Here's my standard reply to my students and why I never let them do this on cars we worked on. "You don't know why that wire failed, and you need to". In the case of a wire shorted to ground, you have to cut the wire off at both ends, so you'll know where that wire goes anyway. With an open wire, running a new one will get the job done, and your customer might think you're a hero, for now, but you don't know where that wire was open or what caused it. Almost all wires are in a harness next to a lot of other wires, and how long will it be before the same thing happens to another one? You may have a harness that has a broken mounting clip and it fell down onto hot exhaust parts. The insulation melted on the oxygen sensor's signal wire. Cutting that wire off and running a new one would stop the diagnostic fault code from setting, but it's just a matter of time before the same thing happens to the heater wire for that sensor. Oxygen sensor heater circuits are often tied in with the same circuits that feed injectors, ignition coils, and fuel pumps. Shorting that wire to ground will blow the fuse for the fuel injection system and it will usually do it intermittently. If the wife's car stalls on a dark, deserted country road, ... Late at night, ... With annoying kids, ... In a blinding snow storm, ... With howling wolves nearby, ... A serial killer on the loose, ... And a dead cell phone, ... Once the search party finds them and the car is diagnosed at another shop and they find that melted harness you overlooked, you can be pretty sure you're going to get a visit from her husband, and it won't be to bring you a bag of cookies, (chocolate chip)! We get accused of trying to sell unneeded parts and services, and we get blamed when we don't do enough to take care of everything that is going to go wrong in the future that we should have psychically known about. There's no need to make that worse by neglecting to determine why something failed so we can prevent it from happening again.
The point is you want to know what caused the problem so you can prevent additional future problems. A coworker ran into multiple failures of a fuse link wire for the radiator fan over the course of a few weeks, even after replacing the fan motor, the obvious cause. On the fourth or fifth visit I noticed when he went to back out, he had no backup lights. Immediate inspection showed his new fuse link was already burned open just from backing up a few feet. Thanks to my superior intelligence, (in this one tiny area, and his lack of understanding electrical), we determined the short occurred when the engine rocked. The wiring harness on the body, running under the battery tray was sliding back and forth each time the engine shifted position. The wire to the radiator fan was bare and the paint on the body was rubbed off, and there was the intermittent short. The backup lights were put on the same circuit because most people will eventually figure out they have no backup lights, but they won't always notice the loss of the radiator fan.
About half a dozen other wires were also close to being bare, but the funny thing was one other wire had already been repaired. That previous mechanic was in the perfect position to do something to prevent future problems but didn't have the presence of mind to do that. Look at all the trouble he caused for the owner and the next mechanic.
Other common things to look for are a harness draped over a sharp metal bracket, a mispositioned harness that is getting rubbed on by something else like a throttle cable, kick-down cable, or hood hinge, and corrosion in an exposed terminal like for your head light, or in a connector.
An even better example is one I'm going to have to deal with a second time on my old rusty trusty '88 Grand Caravan. That's frayed wires between the door hinges. Due to all the power stuff, there's 22 wires in there, and right now all that doesn't work is the right power mirror, but you know all the other wires are just as old and have flexed just as much. I replace all of them when I do this job, and again, thanks to my superior brain, I splice in wires twice as long as necessary and pack half of them into the "A" pillar. This time all I have to do is pull that wad out and my repair is half done. Ahh, it's hard to be humble.
Sunday, October 19th, 2014 AT 9:32 PM