Here's a trick I learned as a tv repairman and used often as a mechanic. Remove the fuse and plug in a pair of spade terminals. Use two pieces of wire to connect those terminals to a 12 volt light bulb. When the short circuit appears, the bulb will be full brightness, and when you do something to remove the short, the bulb will go out, or it will dim if there is something else in the circuit such as a motor or another bulb. You may be able to make the short show up by wiggling wiring harnesses, turning parts, or banging on the body with a rubber hammer. This works especially well for screws for sill plates run through harnesses, wires crushed under the rear seat brackets, frayed and touching wires between door hinges, even corrosion between adjacent pins in a connector.
The bulb you use should be chosen for the circuit you're using it in. A brake light bulb (for a test light) will only allow about 1 amp of current to flow. If you're looking for a short in the brake light circuit, for example, when the short is gone, the test light and brake lights will split the 12 volts from the battery. That means the test light will be dim and so will the brake lights, but both can be observed to see when the short comes and goes. This saves constantly popping fuses.
In high-current circuits, those little bulbs aren't as effective because the normal circuit components draw so much current, the test light will normally be pretty bright. The slight change in brightness when the short appears is so subtle, it can be hard to see. This is where a larger bulb such as a head light bulb works best. When the short is gone, the circuit will get enough current to work somewhat. The fan motor will try to run but it will run slow. When the short appears, the bulb will be full brightness, the fan will stop, and current in the circuit will be limited to a safe 5 amps by the bulb.
There's a twist though to this trick. For it to work with your new fuse holder, the engine will need to be running, the air conditioning will have to be turned on, and the Engine Computer will have to be willing to turn the two relays on. And, the short could be in either the fan circuit or the compressor clutch circuit. An alternative to this is to put the fuse back in and use the test light in place of the relay contacts. This will power up the circuit without even needing to have the ignition switch turned on. Those same spade terminals can be used in the relay connectors. Each connector will have four wires. The two for the relays' coils will be small, typically 14 gauge or smaller. You want to connect to the two larger wires.
Doing this will allow you to power up one leg of the circuit at a time and the bulb will limit current so the fuse won't blow. The compressor clutch will most likely not engage because current will be too low to make a strong magnetic field, but the fan motor will probably try to run but it will be real slow. You will see the bulb go out when the fan motor or compressor clutch are unplugged if the circuit is working properly. When there is a short circuit, the bulb will stay bright even wheh those items are unplugged unless it IS one of those items that's shorted. You replaced the fan motor already so chances are it is ok. The compressor clutch could be shorted, and it could be intermittently shorted, but intermittent shorts occur more commonly from bare wires rubbing on bare metal.
Wednesday, April 28th, 2010 AT 2:16 AM