Engine Starting and Charging Systems Diagnosis and Repair
Battery first, then voltage drop
Every no-crank or no-charge complaint begins at the battery: measure open-circuit voltage (12.6 V is essentially 100 percent state of charge; 12.4 V is roughly half), then load or conductance test it. A battery that reads 12.5 V but cannot deliver cranking current has still failed.
For cranking, measure voltage drops under load: battery positive post to starter stud, and battery negative post to the engine block. Total cranking-circuit drop should stay under about 0.5 V, with each connection contributing no more than roughly 0.1 V. High starter current draw with slow cranking suggests a dragging starter or a tight engine; low current with no crank suggests an open control circuit - check the neutral safety or clutch switch, ignition switch, relay, and solenoid trigger. Never condemn a starter without verifying the control circuit and cable condition.
For charging, test output at 1,500-2,000 rpm with electrical loads applied; expect roughly 13.5-14.5 V. Then measure AC ripple at the battery - excessive ripple means a failed diode. Check belt condition and tension, and confirm the sense circuit sees true battery voltage; a bad ground or open sense lead makes a good alternator undercharge.
Traps: parasitic draw testing requires the vehicle to go to sleep - close latches, wait for modules to time out - and a clamp meter is safer than breaking the circuit. Do not test charging at idle with no load and declare it good.
Rule of thumb: key-off draw should settle below about 50 milliamps; above that, pull fuses one at a time to isolate the circuit.