8.5 CKP/CMP Sensor and Correlation Tests
Key Takeaways
A passive VR sensor produces an AC signal whose amplitude depends on speed and air gap.
A powered Hall-type signal normally switches between its defined voltage levels.
CKP/CMP correlation must be compared with a known-good engine waveform and phaser state.
Note
Sensor waveforms, coil resistance and programming-supply readings in this lesson are teaching examples, not a service specification for every T1 truck. Use the identified engine, component and tool manufacturer's procedure and limits when making a repair decision.
8.3 Ignition Sensors (CKP/CMP), Control Modules (ICM/PCM) & Reprogramming
Distributorless and Coil-on-Plug (COP) ignition systems on medium- and heavy-duty gasoline and LPG trucks rely completely on high-resolution Crankshaft Position (CKP) and Camshaft Position (CMP) signals to calculate engine speed, piston position, dwell duration, spark advance, and sequential firing order. Under ASE T1 Tasks E.6 and E.7, technicians must test CKP and CMP sensors with a Digital Multimeter (DMM) and Digital Storage Oscilloscope (DSO), diagnose Ignition Control Module (ICM) and Powertrain Control Module (PCM) coil driver failures, execute Crankshaft Position Variation Relearn procedures, and perform SAE J2534 module reprogramming.
Crankshaft Position (CKP) & Camshaft Position (CMP) Sensors (Task E.6)
Functional Roles: CKP vs. CMP
Although CKP and CMP sensors use similar magnetic or solid-state sensing elements, they serve distinct functions in engine management:
- Crankshaft Position (CKP) Sensor: Monitors a multi-tooth steel reluctor wheel (tone ring) mounted on the crankshaft or harmonic balancer (
commonly 58X / 60-2 or 36-1teeth with a machined missing-tooth index gap). The CKP sensor is the primary input for engine RPM, exact piston Top Dead Center (TDC) position, base ignition timing calculation, and crankshaft angular-velocity misfire detection. On the example strategy, a complete loss of the CKP signal during cranking results in0 RPMon the scan tool, no primary coil switching (no spark), and no fuel injector pulse (immediate crank no-start). - Camshaft Position (CMP) Sensor: Monitors a target wheel on the camshaft rotating at half crankshaft speed (
1/2 RPM) to identify stroke phase—specifically whether Cylinder #1 is approaching TDC on the compression stroke or TDC on the exhaust stroke. The PCM uses the CMP signal to synchronize sequential fuel injection timing, COP cylinder firing order, and Variable Valve Timing (VVT) cam-phaser closed-loop position. If the CMP signal fails while the CKP sensor remains healthy (DTC P0340orP0341), most truck engines exhibit extended cranking (3 to 5 seconds) while the PCM tests firing phase, then start and run in batch-fire or reduced-power limp mode.
2-Wire Variable Reluctance vs. 3-Wire Hall-Effect Sensor Testing
| Sensor Characteristic | 2-Wire Variable Reluctance (Magnetic Inductive) | 3-Wire Hall-Effect / Magnetoresistive (Digital) |
|---|---|---|
| Wires & External Power | 2 wires (Signal High & Signal Return/Shield); no external power supply required (self-generating) | 3 wires: Regulated 5.0 V or 12.0 V Reference, Sensor Ground, and Digital Signal Output |
| Waveform Shape on DSO | Analog AC Sine Wave (with index signature at missing-tooth gap) | Digital DC Square Wave (0 V to 5.0 V or 0 V to 12.0 V crisp vertical edges) |
| Effect of Increasing Engine RPM | Both signal frequency AND peak-to-peak AC voltage amplitude increase (0.5–1.5 V AC cranking up to 15–25+ V AC at high RPM) | Signal frequency increases with RPM, while peak voltage amplitude remains constant (5.0 V or 12.0 V) over its specified speed range; no repeating edges occur with a stationary target |
| Static DMM Testing | Measure coil winding resistance unplugged (typically 200 to 1,200 Ω at 68°F / 20°C); check AC voltage while cranking (>0.5 V AC) | Never ohm the internal chip; backprobe with KOEO to verify 5.0 V (or 12.0 V) reference, <50 mV ground drop, and 0 V / 5 V signal toggling when cranking |
Important
Why Inductive CKP Sensors Fail Only During Slow Cranking: Because a 2-wire Variable Reluctance (VR) sensor generates voltage proportional to the rate of magnetic flux change, its AC voltage amplitude drops sharply if cranking speed is slow, the sensor air gap (0.020–0.050 in.) is too wide due to rust buildup under the mounting ear (rust jacking), or metallic starter/clutch debris is stuck to the magnetic tip. A 3-wire Hall-Effect sensor switches full 0–5 V amplitude even if the crankshaft is barred over slowly by hand.
