Slow Crank and an Intermittent Single Click on a Cold Morning
A 2016 V6 pickup with 88,000 miles cranks slowly and intermittently fails to crank at all, giving a single click. The battery is a 3-year-old AGM. Resting voltage is 12.4 V at 35 F ambient. During a 15-second crank the battery posts hold 10.6 V and the engine turns about 170 rpm. Voltage at the starter B+ post during the same crank is 9.5 V, and voltage from the starter case to the battery negative post is 0.42 V. The engine starts immediately when jump-started from another vehicle. Charging output at the battery is 14.1 V at 2,000 rpm with accessories off and 13.4 V with headlights, blower, and rear defroster on. The starter solenoid and the ignition switch have both been replaced recently with correct parts.
1. Which circuits are at fault and how does the arithmetic prove it? State the spec you are comparing against.
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Total voltage drop on a cranking circuit, both sides, should be about 0.5 V or less, with each cable and connection held to roughly 0.1 to 0.2 V.
Positive side: battery posts at 10.6 V, starter B+ at 9.5 V, a drop of 1.1 V. That is roughly double the whole-circuit budget on one side alone. Ground side: starter case to battery negative at 0.42 V is also over the limit, typically 0.1 V or so per connection and well under 0.2 V total. Combined loss is about 1.52 V, so the motor sees roughly 9.1 V instead of 10.6 V. That, plus 170 rpm against a usual 200 to 250 rpm cranking spec, explains the slow crank without blaming the starter.
The intermittent single click is consistent: with that much drop, the solenoid can pull in and drop out, or fail to hold, depending on temperature and connection resistance. Look at the engine block ground strap and its attachment point, the battery terminals, and the B+ cable ends, not the parts already replaced.
2. Is the battery the cause, the starter, or neither? Lay out a test sequence that decides it.
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Neither is established yet. 12.4 V resting at 35 F is not automatically a bad battery; AGM rest voltage falls with temperature, and 12.4 V cold can be near full charge. That single number cannot condemn it, and 10.6 V at the posts during crank is low but is being dragged down partly by the cable losses measured here.
Sequence: charge the battery fully at the recommended profile, then run a conductance or load test against its CCA rating and record temperature. With a known-good battery installed, repeat the crank and re-measure all three drops. If the drops clean up and cranking speed reaches spec, the cables were the fault. If the drops remain, repair the positive and ground paths and re-test. Only after the circuit is proven should the starter be current-drawn and bench-checked, and only after that should the intermittent no-crank be traced at the solenoid trigger terminal during an actual failure, since a click with no crank can also be a control-side fault rather than a power-side one.
3. The loaded charging reading is 13.4 V. Is the alternator defective, and how does this interact with the slow-crank complaint?
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13.4 V loaded at 2,000 rpm is not automatically a failed alternator. Many late-model systems use variable or targeted charging, where the PCM commands lower voltage once it decides the battery is at target state of charge, and it reads battery current and temperature to make that decision. A 2016 pickup is likely in that group.
Check the charging voltage at the alternator B+ terminal and compare it to the battery post reading to rule out cable loss on the charge path, confirm the battery current sensor is connected and reporting, and confirm the battery has been registered or reset if the platform requires it after replacement. If the system is commanding low voltage because it believes the battery is full while the battery is actually at partial charge, the truck cranks slow every cold morning and the alternator gets blamed.
So do not sell an alternator on the loaded number alone, and do not skip the charging check because the cranking complaint looks like a cable problem. Both paths can be true at once, and the drop tests decide which one is spending the customer's money.