Intermittent No-Start on a 2016 Cummins ISB
A 2016 Cummins ISB 6.7L in a Ram 2500 cranks but will not start intermittently. The customer reports it happens mostly after the truck sits overnight in damp weather. There are no active DTCs, but a stored P0335 (crankshaft position sensor circuit) is present. During a no-start, a scan tool shows engine speed of 0 RPM while cranking, and injector pulse width is 0 ms. Fuel rail pressure builds to 3500 psi during cranking, which is within spec. Battery voltage is 12.4V at rest and drops to 10.8V during cranking. The CKP sensor is a 3-wire hall-effect type. Resistance from the sensor ground pin to battery negative is 0.2 ohms. The signal wire from the sensor to the ECM has 5.0V with the key on, engine off. A lab scope shows a square wave with amplitude 4.8V when the engine is cranked with the sensor removed and a known-good signal generator is used to simulate the tone ring.
1. What is the most likely cause of the intermittent no-start, and what steps would you take to confirm it?
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The most likely cause is a faulty crankshaft position sensor or a problem with its tone ring (reluctor wheel), given that the ECM sees 0 RPM during cranking even though the sensor circuit tests good with a simulated signal. The simulated signal test confirms the wiring and ECM are capable of processing a CKP signal, so the issue is at the sensor or tone ring. To confirm, first inspect the sensor and tone ring for damage, debris, or excessive air gap. Then, with a lab scope connected to the CKP signal wire, crank the engine and observe the waveform. If no signal is present, check the sensor's power and ground under load (using a load test, not just a voltmeter). If power and ground are good but no signal is generated, replace the sensor. If the sensor is good but the signal is weak or erratic, check the tone ring for cracks, missing teeth, or runout. Also, perform a wiggle test on the harness while cranking to check for intermittent opens.
2. The customer mentions the problem is worse in damp weather. How does this information affect your diagnosis?
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Damp weather can exacerbate intermittent electrical faults, particularly those involving poor connections, chafed wires, or cracked sensor housings that allow moisture intrusion. This suggests the fault may be in the CKP sensor itself (e.g., a cracked potting compound allowing moisture in) or in the connector or harness (e.g., a poor pin fit or chafed wire that shorts when wet). To address this, perform a thorough visual inspection of the CKP sensor and its connector for signs of moisture, corrosion, or damage. Use a spray bottle to lightly mist the sensor and harness while cranking to see if the no-start occurs. Also, check the sensor's internal resistance when cold and when warmed, as temperature changes can affect a failing sensor. If moisture is found in the connector, clean and dry it, apply dielectric grease, and retest. If the sensor is cracked, replace it.