Reluctor Wheel Inspection & Dual-Trace CKP/CMP Correlation (P0016 / P0017)
When a truck sets P0016 (Crankshaft Position – Camshaft Position Correlation Bank 1 Sensor A) or experiences high-RPM timing scatter, connect a dual-trace oscilloscope with Channel 1 on the CKP signal and Channel 2 on the CMP signal:
- Bent or Chipped Reluctor Tooth: Inspect the CKP waveform across two full crankshaft revolutions (
720°). A single bent, cracked, or chipped tooth on the crankshaft tone ring produces a repeating low-amplitude or widened dropout once every360°revolution, which tricks the PCM's misfire monitor into logging falseP0300codes. - Non-Invasive Timing Chain & VVT Phaser Verification: With the engine at warm curb idle and the VVT oil-control solenoid commanded to
0%(using the manufacturer's base-position test; unplugging a solenoid does not guarantee mechanical locking), count how many CKP tooth pulses occur between the CKP missing-tooth sync gap and the rising edge of the CMP square wave. Comparing this tooth count against an OEM known-good waveform overlay supports whether the timing chain has jumped/stretched or a VVT phaser is mechanically stuck—before spending hours removing the front timing cover!
Applied Diagnostic Check
When a correlation trace changes with temperature, compare the same requested phaser state and oil conditions. A hydraulic control that has not reached its target can shift the cam signal without a jumped chain. Inspect sensor mounting and target integrity as separate candidates. A damaged tooth can alter one part of the waveform while the rest looks regular. Capture enough revolutions to distinguish a repeating target event from electrical noise. Mark the known-good reference and actual engine application; a waveform from a different reluctor design cannot supply an exact tooth-count standard for this engine.
Avoiding a Misleading Sensor Identification
Wire count is a clue, not a technology specification. Some active sensors use two-wire current signaling, while a passive magnetic sensor can include a shield in addition to its signal conductors. Read the circuit description and expected waveform before applying resistance testing or an external source. A powered sensor needs the specified supply and return; a missing waveform can result from either of those paths.
Check the target wheel, mounting gap and connector as well as the sensor. Capture a sufficient time window to compare indexed crank and cam events. A phaser that has not reached the specified test position can make a sound mechanical timing set appear incorrectly indexed. Record the test state and known-good reference before judging correlation.
A technician is using a Digital Storage Oscilloscope (DSO) to compare a 2-wire Variable Reluctance (magnetic inductive) Crankshaft Position (CKP) sensor on Truck A against a 3-wire Hall-Effect Camshaft Position (CMP) sensor on Truck B as both engines are accelerated from 700 RPM to 3,000 RPM. How will the two oscilloscope waveforms behave as engine speed increases?
Truck A's 2-wire sensor outputs a digital square wave whose voltage amplitude stays constant at 5.0 V while frequency increases; Truck B's 3-wire sensor outputs an AC sine wave whose voltage amplitude and frequency both increase with RPM.
Truck A's 2-wire Variable Reluctance sensor outputs an AC sine wave whose frequency AND peak-to-peak voltage amplitude both increase with RPM; Truck B's 3-wire Hall-Effect sensor outputs a digital square wave whose frequency increases with RPM while peak voltage amplitude remains constant.
Both Truck A's 2-wire sensor and Truck B's 3-wire sensor maintain a constant 5.0 V peak amplitude at all engine speeds while only the duty cycle percentage changes.
Truck A's 2-wire sensor requires a 12.0 V reference supply from the PCM and decreases in voltage amplitude as RPM rises; Truck B's 3-wire sensor generates its own AC voltage without a power supply.
Sections you finish are checked off in the contents.