TO FIND A SHORT
or battery drain, first switch off and disconnect the obvious devices (eg. taillights if blowing taillight fuse) to see if the fuse still blows or the battery current drain disappears. (See below “FOR THE CASE OF AN UNKNOWN BATTERY DRAIN” for more on how to detect a battery drain.) Also try disconnecting everything that is or may be on that fuse’s circuit or anything suspicious or recently worked on that’s electrical or not stock (eg. aftermarket alarm or radio). Disconnect anything electrical that doesn't work since it may be the problem. Examine and wiggle the wiring and connectors on the things you disconnect to see if they are frayed, broken, melted, wet, or touching other wires or metal (and while you’re disconnecting a connector this is a good time to check if it’s pins are all still locked into place). Also check wiring on anything that was added, changed or tied into for power (eg. stop light, radio, and cigarette lighter are typical aftermarket power sources). Probably 95% of the time you will find the short or battery drain using the above techniques which is also the quickest method. Lastly, if all else fails, try unbolting the engine fuse box (disconnect battery first) and examine the cluster of wires underneath. Some may be melted, broken, or shorting against others.
If the fuse still blows you will have to use a tougher technique. Replace the fuse that blows (temporarily and only for testing purposes) with an actual 12 volt incandescent light bulb like a sealed beam headlight, tail light, automotive tester light, or dome light (don't try to start the engine with it in if it’s an engine circuit). If you don’t have a tester light put a bulb in a socket that has wires (or you can solder wires on a bulb). Then insert the wires into the slots where the fuse was. Try to use a light that draws between 3 and 8 amps. More than that can melt wires. Less than that may leave the test light on even after the short is eliminated should there be other lower resistance loads (eg. lighting) that are normally on and drawing power on that circuit. Using a test light drawing between 3 and 8 amps it will go from bright to dim when the short is eliminated and you have other lower resistance loads present on that circuit. Don't use an LED as this will glow even when you only have 30mA of current flowing which is normal for ECU and radio memories. LED’s also only conduct in one polarity so it’s misleading if it’s backwards.
[Note: Instead of using a bulb you can use the ammeter part of a multimeter if the current draw will be less than 10 amps. More than that can damage the meter (check it's maximum) or possibly the car's wiring or blow an internal ammeter fuse if it has one (warning: not all do). You should use a 10A scale if possible since lower scales will add enough resistance to affect the circuit's current draw giving a misleading reading. If you don't know if it's going to be over 10A (which often is the case) you may want to start with the bulb method. Also the bulb limits the current flow to safe levels - the ammeter doesn't since it’s basically a dead short].
If you have very weird symptoms (eg. headlights never go off, or turning on/off one device causes another device to turn on/off), check for open grounds on the devices affected (also any isolation diodes that are an open or short circuit in both directions). Without the device ground the current will find another way to ground, often through another unrelated device which can cause strange happenings! Also check all fuses as sometimes a device may be getting power through a different fuse when its one is blown. If the problem involves an exterior light (headlight, taillight, turn signal, brake light) check to see if any dual filament bulbs have their filaments shorted together (or a single filament bulb is where a dual one should be). This can cause back feeding the other circuit (eg. taillights on backfeeds brake circuit cancelling cruise control). Just removing the bulb will identify this situation (weirdness stops). If an aftermarket radio or alarm power was tapped into a not true power source, strange things will happen. A not true power source is one where you measure +12V but it’s because you are on the load side of something that’s turned off (ie. open circuit voltage). As soon as the device is turned on there won’t be +12V there anymore since it causes a voltage drop. Often using a light bulb in addition to a voltmeter is a good way to find true power sources if you don’t have wiring diagrams (light will be dim on not true sources).
FOR THE CASE OF AN UNKNOWN BATTERY DRAIN
, disconnect the negative battery cable terminal and connect the test light (or ammeter but read precautions above) between that cable terminal and the negative battery post.
[Note: Some aftermarket alarms have a surge current when battery is first connected so if using an ammeter instead of the light, connect a jumper between negative battery cable terminal and negative battery post first. Then connect ammeter in parallel to jumper and finally remove jumper. This will prevent any damaging surge from going through ammeter.]
Be sure the light is the only thing connected to the post - all other battery negative cables go on the other side of the light. The light is now the only thing connected to the battery in series with the rest of the car's electrical (don't try to start the engine with it in). The bulb will limit the current so you now don’t have to worry about any short melting the wires. Don't use an LED as this will glow even when you only have 30mA of current flowing which is normal for ECU and radio memories. LED’s also only conduct in one polarity so it’s misleading if it’s backwards.
[Note: If you are also going to be measuring voltages to ground you may want to use the positive post for the test light instead so you don’t "raise or float the ground" for the measurements (there will be a voltage drop across the light if it’s on, of course, to consider). Just be careful not to touch any metal when working with the positive battery terminal.].
THE LIGHT WILL GLOW BRIGHTLY if there is a full short. An open door will appear as a full short so it should either be closed, or it's door switches disconnected, or the fuses removed that operate items (eg. dome/foot lights) caused by an open door (if the test light goes out when removing these fuses, that is the circuit with the short).
THE LIGHT WILL GLOW DIMLY if either you have a partial short or normal devices are turned on (which you should turn off to then see how it glows).
THE LIGHT WILL NOT GLOW AT ALL if nothing draws any significant current (30-40 mA is an acceptable draw to power the ECU memory, radio memory, etc.).
THEN START DISCONNECTING, and removing fuses/relays, and switching off, one at a time (1) things that don’t work, (2) suspected things, and (3) everything on that circuit. When you disconnect, or switch off, the item that is causing the short, the light will go out (so it's a great detection device). Keep in mind the short may be in the wiring or fuse box which is harder to find, but first check all the easier, more obvious devices and places. If the light always stays on no matter what item you disconnect, you probably have a short in the wiring. In that case switch off all devices one at a time to see if light goes out. If it does, then the short is between the switch and the device. If you still can't find the short, try removing all fuses and relays in both the engine and inside fuse boxes. If the light goes out then start replacing the relays and fuses one at a time until the light goes on (identifying the circuit with the short). If the light is always on, you will need to get a circuit diagrams manual to start tracing and disconnecting wires and unplugging harnesses until the light goes out or, without the light connected, start measuring voltages and circuit continuities.
If you know the short is in a specific harness or wire somewhere but don't know where along that harness or wire, here is a technique that uses magnetism to locate it: http://www.inliners.org/tech/tech6.html
If you don't know how to use a multi-meter, here's a tutorial: Multi-meter usage primer