How to find a short in a modern car fast and easy (The correct way)

Introduction

This tutorial will guide you through the process of finding an electrical short in a modern car, which can drain your battery overnight. By following these steps, you will learn how to perform a parasitic current draw test using a multimeter, identify electrical shorts, and understand automotive relays and fuses.

Step 1: Understanding Electrical Shorts

• An electrical short occurs when current flows through an unintended path, which can drain the battery.
• Signs of a short include:
  • The car not starting after sitting overnight.
  • Battery drain without any electrical components being used.

Step 2: Preparing for the Test

• Ensure you have a fully charged battery before testing.
• Gather necessary tools:
  • Multimeter (a cheap one will suffice).
  • Alligator clips (optional for easier connections).

Step 3: Checking Battery Voltage

1. Set the multimeter to 20 DC volts.
2. Connect the black lead to the negative terminal and the red lead to the positive terminal of the battery.
3. Check the voltage reading; it should be between 12.4 to 12.6 volts. If it's lower, charge the battery.

Step 4: Disconnecting the Battery

• Disconnect the negative terminal of the battery and keep it isolated.

Step 5: Setting Up the Multimeter for Current Draw

1. Set the multimeter to 10 amps.
2. Connect the red lead to the amp jack on the multimeter and the black lead to the common jack.
3. Connect the leads to the battery terminal and the negative battery cable to measure current draw.

Step 6: Measuring Parasitic Draw

• In modern cars, a normal parasitic draw is between 35 to 50 milliamps.
• A reading above that (e.g., 1.3 amps) indicates a significant drain.

Step 7: Identifying the Circuit Causing the Drain

1. Start pulling fuses one at a time from the fuse box while monitoring the multimeter reading.
2. Look for a drop in the current reading; this indicates the circuit causing the drain.
3. Note the position of the fuse that caused the drop.

Step 8: Troubleshooting the Faulty Circuit

• If the reading dropped when pulling a fuse, investigate that circuit further:
  • Disconnect the component associated with the fuse (e.g., radio).
  • Check if the reading goes back to normal.
• If the reading remains high, the short may be in the wiring harness.

Step 9: Using a Circuit Tester Kit (If Needed)

• If you suspect a wiring issue, use a short/open electrical circuit tester.

1. Connect the transmitter to the fuse.
2. Use the receiver to trace the signal along the wires.
3. Follow the wire until the receiver stops detecting the signal, indicating a short.

Step 10: Handling Modern Car Complexity

• Modern cars have multiple computers that draw current, making it challenging to identify shorts.
• To avoid false readings:
  • Ensure the car is locked, and all doors and windows are closed.
  • Use a clamp to hold the door switch down if needed.

Step 11: Final Checks

• After testing all fuses, if no shorts were found but battery drain continues, consider:
  • An old battery that can’t hold a charge.
  • A faulty alternator diode.

Conclusion

You now have a comprehensive understanding of how to find an electrical short in a modern car using a multimeter and systematic troubleshooting. If you encounter ongoing battery issues, consider testing the battery and alternator, as detailed in additional resources. With practice, you will become proficient in diagnosing electrical problems in your vehicle.