Starter Motor Won't Crank
. Ensure the battery is fully charged, before diagnosing starter problems. Then, test each cell, with a hydrometer or multimeter, to ensure that it’s good. This important step is often overlooked.
. Test all under hood and under dash fuses with a test light or multimeter.
. Ensure the gearshift is in Neutral or Park (automatic transmission), or the clutch pedal is depressed (manual transmission), if the starter motor does not turn at all when the ignition switch is turned to Start.
1. If the starter motor doesn't turn, but the starter solenoid (relay) "clicks" when the ignition switch is turned to Start, the problem is with the battery, starter solenoid, or starter motor connections.
2. Disconnecting the negative battery cable first, clean battery terminals and cable connections with a battery terminal brush, No. 240 abrasive cloth, or steel wool. Also, check the battery ground cable connection to the body and engine. Apply silicone dielectric grease, when finished. Examine the cables carefully for hidden corrosion or internal breaks.
3. While the negative battery cable is disconnected, clean starter motor and starter solenoid connections. Apply silicone dielectric grease, when finished. Make sure all connections are secure. When installing the starter cable, make sure that the crimped side of the ring terminal is facing out. Reconnect the negative battery terminal last.
4. If the starter motor spins but the engine is not cranking, the overrunning clutch in the starter motor is slipping and the starter motor must be replaced. Also, the ring gear on the flywheel or drive plate may be worn.
5. If the solenoid plunger cannot be heard when the switch is actuated, the battery is bad, the fusible link is burned (circuit is open), or the solenoid is defective.
6. In order to check the solenoid, put the transmission in Park and remove the push-on connector from the solenoid “S” terminal. Connect a jumper lead (12 AWG), or preferably a remote starter switch, between the positive battery terminal and the exposed “S” terminal on the solenoid. If the starter motor now operates, the solenoid is fine and the problem is with the ignition switch, neutral start switch, or wiring. If the starter motor still does not operate, replace the solenoid.
7. If the starter motor cranks the engine slowly, ensure the battery is fully charged and all terminal connections are clean and tight. Then, check connections at the starter solenoid and battery ground. The engine will also crank slowly, if it’s partially seized or has the wrong viscosity oil in it. Synthetic engine oil allows starter motors to turn the engine over easier and faster, thus promoting starting and reducing battery drain.
Ohm's Law states Amperage = Volts/Resistance. As resistance (Ω) doubles, amperage is cut in half. Resistance is increased by corrosion, smaller cable size, poor cable material, and heat. Electric motors (e.g., starter motor armatures) will have greater impedance and generate less torque when they are hot.
Taking this a step further, focus on corrosion and cable size. Smaller cables and corrosion greatly reduce the effective cross-sectional area of grounds. Based on the formula Area of a Circle = Pi x R-squared, as the effective cross-sectional area of a cable gets cut in half, the resistance goes up four fold. This means that the amperage available to the starter motor would be 1/4 of what it would be otherwise, when you plug these values into Ohm's Law. That's why all of these factors influence starter motor performance, especially when they are failing.
WARNING: Do not jump a modern-day car. It's easy for a voltage spike to damage the ECM, alternator, and sensors.