Spade fuses would have been a modification that a previous owner would have done.
A relay is an electrically controlled switch that uses a small amount of current to control a larger amount of current.
Fuses are installed to protect the wiring from the heat that would be created by higher amounts of current flow than the wiring is capable of safely carrying.
Adding additional circuits to existing circuits will increase the amount of current flowing through the fuse for that circuit. A small increase, such as the amount of current needed to operate the control coil of a relay, is usually not an issue. Adding a significant current load to the circuit can cause issues.
In general, it is a bad idea to replace a fuse with another fuse of a higher current rating, as the fuse protects the wiring, not the components of the circuit. A reasonable exception to the rule would be to replace the 15A headlamp fuse with a 20A fuse, as the KLR headlight switch has a tendency to momentarily burn both filiments in the bulb during switching from low to high / high to low which can cause the 15A fuse to fail on a regular basis.
A "short" is a term used to describe an electrical circuit fault where the path of current flow by-passes the electrical load (component) with the result being an over-current condition within the circuit resulting in a failed fuse and/or melted wiring.
Adding excessive current demand to an existing circuit can, and often does, result in fuse failure due to the higher amount of current, a 'fault' has been created, but it is not a "short".
Either your wiring modification has created a "short" - bare wiring contacting ground or inadvertantly by-passing a load, or your modification has increased the demand placed on the circuit for what that circuit's fuse (and wiring) was designed to carry.
As an example, a bulb rated for 35W @ 12V would have a current demand of 2.9Amps at 12 volts. Two bulbs connected in parallel (an example like your turn-signal / running light modification, although the rating of your turn signal bulbs is not what I am arbitrarily using in my example here) would then increase the current load to 5.8A at 12 Volts. Increase the voltage with the engine running to say, 13.6Volts, and your circuit draw will increase to 6.57A.
Pretty safe to say that a circuit protected by a 15A fuse, that most likely protects a circuit that normally flows 7 to 8 Amps is going to have an issue when you add another nearly 7 amps to the mix. It is also necessary to understand that with bulbs, the initial amount of current that flows the instant before the bulb filiments begin to 'burn' will be a significantly higher amount of current flow than what flows once the resistive effect of hot filiments takes over and reduces the current flow to normal operating levels.
It's your motor, and your money. I would not discourage you from learning how, and why electrical circuits behave the way that they do. However, if you want to learn those lessons empirically on your own vehicle, the most important piece of knowege I could pass on to you is that: electricity flows faster that you can say "Oh Shit!".
"If you simply take up the attitude of defending a mistake, there will be no hope of improvement."
(not my site)
Oh, and I ran two quarts of hot oil through before filling her up cause I'm anal.
Last edited by Some Mook; 04-24-2011 at 07:58 AM.