Intermittent No-Crank with Voltage Drop Clues
A 2018 medium-duty truck with a 12V system and a Delco 28MT starter intermittently fails to crank. The customer reports that sometimes turning the key produces a single click, and other times the engine cranks normally. The battery is new, rests at 12.6V, and passes a conductance test. During a no-crank event, battery voltage at the terminals is 12.4V, but voltage at the starter solenoid 'S' terminal is only 9.8V when the key is held in START. The starter control circuit includes a neutral safety switch, a clutch switch, and a 20-foot harness with several connectors. A voltage drop test from battery positive to the starter solenoid main stud shows 0.05V during crank. The ground circuit from battery negative to engine block shows 0.02V during crank.
1. What is the most likely cause of the intermittent no-crank, and what is the next diagnostic step?
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The voltage at the 'S' terminal (9.8V) is significantly lower than battery voltage (12.4V), indicating excessive resistance in the starter control circuit. Since the main power and ground circuits show negligible voltage drop (0.05V and 0.02V), the fault lies in the control circuit components: neutral safety switch, clutch switch, ignition switch, or wiring/connectors. The next step is to perform a voltage drop test across each component in the control circuit while the key is held in START, starting from the ignition switch output to the neutral safety switch, then to the clutch switch, and finally to the starter solenoid. This will localize the high-resistance point.
2. If the voltage drop is found across the neutral safety switch, what does that indicate and how should it be repaired?
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A voltage drop across the neutral safety switch indicates high resistance inside the switch, likely due to worn contacts or corrosion. This resistance reduces current to the starter solenoid, causing intermittent no-crank. The switch should be replaced. After replacement, re-test the voltage at the 'S' terminal during crank; it should be within 0.5V of battery voltage. Also, check the switch adjustment to ensure proper engagement in Park/Neutral.