Intermittent No-Crank on a Day Cab
A 2017 Class 8 day cab with a 12.7L engine and automated manual transmission will not crank intermittently - roughly 1 in 5 key cycles. The driver reports it happens more often after the truck sits overnight in cold weather and after fueling. Battery voltage at rest is 12.4V; during crank attempt at the starter stud it drops to 10.9V. The starter relay clicks on every key cycle. The transmission is in neutral and the park brake is set. The shop has a DVOM, a load tester, and a scan tool. The battery terminals look clean but the negative cable at the frame ground shows slight corrosion under the bolt head. Ambient is 28°F.
1. Given the symptoms, list the diagnostic steps in priority order and explain why each is placed where it is.
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- Verify the complaint and capture the failure - the fault is intermittent, so any test done when it cranks proves little. Reproduce it (cold soak, multiple key cycles) before condemning parts.
- Battery condition first - 12.4V rest is about 50–60% state of charge; at 28°F capacity is already reduced. Load-test both batteries individually and check for a parasitic draw if the truck sits overnight. A weak battery explains the cold/overnight pattern.
- Voltage-drop test the cranking circuit - measure from battery negative post to starter case and battery positive post to starter stud while cranking. Spec is typically ≤0.5V per leg. The corroded frame ground is a prime suspect; clean and retorque, then re-measure.
- Neutral/safety switch and clutch/transmission interlocks - the AMT controller must confirm neutral. Scan tool data will show the neutral switch state; an intermittent neutral signal will block crank even though the lever reads neutral.
- Starter relay and control circuit - the relay clicks, which proves the coil is energized but not that the contacts carry current. Check for voltage at the starter S-terminal during a failed crank.
- Starter draw - only after supply and control are verified.
Placing battery and voltage-drop first is deliberate: they are cheap, non-invasive, and explain the cold-weather pattern better than any single component.
2. The technician finds 0.9V drop on the negative leg from battery post to frame. What does this tell you, and what is the correct repair and verification?
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0.9V on the negative leg exceeds the typical 0.5V limit and means high resistance in that path - consistent with the corrosion seen under the ground bolt. High resistance steals cranking voltage from the starter and gets worse in cold weather as current demand rises.
Repair: Disconnect batteries, remove and clean the frame ground to bare metal, replace the cable if the conductor is corroded under the insulation, apply anti-corrosion compound, and torque to spec. Inspect the positive leg and engine-to-frame strap at the same time.
Verification: Re-run the voltage-drop test under crank; both legs should be ≤0.5V. Confirm rest voltage recovers after charging and load test passes. Perform multiple cold key cycles to confirm the intermittent no-crank is gone. Document before/after readings.