Crank No-Start With Good Compression and Spark
A 2014 medium-duty truck with a gasoline V8 cranks but will not start after a fuel pump replacement. The technician confirms strong spark at all coils with a spark tester, compression is 150 psi on all cylinders, and the engine will run briefly on starting fluid. Fuel pressure at the rail is 58 psi, which is within spec, and it holds after key-off. The scan tool shows no RPM signal during cranking, and the tachometer does not move. The technician replaced the crank sensor before the pump and the cam sensor is original. Battery voltage drops to 10.5 V during cranking.
1. The engine runs on starting fluid and has fuel pressure, yet the scan tool shows no RPM during cranking. Explain the significance of the missing RPM signal and how it fits with the other findings.
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The missing RPM signal is the key. The PCM needs a crank signal to fire injectors and establish timing; many systems will still spark in a default or waste-spark mode with a cam signal, which explains strong spark at the coils while the engine will not run on fuel. Running on starting fluid only proves the ignition can fire and compression is adequate; it does not prove the PCM is sequencing injectors.
Fit with other findings:
- Fuel pressure at 58 psi is a static check; it does not prove injector pulse. Without RPM, the PCM may not pulse the injectors at all.
- Battery at 10.5 V during cranking is low but usually adequate for cranking; it is worth noting because a weak crank signal can be caused by low voltage or a poor ground, not just a bad sensor.
- The tachometer not moving is consistent with no RPM data reaching the cluster.
The technician should not chase fuel delivery until the RPM signal is restored.
2. The crank sensor was already replaced. List the causes you would check for a missing RPM signal and describe how to test each.
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Causes and tests:
- Sensor air gap or reluctor damage: inspect the reluctor wheel for missing or damaged teeth and verify the air gap with a feeler gauge or depth measurement.
- Wiring and connector: back-probe the sensor connector and check for reference voltage, ground, and signal. Wiggle-test the harness; a chafed wire near the pump replacement is plausible since the work was done there.
- Sensor type mismatch: a Hall-effect sensor needs a pull-up/reference voltage; a wrong sensor or a sensor installed with the wrong orientation will not signal.
- Poor ground or low cranking voltage: load-test the battery and check engine-to-chassis grounds. At 10.5 V, verify the PCM and sensor have clean power and ground.
- PCM driver/input fault: if the sensor, wiring, and reluctor are good and a known-good sensor still produces no signal on a scope, suspect the PCM.
- Cam/crank correlation: if the crank signal is present on a scope but the PCM does not report RPM, check for a cam-crank sync fault that inhibits startup.
Use an oscilloscope on the signal wire during cranking; a scope pattern will show whether the sensor is producing a clean waveform and whether the reluctor is damaged. Fix the signal before re-evaluating fuel.