 8 platforms.
What is the correct air gap for a crank sensor?
For magnetic pickup type crank sensors, the air gap should be 0.020–0.060 inches (0.5–1.5mm) between the sensor tip and the reluctor ring teeth.
What causes a crank sensor fault?
Common causes include incorrect air gap, damaged reluctor ring teeth, open circuit in sensor wiring, internal sensor failure, or oil contamination from a front seal leak.
What fault codes are caused by a failed crank or cam sensor?
Common fault codes include SPN 636 FMI 8 (crankshaft position — abnormal frequency), SPN 637 FMI 8 (camshaft position — abnormal frequency), and SPN 723 FMI 8 (cam position — abnormal update rate).
Snap a photo of your scan tool screen and let Diesel Vision interpret the fault instantly — no manual code lookup required.
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