No-Crank on a Cold Morning
A 2018 transit bus with a 6.7 L diesel will not crank on a 20 F morning. The dash shows 12.4 V at rest. When the key is turned to start, the starter solenoid clicks once and nothing happens. The battery load test shows 11.9 V on each of two batteries under a 500 A load for 15 seconds, which is above the minimum for a healthy battery at this temperature. The starter relay coil receives 12 V when the key is turned. The starter ground strap shows 0.4 V drop from block to frame during a crank attempt. The positive cable from battery to starter shows 0.2 V drop. The engine turns freely by hand at the crank bolt.
1. Given the voltage-drop readings, where is the fault and why?
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The batteries pass a load test and the positive cable drop (0.2 V) is acceptable (generally under 0.5 V is acceptable; under 0.3 V is good). The starter ground strap drop of 0.4 V is high for a ground circuit and is the standout reading. A high ground-side drop means the starter is not getting a solid return path, which can cause a single click and no crank even with good battery voltage.
- Check the block-to-frame ground strap and the battery negative cable to block. Clean and retorque.
- Also verify the starter solenoid control circuit: the relay coil gets 12 V, but confirm the relay contacts pass full voltage to the solenoid terminal under load.
- The single click plus no crank with free-turning engine and good battery suggests high resistance in the ground path or a failed solenoid contact. The 0.4 V ground drop points to the ground path first.
2. After cleaning grounds, the engine cranks slowly and labors. What else should you check?
Read the worked answer
Slow, labored cranking after fixing grounds points to the starter itself or a mechanical drag.
- Measure cranking voltage at the starter B+ and at the battery during crank. A drop below about 9.6 V at the battery during crank indicates a battery or cable problem; a large difference between battery and starter indicates cable resistance.
- Measure starter current draw with a clamp meter. Excessive draw with slow cranking suggests a dragging starter, a bad armature, or mechanical drag.
- Check engine oil viscosity for the 20 F ambient; the wrong oil grade can make cold cranking labored.
- Verify the starter drive engages cleanly and the flywheel ring gear is not damaged.
- If current draw is high and voltage is good, replace or rebuild the starter.