A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.
Common things to look for are a chewed up trailer harness, crushed wires in the trunk, and a mis-wired aftermarket radio.
March, 21, 2013 AT 6:59 AM
Thanks for the quick response. Could I use my voltmeter instead of a light bulb? Would I read 12V across the fuse terminals when the short is present?
March, 21, 2013 AT 8:12 AM
Nope. Won't work like you would think. Current flows through an amp meter or a light bulb. The two problems with an amp meter are it has no resistance so there's nothing to limit current flow to a safe level, and it will show a real high current which is meaningless because you already know there's way too much current. A light bulb has resistance that will limit the current to a safe level, about one amp with a brake light bulb, and about five amps with a head light bulb, and the brightness will give an indication of when the short is gone.
A voltmeter ideally has infinite resistance and no current will flow through it. That's the same as the blown or removed fuse. The voltmeter works like the pressure gauge on a compressed air line or air compressor. No air flows THROUGH the gauge for it to do its thing.
What you are probably thinking is there's 12 volts on one fuse terminal and the short is creating a ground on the other side so the voltmeter will read 12 volts. That is correct but you don't need a dead short for that to occur. If you understand electrical theory you know that if there is no current flow, there is no voltage dropped across any resistances in the circuit. Since we're working in the tail light circuit, imagine if there was no short and the circuit is working properly. If you stick a voltmeter in place of the fuse and turn the switch on, no current will flow through the voltmeter so the lights will be off. 12 volts will be on the hot fuse terminal and the total resistance of all the light bulbs in parallel will be only a couple of ohms. For all practical purposes that terminal is ground. That means the voltmeter will read 12 volts in a good circuit, and 12 volts if a short bypasses those couple of ohms. The only way the voltage could go to 0 volts is if there was nothing but an open circuit. That means no short AND no light bulbs plugged in. It is not practical to remove all the tail light bulbs, front running light bulbs, license lamp, and all the dash light bulbs. And you'd still have the radio in the circuit. You don't have that kind of time to troubleshoot that way.
Even in higher resistance circuits like for radios, it's resistance will be WAY lower than the resistance of the meter so thanks to the couple of micro-amps that really will flow through the meter, all the voltage will still be dropped across the higher resistance which is the meter by a long shot. You'll see 12 volts even though there's no defect in the circuit.
What you could do is find the fuse terminal that has the 12 volts on it, then go to the other one and measure from it to ground with an ohm meter. The problem with this is you'll be looking for the change between when the short is present in the circuit and when it's gone. Suppose there's 8 tail light and running light bulbs and each one has 12 ohms of resistance. The total resistance of all of them in parallel will be 1.5 ohms. Add in lets say another 1.5 ohms of car wiring for a total of 3 ohms. Digital volt / ohm meter leads can easily have 3 ohms themselves so you'll read a total of six ohms. Now bypass the light bulbs with a short circuit. Usually that doesn't occur in protected areas near the fuse box. Shorts are more likely to occur way down near a light bulb so you still have that 1.5 ohms of wiring resistance. The difference between when the short is present and when it's gone will be the difference between 4.5 ohms and 6 ohms. Your meter display will bounce around and change that much on its own so the readings will be of no help.
A bigger drawback to the ohm meter is you have to run to the back of the car, unplug something, run back to look at the meter, then start all over again. You'll wear out your shoes and still never find the short. I made a test light harness out of a Dodge Viper tail light harness that has three bulb sockets with a total of five filaments. I tied them all together then attached the two wires to a blown fuse so I could plug it in quickly. The real advantage was I could stick in as many bulbs as necessary to get the desired current flow for the circuit I was working on, but mainly I could hang it over a rear-view mirror. No matter where I was working I could wiggle a wire, unplug a connector, or bang on the car with a rubber hammer and see instantly when the short was affected by the brightness of the bulbs.
In one case someone had removed a rear seat years earlier and when they reinstalled it the wire harness wasn't clipped in place and got crushed under part of the frame. That one didn't act up until after passengers were sitting in the rear for a while.
On another one a fellow installed mud flaps and a screw went through the wiring harness. Found that with the rubber hammer. The guy said the tail light fuse would blow when he hit a big bump in the road. That short was NOT present most of the time which made it impossible to diagnose while standing still in the shop. After installing my test light I walked around banging on the car with the hammer. When I hit the mud flap the test light flickered brighter for an instant telling me I was in the right area.
On another one a guy installed a trailer harness on a Dodge van. When he bolted the tail lights back in he caught a wire under one of the screws. That was for the backup lights. With the ignition switch on and the transmission in reverse, the backup lights were off and my test light was full brightness. After working around for a while when I started to unbolt that light housing I saw the backup light start to glow dimly, then noticed my test light had gotten dim. Since the 12 volts was being dropped across multiple bulbs, each one had roughly 6 volts; that's why they were dim. Replaced my test light with a fuse and the backup lights were full brightness again.
A few people incorrectly think that test light is going to explode or at least burn out because of the short but that is not the case. You simply have full battery 12 volts on one side, like normal, and thanks to the short you have ground on the other side, like normal, ... And the bulb will be full, normal brightness. It's no different than connecting a bulb right across the battery terminals. Since the size of the bulb determines how much current it will allow to flow, you don't have to worry about overheating any wiring.
March, 22, 2013 AT 12:54 AM
WOW ! Thank you so much for your in-depth answer. Even though I work all day repairing electric circuit boards down to the micro chip level, I still get overwhelmed by auto electronics. (The wires are so big! ) Anyway here's what I did today. My son brought the Escort home after work and he said he had gone through about 5 fuses. Well woudln't you know it when I put one in, the dash lights and the tail lights came on and stayed on even after banging and tugging and twisting and jiggling and unplugging etc. But here is what has been happening. The audio signal from the ignition cylinder has been staying on intermittently even after the key has been removed. I could turn the steering wheel slightly and it would go out for a second then come back on. So I took off the cover for the steering column and started poking around and found a cut-off switch that is mounted right under the cylinder. When I jiggled the wires, the signal would shut off. Removed the switch and problem solved(so long as he remembers to not leave his keys in the ignition before locking the door). But is it possible that the switch could have been causing the dash light fuse to fail? Or was it just dumb luck that I might have jiggled enough to keep the circuit from shorting? Either way I now have a handy dandy little troubleshooting light that I can plug directly into the fuse box if the problem arises again. Thanks again for the advice. Now I will make a donation to the website. THANKS
March, 22, 2013 AT 3:54 AM
You're a wonderful human being! I don't want to spoil the fun but I suspect the two things are not related. I also don't have a diagram to look at but typically they will not run a 12 volt wire up the column, then switch on another wire with 12 volts to run the chime. They normally feed 12 volts to the chime module, the turn it on by grounding a single wire running to the switch.
What kind of circuit boards do you work on? I did tv / vcr repair for over 35 years while also working full time as a suspension and alignment specialist for ten years at a very nice Chrysler dealership. I've worked on a real lot of car radios for two dozen dealers in my state.
You might consider replacing the fuse with an auto-resetting circuit breaker until you find the cause of the problem. The disadvantage is they don't trip as fast as a fuse blows because they have to heat up first, but they should still protect the wiring. 6 and 30 amp breakers are easy to find in any salvage yard. They make 20 amp breakers too but they're harder to find. The advantage is the lights will come back on faster than if you stop and fiddle around replacing the fuse.