* Alternator: The most likely culprit. The alternator charges the battery. If it's failing, it might initially charge the battery enough to start the car, but after 20 minutes of running, it can't keep up with the electrical demands, causing the voltage to drop, leading to electrical component failure and finally, the engine stalling because the computer and fuel pump lose power. Check the alternator's output voltage with a voltmeter while the engine is running. It should be around 13.5-14.5 volts. A lower voltage indicates a problem.
* Battery: A weak or failing battery might initially have enough charge to start, but it quickly depletes under load and when the alternator isn't functioning correctly, causing the system to fail. Get the battery tested for capacity and cranking amps.
* Wiring harness: Heat, age, and corrosion can cause wires to short circuit or create high resistance connections. After running for 20 minutes, heat might exacerbate these problems, leading to complete electrical failure. Inspect the wiring harness, especially in areas that get hot like near the engine and exhaust manifold. Look for melted insulation, broken wires, or corrosion.
* Voltage regulator: This component regulates the alternator's output voltage. A faulty regulator can cause the alternator to overcharge or undercharge the battery, leading to the symptoms you describe.
* Ignition switch: While less likely, a failing ignition switch could intermittently cut power to the electrical system after a period of use due to heat or wear.
* Fuse/Relay: A fuse or relay that's failing due to heat could break the circuit after a while. Check all fuses and relays, especially those related to the ignition system and major electrical components.
* Computer (PCM): A less common possibility, but a failing PCM could overheat and shut down after a period of operation, though this would likely cause more complex problems than just electrical failure.
Troubleshooting Steps:
1. Check the battery voltage: Test the battery voltage with a multimeter both when the car is off and after it has run for about 10-15 minutes.
2. Test the alternator output voltage: With the engine running, test the voltage at the alternator output.
3. Inspect the wiring harness: Carefully examine the wiring harness for any signs of damage.
4. Check fuses and relays: Replace any blown fuses or faulty relays.
5. Have the alternator and battery tested professionally: An auto parts store can usually perform these tests for free.
If you're not comfortable working on your car's electrical system, take it to a qualified mechanic for diagnosis and repair. Ignoring the problem could lead to further damage or even a dangerous situation.
