No-Crank on a School Bus With Good Batteries and a Suspect Starter Circuit
A 2017 school bus with a 6.7L Cummins will not crank. The dash lights come on, the headlights are bright, and the batteries measure 12.7V at rest and 12.4V when the key is turned to start. The starter solenoid does not click. The transmission is in park, the park brake is set, and the neutral safety switch reads correct on the scan tool. The battery cables were replaced last year. The starter relay in the under-hood box was swapped with an identical relay and the symptom did not change. A voltage drop test from battery positive to the starter B+ terminal reads 0.2V while cranking is attempted; the ground side from engine block to battery negative reads 0.3V. The ignition switch start signal at the relay coil reads 0V when the key is in start.
1. Given the voltage drop results and the missing start signal at the relay, where is the fault and how do you prove it?
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The voltage drops (0.2V positive, 0.3V ground) are acceptable, so the heavy cables and grounds are not the problem. The starter does not click and the relay coil has 0V on the start signal when the key is in start - so the starter and relay are not being commanded. The fault is in the start-signal path: ignition switch start contacts, the neutral/park switch circuit, the start inhibit (e.g., hood/emergency door/brake interlock), or the wiring between them.
Prove it by back-probing the start circuit at each junction with the key held in start: ignition switch output, then the neutral safety switch input and output, then each interlock in series. The point where voltage is present on the input but absent on the output is the open component or connection. Because the scan tool shows the neutral switch 'correct,' verify the switch's start-contact circuit separately - a scan-tool status can be derived from a different contact than the one that carries starter current.
2. The open is found at an emergency-door interlock switch. What should be verified before returning the bus to service, and what is the safety implication?
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The emergency-door interlock is a safety device that prevents the bus from being driven with a door open. Before returning to service, verify the switch operates correctly in both positions (door closed = start enabled; door open = start disabled), that the wiring repair is secure and routed away from door pinch points, and that no other interlock in the start circuit was bypassed or left disconnected.
Safety implication: a failed-open interlock causes a no-crank, which is a safe failure; but if a technician bypasses it to 'fix' a no-start, the bus could be driven with an emergency door unlatched, which is a serious hazard. The repair must restore the interlock function, not defeat it. Document the repair and perform a functional check of all door interlocks before the bus carries students.