Hot Slow Crank: Battery or Cable?
A 2015 3.0 L diesel pickup has a hot slow-crank complaint. The battery is rated 800 CCA and 90 Ah, and is four years old. Rest voltage is 12.65 V. During cranking the battery posts measure 9.6 V, the starter positive terminal measures 7.8 V, and an inductive clamp reads 340 A. Voltage drop tests during cranking show 1.2 V on the positive cable from battery positive to starter terminal and 0.35 V on the ground cable from battery negative to engine block. A conductance tester reports 610 CCA at 35 degrees C. The starter solenoid engages firmly and there is no repeat click. The starter was replaced 18 months ago. The customer wants the starter replaced again because the engine cranks slowly. No aftermarket accessories are fitted. The vehicle has 180,000 km.
1. Calculate the resistance of the positive and ground cables. Compare the results with a typical cranking-circuit voltage-drop limit and decide whether the cables are acceptable.
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Positive cable resistance is R = V/I = 1.2/340 = 0.0035 ohm, or 3.5 milliohms. Ground cable resistance is 0.35/340 = 0.0010 ohm, or 1.0 milliohm. Total cable drop is 1.2 + 0.35 = 1.55 V, which is 4.5 milliohms at 340 A. A common guideline for a complete cranking circuit is no more than about 0.5 V total drop, with the positive side often limited to about 0.2 to 0.3 V and the ground side similar. The positive cable alone drops 1.2 V, more than twice the total guideline, and the ground side is also high. These cables are not acceptable. The starter is receiving only 7.8 V, so slow cranking is expected even if the starter is healthy. Repair or replace the positive cable and clean the ground connections before condemning the starter.
2. The conductance tester shows 610 CCA against an 800 CCA rating. Can the battery be condemned from this result alone? Explain how you would decide.
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No. 610 CCA is about 76 percent of the rating, and conductance testers are sensitive to temperature, connection quality, and recent charging. At 35 degrees C the battery is warm, which normally helps cranking performance, yet the posts still fall to 9.6 V at 340 A. That voltage is low, but the battery has not been proven bad by conductance alone. First repair the excessive cable drop, confirm the battery is fully charged and at a known temperature, and perform the manufacturer's specified load test or a capacitance test with the correct CCA setting. If the battery fails the load test, replace it. If it passes, recheck cranking voltage at the starter and starter current. The decision should be based on a passing load test, not on one conductance number or on the customer's request.
3. The customer insists that a new starter will fix the slow crank. Write a reasoned response that explains why the starter replacement may not solve the problem.
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The starter is seeing only 7.8 V at its positive terminal while the battery posts are at 9.6 V. That 1.8 V difference is mostly cable and connection loss, and the measured positive cable drop alone is 1.2 V at 340 A. A starter designed for 12 V operation will crank slowly and draw abnormal current when it receives 7.8 V. Replacing it without repairing the circuit could produce the same complaint and may damage the new starter through low-voltage/high-current operation. The correct sequence is to repair the positive cable and ground path, verify less than about 0.5 V total drop during cranking, recharge and load-test the battery, and then evaluate starter current and cranking speed. Only if the starter still performs poorly with good voltage and a proven battery should it be replaced.