Automotive electrical systems have grown more complex over the years, but the basic principles behind them are similar. The alternator or generator will power the car while recharging the battery at the same time. A common problem with some systems is the tendency to "strobe," or make the lights flicker. Diagnosing the problem could take the backyard mechanic a few minutes, fixing it could take up to an hour.
Check the alternator/generator with a voltmeter. The most common reason for flickering lights is a worn out alternator, as one of the three rotating plates that generate electricity wears out. So as the unit hits the "dead spot," the power wanes, causing the lights to flicker. The voltage should be over 13, ideally over 14 volts. Automotive parts shops will likely have a machine to test the alternator for free. Replace the alternator if it fails to produce more than 13 volts, or tests bad. Most alternators since the 1970s have internal voltage regulators, but for external models this unit is mounted close to the alternator and can cause part failure.
Check for loose or disconnected ground cables. Some on-board engine computer systems are sensitive to loose connections, affecting the ignition system as well. A ground strap or cable is on the engine and sometimes on the transmission. Many electrical items on the car, including the ignition system, will depend greatly on these ground connections. Wobbling bolts or intermittent connections can cause the lights to flicker. Tighten these grounds, as well as the primary battery ground wire.
Check for loose or disconnected ignition wires. On most cars, there is one wire for each spark plug. When they come loose, up to 70,000 volts of electricity can ground out every time the wire is fired by the distributor. This can cause severe light flickering and may damage sensitive electronics.
Check the battery's acid level, and test separately for charge capacity. A battery losing its ability to charge correctly may cause a weak flicker in the lighting system. On some models, a separate positive lead wire goes into a junction box nearby. This box can corrode, as it is often underneath the battery where acid collects. Once corroded, the junction box does not allow for full current transfer, and when the car is in motion it may wobble or bump.
Disconnect the battery before testing it or working on the vehicle. Use safety equipment when working on a vehicle.
Do not touch the red cables or the ignition wires while the car is running.