As an automotive technician, I often come across complex wiring issues that leave me scratching my head. After years of experience and continuous learning, I've discovered some lesser-known problems that can cause major headaches if not addressed properly. In this guide, I'll walk through some of the hardest automotive wiring gremlins I've encountered and provide tips on how to diagnose and repair them.

Intermittent Power Loss to Fuel Pump or ECU

Losing power intermittently to critical components like the fuel pump or engine control unit (ECU) can be incredibly frustrating. Here are some likely culprits and solutions:

Faulty Ground Connection

• Check ground connections for corrosion or looseness. Clean contacts and tighten down screws.

Bad Main Relay

• Main relays switch power to major systems. When they fail intermittently, diagnose by swapping with a known good relay.

Damaged Wiring Harness

• Inspect wiring harness for damage like chafing, cuts, or pinched wires. Repair or replace damaged sections.

Weak Battery Terminal/Fuse Box Contacts

• Loose battery cables or fuse box terminals can cause high resistance and voltage drop. Clean contact surfaces and tighten hardware.

Electrical Shorts that Are Impossible to Find

Tracking down electrical shorts can make you feel like you're searching for a needle in a haystack. Here are some tips:

Use Schematics

• Consult wiring diagrams to understand circuit layouts and identify possible short locations.

Check Grounds First

• Many times, shorts are from frayed ground wires touching conductive surfaces. Inspect all ground points.

Divide and Conquer

• Systematically disconnect electrical connectors and components to isolate the shorted circuit.

Wiggle Test

• With the ignition on, wiggle wiring harnesses while monitoring for voltage drops. This can expose damaged insulation.

Follow the Symptoms

• Determine what systems are affected by the short and trace their wiring paths.

Mysterious Draining or Dead Battery

A car that goes dead overnight or has a battery that runs down prematurely can be perplexing. Several culprits could be at play:

Parasitic Battery Drain

• Use an ammeter to measure current draw with the ignition off. Draw over 50mA indicates a parasitic drain.

Faulty Alternator Diode

• Test alternator diodes with a digital multimeter. Replace alternator if diodes are shorted or open.

Loose Battery Terminals/Cables

• Check for loose battery and ground connections. Tighten and clean as needed.

Damaged Cell in Battery

• Test battery and individual cells with a hydrometer. Replace if any cells are dead or weak.

Wrong Battery Size

• Verify battery is the correct cold cranking amps (CCA) rating for the vehicle.

Electrical Gremlins After Jump Starting

Improper jump starting can introduce a range of weird electrical issues. Here's what to look out for:

Reversed Battery Cables

• Crossing positive and negative cables can fry computer components. Check for blown fuses.

Overvoltage Spikes

• Unsmooth disconnect of jumper cables creates spikes. Check for damaged modules.

Welded Jump Points

• Clean jump points well after use, especially ground point. Rust buildup causes issues.

Faulty Ground Point

• Use ground point directly on block or battery negative post, not accessory brackets.

Remove Battery Connections

• Always disconnect battery prior to charging battery or jump starting. Reconnect when done.

Key-Off Battery Drain or Parasitic Draw

Excessive current draw with the key off can quickly kill a battery. Isolate the source with these tips:

Check Fuses

• Pull fuses one by one while monitoring drain. The last fuse pulled is the circuit with the draw.

Consult Schematics

• Use wiring diagrams to identify components on the suspect circuit.

Disconnect Components

• Unplug connectors of components on circuit individually to pinpoint draw source.

Target Common Culprits

• Door lights, gauges, engine computers, and radio memory are common parasitic draw causes.

Consider Intermittent Problems

• Use an ammeter, don't just pull fuses, to catch components turning on occasionally.

Conclusion

Automotive electrical problems often seem like mysteries, but armed with the right troubleshooting techniques and diagnostic tools, you can hunt down the source systematically. Reference wiring diagrams, inspect grounds and connections, perform resistance or draw testing, and isolate circuits one-by-one. With some perseverance and deductive savvy, you can solve any difficult wiring gremlin that comes your way.