Intermittent No-Crank on a 2016 Type C Bus
A 2016 Type C conventional school bus with a 6.7 L diesel and an automatic transmission will not crank about one morning in five. The driver reports a single click from the right frame rail and then nothing; sometimes a second key cycle starts it normally. The shop has already replaced both batteries (12.6 V each, 12.65 V at rest after a load test) and cleaned the battery terminals. Voltage at the starter solenoid control terminal measures 11.9 V during a failed crank attempt, and battery voltage at the starter B+ stud stays at 12.4 V. The starter relay coil draws 0.18 A and its contacts show 0.4 V drop when energized. The transmission range sensor was replaced last month. Ambient temperature is 28°F and the bus sits outdoors overnight.
1. The technician wants to condemn the starter because it clicks but does not crank. Evaluate that conclusion using the voltage readings, and state what the 11.9 V at the control terminal tells you.
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A click with 11.9 V present at the S terminal means the solenoid is being commanded and is pulling in, but the motor is not turning. That does not by itself prove the starter is bad. Two facts matter: battery voltage at the starter B+ stud is only 12.4 V while the batteries are at 12.6 V, and the control terminal is 11.9 V.
- A 0.2 V drop from battery to starter B+ is acceptable for a cranking circuit at rest, but the real test is under load. Static voltage cannot confirm cable integrity.
- 11.9 V at S is marginal but generally enough to pull a healthy solenoid. If the solenoid pulls in (click) but the motor does not spin, suspect the solenoid contact disc, the motor brushes, or a mechanical bind - not the control circuit.
- Do not condemn the starter from a click alone. Perform a voltage-drop test on the B+ cable while cranking (spec typically <0.5 V total) and a starter draw test. If B+ holds and the motor still will not turn, the starter is the fault. If B+ collapses under load, the cable or ground is the fault.
2. The intermittent nature and cold weather are suspicious. Describe a diagnostic sequence that separates a control-circuit fault from a starter fault without replacing parts.
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Use a staged, measured approach rather than substitution:
- Reproduce the failure. Leave the bus overnight as the driver does; do not warm the shop. Intermittent cold-only no-cranks often involve connections, not the starter.
- At the moment of failure, measure voltage at the S terminal and at the starter B+ stud simultaneously. A helper or a min/max recording meter is needed because the event is brief.
- If S is present (≥10.5 V under load) and B+ holds, the control circuit and cables are good; the starter is the prime suspect. Bench-test or replace.
- If S is low or absent, work backward: neutral safety/range sensor input, starter relay contacts, ignition switch, and the anti-theft or body controller inhibit. The 0.4 V relay contact drop is within reason but should be rechecked cold.
- Perform a cranking voltage-drop test on both the positive and ground sides. A ground strap with corrosion can pass a static test and fail under 200+ A.
- Wiggle-test harnesses at the frame rail and firewall while cranking to expose a chafed or loose connector.
Document each measurement. The goal is to prove where the voltage is lost, not to guess which part is weakest.