* Weak or dead battery: This is the most common cause. Even if the battery seems okay, have it load tested at an auto parts store. A battery might show enough voltage to power accessories but lack the cranking amps needed to turn the starter.
* Corroded or loose battery cables: Inspect the cables from the battery to the starter and the battery to the chassis. Clean the terminals thoroughly with a wire brush and baking soda solution. Make sure the connections are tight. Loose or corroded connections create high resistance, preventing sufficient current flow.
* Bad ground connection: The engine block needs a good ground connection to the chassis. Trace the ground cable from the engine to the chassis. Ensure it's clean and securely fastened. A poor ground can prevent the starter from receiving the necessary current.
* Faulty wiring: Somewhere between the battery, ignition switch, relay, and starter, there might be a broken or damaged wire. Visually inspect all wiring related to the starting system. Pay particular attention to areas that may have experienced wear and tear.
* Internal starter motor problem (despite replacement): While you've replaced the starter, there's a slim chance the new one is defective or the problem wasn't the starter itself to begin with.
* Excessive current draw: Something else in the electrical system might be drawing too much power, leaving insufficient current for the starter. Check for any other components that are malfunctioning and drawing excessive amperage.
* Ignition System Issues (unlikely given replacements): While you replaced the ignition switch, double-check the wiring related to it and the ignition system. A fault here could be sending the wrong signal to the starter relay even if the relay is new.
Troubleshooting Steps:
1. Battery Test: This is the absolute first step. Get a load test.
2. Cable Inspection: Thoroughly inspect and clean all battery cables.
3. Ground Check: Check the engine-to-chassis ground connection.
4. Voltage Drop Test: Use a multimeter to check the voltage drop across the battery cables while cranking. A significant voltage drop indicates a high-resistance problem in the cables or connections.
5. Wiring Inspection: Carefully examine all wiring in the starting circuit.
6. Relay Check: Even if you replaced it, double-check the relay is correctly installed and functioning. You could temporarily test with a known-good relay (though you say you've already replaced this).
7. Starter Check (again): While unlikely, consider getting the new starter checked, or if possible, borrowing a known good starter for testing.
If you're not comfortable working with car electrical systems, it's best to take it to a qualified mechanic. Continuing to try and crank a car with a weak battery can cause further damage.
