Diagnosing Car Electrical Problems

By June 13, 2012Electrical
Car Electrical Fuses

Electrical problems in your car can run the gamut from the obvious to the head scratchers. A lack of schematics and proper tools can make electrical diagnosis an even more challenging problem. However, just about anybody can find and fix most electrical problems without the expense and time loss of taking their car to a shop.

Diagnosing a Consistently Flat or Dead Battery

We’ve all experienced this scenario: You go to start your car in the morning, or after it has sat for a while, and ruh, ruh, ruh, VROOM!, it feels like the battery just barely has enough juice to turn the engine over and get it started. You’ve had the battery, alternator, and starter tested and all are in perfect working order. So what gives with your sleepy battery? This is normally caused by what’s known as a parasitic draw. These occur when something isn’t being switched off properly and can be verified by pulling one cable off the battery and looking for a spark. If you see a spark (it could be quite small), the next steps should help you find the cause of the draw.

  • Turn everything off and walk around the car, looking for any lights left on that should be off.
  • Install your meter between the positive cable and the battery and set it to read current. With everything off, there should be a maximum of 1 to 1.5 Amps of draw (computer and stereo memory, plus security systems).
  • Remove and replace fuses in the convenience center (main fuse box) one at a time, referring to the meter each time a fuse is removed. You could also do the “spark test” or use the test light if you don’t have a meter.
  • When the test light turns off or the meter reading drops significantly, or the “spark test” returns no spark, you’ve found the problem circuit. Different circuits have different procedures for fixing, but normally they all boil down to making switches completely disconnect power, or fixing a small short to ground.

Courtesy Light(s) Not Coming On When Doors Are Opened

You walk out to the car late at night, open the door, but the courtesy light(s) doesn’t come on. Music from a B-level slasher flick starts to play in your head, but luckily because you already fixed your battery drain problem you’re able to start the car right up and escape to mod another day. This is another common electrical problem that’s usually pretty easy to diagnose.

  • Check the fuses to make sure the dome/courtesy lamp fuse is in good shape.
  • The plastic cover on the dome lights just pop out of the housing. You might need a small flat screwdriver, though.
  • Look at the bulb to see if you can verify the filament is in one piece.
  • Grasp the bulb with a clean tissue or napkin (finger oils can accelerate lamp degradation) and pull it out of the lamp holder.
  • Use a continuity tester or multimeter to verify continuity through the lamp. If there is none, replace the bulb, being sure to hold the new bulb with the tissue or napkin too.
  • Use the test light or meter to verify that the lamp holder is receiving power and has continuity to ground. Check all doors. A lack of power (with good fuses) or ground means a bad door switch normally, although it can mean a broken or cut wire.

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