Hot Slow-Crank on a Petrol Car
A 2.0-litre petrol car cranks slowly only when the engine is hot; from cold it starts normally. The battery is 12.6 V at rest and passes a load test. During a hot crank the battery terminal voltage falls to 10.4 V, the starter main terminal reads 9.6 V, and the starter draws 180 A. The voltage drop on the positive cable from battery positive to starter main terminal is 0.80 V. The engine-to-battery negative drop is 0.06 V. The solenoid control terminal reads 11.8 V while cranking. The starter is rated at 1.4 kW. The positive cable is original, with a slightly green terminal at the starter end. The owner says the problem began gradually over two months and is worse after a long drive.
1. Calculate the resistance of the positive cable and the power dissipated in it at 180 A. Compare the result with an acceptable maximum drop.
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Calculation
R = V / I = 0.80 V / 180 A = 0.00444 Ω, or 4.4 mΩ.
Power lost = V × I = 0.80 × 180 = 144 W.
Comparison
A typical acceptable positive cable drop at cranking is about 0.2 V or less, with the total starter circuit drop under about 0.5 V. The measured 0.80 V is four times the per-cable guide. The starter sees 9.6 V instead of 10.4 V. For a given motor, torque falls roughly with voltage squared, so the starter has about (9.6 / 10.4)² = 0.85, or 15% less torque. That is enough to cause a hot slow crank. The cable is therefore a genuine fault, not a marginal reading.
2. Decide whether the starter or the cable is the primary fault, and explain how temperature affects the evidence.
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The cable is the primary fault. A 0.80 V drop at 180 A is excessive, and the green terminal indicates corrosion. The 0.06 V ground drop is good, and the solenoid control at 11.8 V is adequate, so the control circuit is not the main problem.
Temperature matters because copper resistance rises by about 0.4% per °C, and hot connections can soften and lose tension. A marginal cable therefore becomes worse after a long drive. The starter also needs more current when hot. However, the cable drop already exceeds the limit during the test, so repair the cable first. If the crank is still slow after repair and the cable drop is normal, then test starter current and hot cranking voltage before condemning the starter.
3. Outline the repair and the post-repair verification, including a check of the solenoid control circuit.
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Repair
Replace the positive cable, or cut back and fit a new crimped terminal if the cable is otherwise sound. Clean the starter stud and battery post, tighten correctly and apply protective grease.
Verification
- Crank the engine hot and measure positive cable drop: should be under about 0.2 V at 180 A.
- Measure engine-to-battery negative drop: should be under about 0.1 V.
- Battery cranking voltage should stay above about 9.6–10 V.
- Check control-circuit drop from battery positive to solenoid terminal: should be under about 0.5 V.
- Repeat the hot test after a road drive, not just from cold.
Also check charging voltage and battery condition, because a weak battery can mask or mimic a cable fault.