You already know the circuits that are involved. They're the items that don't work. Knowing the wire colors isn't going to help right now. Most people who understand basic electrical theory think using an ohm meter at the fuse box is the place to start, but all that will do is show 0 ohms to ground. You already know the resistance on that circuit is too low because the fuse pops.
People who know slightly less about electronics but enough to make some progress will keep putting in new fuses after trying something. At some point you have to ask yourself, how many fuses are you willing to blow before you realize there must be a better way. I've used the light bulb trick many times on cars I was not familiar with, didn't know which wire was which, and didn't even have a service manual. I think this will work for you too if we approach this logically. Electrical was one of my specialties when I worked for a very nice Chrysler dealership, but we didn't have service literature for all the trade-in cars, and usually didn't need them.
Even if you knew the wire colors, you still have to find plugs to disconnect to isolate the short from the fuse. Then you unplug the connector and install a new fuse. Pop! Find another plug to disconnect, install a new fuse. Pop! Now, ... If you're smart enough to read a wiring diagram, (most people can't), you are also smart enough to put an ohm meter between the fuse holder and ground. I might be wrong, but I get the impression that's what you want to do, then try things to make the short go away. That's the logical plan of attack but the one big disadvantage is each time you change something, you have to run back and look at the meter.
The advantage of using a light bulb first of all is the ease of seeing the change in brightness from where ever you're working on the car. The less obvious benefit is you can do things you can't with a meter such as bang on things with a rubber hammer. You can bounce the car, sit on the seat, etc. This works best for those intermittent shorts that pop a fuse when you drive over a pothole. With a direct short like you have, you might slam a door, push on a wiring harness, or remove a screw, and you'll see the test light flicker. AHA! A clue that would never show up with an ohm meter.
It's possible for a wire to rub through on a sharp metal bracket or from sliding on the inner fender from the normal rocking of the engine, but those are not the most common types of failures, at least for a dead short. Are you aware that you may have already eliminated a big percentage of the circuitry? I'm going on the assumption that all the inoperative items are on the same circuit. If I'm wrong, you have at least two different problems, but let's handle just one of them first.
Take the map light for example. If the fuse still blows if the map light switch is turned off, the short is not after the switch. If the fuse blows, or the test light is bright, only when the map light switched is turned on, you know the short is after the switch. I don't know what is meant by "signal repeater". If I had to guess, I would say it has something to do with a trunk-mounted mobile phone. That unit could have an internal short. Unplug it. If the test light goes out, you found the problem.
Was the fuse blowing before you replaced the right headlamp or did the fuse blow only after replacing the headlight? If it started blowing after, the headlight is the logical place to look. I can think of a bunch of things you might find, and I can share stories that will help illustrate the point.
Ok, that's the "short" version of my story. If you understand why this should work for you, there is no need to bore yourself by reading further except for the entertainment value. If this method still isn't clear, these examples that happened to me might help it make sense. Between this and my other career as a tv repairman, I loved collecting and using service manuals for the wiring diagrams, but except for the last problem, I didn't need the wiring diagrams to find the problem. I used the service manual for the last car only because I couldn't figure out what else was on the shorted circuit.
So, here's "the rest of the story"!
Car stalled when driving over large bumps or potholes. Found the fuel pump fuse blown, but in the shop, the pump always worked fine. With the test light in place of the fuse, the pump would run slow and the bulb was dim. Used a rubber hammer to simulate bumps, and found that banging on the left rear sill plate would make the light flicker brighter for just a fraction of a second. Sitting on the rear seat also caused the light to go bright. Found a sheet metal screw for the left rear sill plate run through the wiring harness going to the trunk. When the rear seat frame moved, it pulled the bare wire against the screw.
Tail light fuse blows as soon as the headlights are turned on. The problem has to be after the switch, but it could be related to any of the four running lights, the radio, (that circuit tells the display to dim at night), or the dash lights. The van had minor damage to the left front fender so logically, that's where I started. Unplugged the harness and found no change. Eventually ended up at the left rear tail light assembly. As soon as I removed the screws to inspect the wires, the test light went dim and the other lights on the circuit came on dim. The owner had it apart and crushed a mispositioned wire under the edge of the lens. If he had told us he had just connected wires in there for a trailer hitch harness, that's the first place I would have looked, and it would have saved us time and him dollars.
Minivan came in overheating due to radiator fan not running. Coworker found a burned fuse wire for the fan motor. Logic says intermittently shorted fan motor so he replaced it and the fuse wire. A week later it's back for the same problem. New fuse wire, which takes about 20 minutes to repair, and it worked again. When it came back for the third time, as one of the courtesies we did for each other, I mentioned his vehicle had no backup lights when he drove out of the shop. When he fixed it the fourth time, it did have backup lights, ... Until he moved the van. Luckily I happened to be watching when it happened. Found a wire in the harness under the battery tray sliding back and forth as the engine rocked that was bare, and the paint was rubbed off the body. When the van was shifted into reverse, the harness moved and that wire shorted. It was for the backup lights, but the fan was on the same circuit. They do things like that for a reason. It's a safety circuit for other drivers, but how likely are you to notice when they're not working? You WILL notice the overheating.
My most recent fix was on a friend's son's truck. On the way home from college, the right signals began not flashing whenever the headlights were on, and the radio display dimmed when the right signal was turned on. Unplugged connectors until we isolated the short to the right front bulb socket. Had to look twice before we found the button of solder from one of the bulb terminals had cracked off and got stuck between the two contacts for the signal and the running light. The socket was full of grease so we didn't see the short right away. This problem happened all by itself while he was driving. As soon as we pulled that bulb terminal from between the contacts in the socket, the test light went out.
Here's my favorite of all time. No dome lights and fuse blown. Service advisor finds and replaces the fuse and sees the fuse blow again when slamming the left rear door. Must be a bad switch. He installs the new one himself; customer is happy. Next day he's back with the same problem so now I get it. After lots of frustration, I put the test light in place of the fuse and found it would get bright when hitting the brakes, but it would STAY bright when I released the brakes. The light would dim when hitting the gas. This one I did have to resort to the service manual. After lots of testing and inspecting, I finally found a wire with the insulation hardened and cracked off where it came through the arm for the sunvisor. There was just enough play in the arm for it to move forward 1/4" when hitting the brakes, (shorted), and it would move back when hitting the gas, (short gone). As soon as I touched the visor, the test light flickered on and off. Problem found.
This is an entirely different approach than most mechanics were taught. It is unrealistic to think it will work for every short circuit, but it will work for the majority of dead shorts, and it is the most effective way to find intermittent shorts.
Saturday, March 27th, 2010 AT 1:09 AM