Hot no-start on a turbo-diesel: separating a high-resistance cable from a dragging starter
A 2.8 L turbo-diesel ute with 210 000 km is hard to start when hot; cold starts are normal. The battery is a 12 V 95 Ah 800 CCA AGM, 14 months old, resting at 12.68 V, and it holds 10.2 V at 400 A for 15 s on a carbon pile. Cranking voltage at the battery posts falls to 10.4 V; at the starter solenoid main stud it falls to 8.9 V while the engine block reads 0.22 V above battery negative. A clamp meter shows 320 A cranking against a spec of 200–280 A. The customer fitted a rebuilt starter 3 000 km ago. Terminals are clean and tight. Block-to-body strap reads 0.35 V during crank; block-to-battery negative reads 0.18 V. Ambient is 30 °C and the fault only appears after a 20 minute run.
1. Rank these measurements by significance, calculate the positive-side voltage drop at cranking current, and state whether the cranking current is acceptable.
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In order of concern
- Positive side: 10.4 V at the posts minus 8.9 V at the solenoid stud = 1.5 V lost at 320 A, which is 4.7 mΩ. A healthy 12 V starter circuit should lose well under 0.5 V total at cranking current. This is roughly three times too high and is a real fault - cable, crimp, terminal or solenoid stud.
- Cranking current: 320 A against a 200–280 A spec is 15–60 % high. That points at the starter or at engine drag, not at the battery.
- Battery: 12.68 V resting and 10.2 V held at 400 A for 15 s passes a carbon pile test (typically ≥9.6 V at half CCA, corrected for temperature). The battery is not the cause.
- Block-to-battery negative: 0.18 V ÷ 320 A = 0.56 mΩ. Acceptable.
- Block-to-body: 0.35 V. That strap does not carry cranking current, so it cannot cause this fault, but it is corroded and will upset accessories and sensor references.
Verdict
The 1.5 V positive drop must be repaired first. A starter fed 8.9 V turns slowly, draws more current for the same mechanical load and gets hot, so the high current reading may be an effect as well as a cause. Re-measure current after the cable repair; only then judge the rebuilt starter.
2. Explain why the fault appears only when hot, and list what you would check on the starter itself.
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Why hot only
- Copper, brushes, commutator film and solenoid windings all rise in resistance with temperature. A joint that drops 0.7 V cold can drop 1.5 V after heat soak, especially a crimp that is already marginal.
- A rebuilt starter with tight bushings or armature drag can bind as the housing expands, and cranking current climbs.
- A hot diesel has higher compression and a hot injection pump, so it is genuinely harder to spin.
Starter checks
- Voltage drop across the solenoid main contacts while cranking - should be under about 0.2 V.
- Brush length and spring tension, commutator wear and mica undercut.
- Armature end float and bushing play; turn the pinion by hand.
- Cold versus hot cranking current with a clamp, and repeat the positive drop test hot. If the drop grows when hot, the fault is in the cable or a crimp, not in the starter.
- The control circuit: ignition switch, starter relay contacts and the solenoid pull-in winding. A hot relay that fails to pull in gives a no-crank, not a slow crank, so this is a secondary check.
3. Evaluate the two earth readings: is 0.18 V at 320 A acceptable, and what does the 0.35 V block-to-body reading tell you?
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0.18 V ÷ 320 A = 0.56 mΩ. For a cranking earth this is fine - the usual pass is under 0.1–0.2 V at cranking current - so the main earth path is not causing the 1.5 V loss.
The 0.35 V block-to-body reading is taken while cranking current flows through the main block-to-battery strap, so the body strap is only seeing stray and accessory current. A 0.35 V reading under that tiny current means high resistance, almost certainly corrosion under a terminal or a strap that is fraying inside its insulation. It will not stop the engine cranking, but it will drag down lighting, ECUs and sensor references and it will get worse. Clean to bare metal, fit star washers, seal the joint and re-test. Do not use the body strap as the cranking return when you rewire the main earth.