1) First of all, check to make sure the batteries are good. Although it is an obvious first step of flashlight troubleshooting, it is important to replace the batteries with a fresh set. Taking a few minutes now can save a lot of time throughout flashlight troubleshooting. Also, reverse the batteries to make sure they are not in the wrong direction.
If you have not opened or messed with the flashlight recently, troubleshooting often points to bad batteries. Make sure the batteries you are replacing are new. If you have access to a battery tester or a DMM, check the batteries to make sure they are at full strength. If they are alkaline batteries, they should be at or just less than 1.5v.
2) The next step of flashlight troubleshooting is to test the bulb.
Remove it from the flashlight carefully, being sure not to touch the bulb directly if possible.
High performance bulbs that get hot during use must not have any skin oils on them. Any residue on the surface of the bulb can result in the glass cracking and breaking. Pressurized bulbs will explode if this happens, sending glass shards throughout the flashlight. If you do touch the bulb, clean it with rubbing alcohol (isopropyl alcohol).
Set the bulb on a flat surface, and line the batteries up in series behind the bulb, touching its rear. Find some wire or a paper clip, and connect the circuit from the last battery to the bulb. If the flashlight has any circuitry between the batteries and bulb, be careful when attempting this step. Check the bulb's preferred voltage to make sure you are applying the correct amount of voltage. If not, you may blow the bulb.
This wire needs to touch the outside edge of the bulb stem, where the flashlight body would normally connect the circuit. Be sure the batteries are all touching each other and the bulb, and move the wire around the bulb's metal to find other areas it may work at.
Check to make sure the batteries are oriented the same and correct direction. Reverse direction to check for opposite polarity.
If the bulb does not light up, the filament may be bad. Look inside to see if the filament is still intact. If it isn't, or if there are any objects loose inside the bulb, it needs to be replaced.
If the bulb lights up only in flashlight troubleshooting, but not in the flashlight, then you have a loose connection. Check to see how the bulb connects to the flashlight. You may need to your own wire connection to make contact across the bulb.
Another possibility is that the outer edge of the bulb stem is not contacted to the rest of the bulb. If flashlight troubleshooting only works when the wire contacts the upper half of the bulb stem, but not on the lower half, you may need to use wire or solder to connect the gap.
3) There are two methods for flashlight troubleshooting the switch. You can either substitute the switch into a flashlight or similar volted electrical device to make sure it works, or you can use a meter to test the current flow over the switch.
To substitute it into another device, find an electrical item that uses the same DC volts. If battery chemistry is the same, find one that uses the same number of batteries. Find a way of substituting the switch into the working circuit. You can position it between batteries with wire.
Test to see if the switch allows the device to still work. If so, skip to the next step of flashlight troubleshooting.
The other flashlight troubleshooting option is to use a meter to measure the current across the switch.
If you don't have a digital multimeter, they can be found at hardware stores for around $25-$30. They are very useful if you ever intend to be involved with electricity.
Keep the makeshift setup intact, but replace the bulb with the switch. Have the meter ready, and set it to measure the current (amps) of the wire. Be sure the switch is turned off. Use the wire to connect to the circuit together to the batteries and the switch. Set the probes on opposite sides of the meter on the wire.
The DMM should read zero. If it does not, you have a short in the wire, and you need to disconnect the circuit immediately. The batteries had zero resistance in the wire, and were getting warm from the unlimited current draw. Reconfigure the circuit, and be sure the switch is off.
Once the DMM reads zero, turn the switch on. If a sufficient number of amps are shown on the DMM, turn the switch off quickly, and disconnect the circuit before the batteries and wire get hot.
If the DMM did not read any amps, the switch is the problem. Try cleaning the contacts if they are worn or rusted. Check any wires or connections to find a break in the circuit. Other than DIY fixing techniques, the switch is very hard to troubleshoot, and you should seek to replace the switch.
4) By now, you have completed flashlight troubleshooting. If you still have not found the problem, check again to make sure your batteries are fresh and installed in the correct direction.
If any contacts on the flashlight are rusted or not shiny, use cleaner to shine them up. The rust may be providing resistance to the current, restricting the current from flowing. Just cleaning the contacts may be enough to fix flashlights.
If the flashlight circuit involves a metal tube on the inside of the flashlight, perform these tests to troubleshoot the flashlight.
Check to make sure the switch is making contact with the metal tube on the inside. Use wire to connect from the switch to the batteries and metal tube to make sure it is making contact.
Check the bulb to see if it is also making contact with the metal tube. Perform the same test by using wire to connect the bulb to the metal tube and batteries.
If all parts of the flashlight work, flashlight troubleshooting only points to a loose connection somewhere. Carefully inspect the entire flashlight for a connection that is not successful when you connect the flashlight together. Fix the flashlight by repairing loose connections.
