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Common Problems. Common Solutions. Common Sense.
This set of problems and solutions is common to most cars, and dealing with them doesn't require a lot of specific electrical knowledge, just some common sense. Of course, these are very general and may not work for some specific makes and models. If these don't work, consult a manual or an expert.
Dead Battery
Charge the battery for at least one hour. Check for clean connections at the battery terminals, starter and grounds. Use the starter to crank the engine over five or six times. Attach a voltmeter to the battery and watch its reading as someone cranks the engine several times. The voltage should stay at 12 volts when the engine is not cranking. If the voltage drops below eight or nine volts while cranking, or the engine won't crank any more, suspect the battery. If you suspect the battery, and it's not very old, charge it longer and test it again.
Slow Battery Drain
If the battery is draining overnight or over the course of a few days, some device is still turned on and draining it. To find the cause, disconnect the negative battery connection. Use your test light to jump the negative battery cable to the negative post on the battery. If the light glows, something is turned on. Disconnect fuses and/or circuits one by one until the light goes out.
Trace the circuit that was causing the light to glow to find which device is still on. Dome lights, trunk lights, alternators, and non-factory accessory circuits are common causes of such drains. Radio memories and dash clocks usually are not drains and will not make the light glow for this test.
Alternator Over- or Undercharging
Attach a voltmeter to a good ground and a good positive lead. (Usually, the battery works best for this.) With the engine switched off, the battery voltage should read 12 volts. With the engine running, the voltage should read 13.5 to 14.5 volts.
Below 13.5 volts usually signals a non-working charging circuit. Check for a tight belt, and clean connections at the alternator and the battery. Also make sure the engine is properly grounded.
Above 14.5 volts usually signals a bad voltage regulator. Either way, the solution is usually a new or rebuilt alternator.
Crank Starter, Everything Goes Dead
Sometimes everything will seem just fine until you crank the starter, then nothing will work, not even the dome light. Starting with the battery terminals, remove them and give them a good cleaning. Then clean the ground strap to the body and to the engine. Then clean the positive connection to the starter. One or more of these connections is corroded. The load of the starter causes arcing at the corroded connection, which weakens the connection. Since these connections are the main power connection for the whole car, they shut everything else down when they get too weak.
Sticking Heater, Accelerator, Clutch, or Choke Cables
What does this have to do with electrical problems? Plenty. If the engine ground strap goes bad, the engine will seek another ground through these cables. Often, the car will run and start just fine. Over time, however, these cables will melt themselves to their housings. Replace the affected cables and clean or replace the engine ground strap.
Dim Headlight
Sometimes, one or both headlights will be dim. One of the headlights has a bad ground and is grounding itself through the other headlight. In doing so, the headlights change the wiring configuration from parallel to series. When wired in series, they each share half the voltage and glow dimly. Clean or replace the ground(s).
Brake Lights Turn off Taillights
This is a variation on the dim headlight problem. A bad ground is causing the brake lights to ground themselves through the taillight circuit and vice-versa. Clean up the grounds, and everything will work fine.
Turn Signal Problems
When a bulb burns out, most turn signals will either flash quickly or not at all. Sometimes, they do so even though all bulbs appear to be working. Other times, they may flash, but very slowly. If both left and right circuits act the same, suspect the flasher unit or the switch. If only one side has a problem, corrosion is at work. The solution is to first check and clean all the grounds, which often requires removing lamp assemblies to clean the bolts and attachment points with a wire brush. Sometimes, the base of a bulb will corrode, and simply replacing the bulb will solve the problem. Other times, the bulb socket is corroded and should be cleaned.
Blown Fuse
Finding the cause of a blown fuse can be difficult. A component in the circuit is either dead-shorted to ground, or is causing too much load on the circuit. If something is dead-shorted, fuses will blow the instant they are replaced and the circuit is turned on.
Physically search the wiring in the circuit, then disconnect components attached to the circuit one by one until you find the short. If something is generating too much load, the diagnosis is similar, but more difficult. Try to isolate any device on the circuit and see if its use blows the fuse. If you still don't find the problem, check a manual or consult an expert for testing each device and ensure each device is in spec.
Intermittent Problems
Intermittent problems are the hardest to solve. If you can't get the problem to happen while you're looking for it, shake the car or the wiring harness and see if that causes it. Loose or corroded connections are common causes for intermittent problems and such shaking will often bring them about. If you still can't solve it, call in an expert.
Wiring Additional Circuits
First and foremost, follow the accessory manufacturer's instructions. However, many instructions suggest wiring directly to the battery to ensure a good power supply. Avoid this if possible. First see if there is an available accessory circuit in the factory wiring that can handle the necessary current. Many factory systems have extra accessory circuits and fuses built in for owners to expand. If you still want to wire directly to the battery, make sure you have a fusible link, fuse, or circuit breaker as close to the battery as possible. Also, avoid the self-resetting circuit breakers since they may reset before you know there is a problem.
The end.