Locating wiring problems, especially shorted circuits, can pose a challenge. These tips should help make life a little easier.
"When our guys are looking for a short circuit, they put in a bigger fuse and look for the smoke." I actually had a newly graduated technical school student tell me that once, referring to the troubleshooting method used by some of the technicians he now was working with — and learning from. (Of course, I present this statement to you for your amusement and I certainly don't recommend or approve of this particular technique.)
However, chasing down wiring faults can be considered one of the tougher tasks most technicians face. Let's see what we what we can do to make this one challenge a bit easier.Open?
You have work order in hand and a "blower motor inoperative" (or similar) complaint to deal with. Any electrical component will not work properly, if at all, if current flow through that component is less than what it should be. The total resistance in the circuit affects the current flow, and the greatest source of resistance should be the component itself.Locating the open circuit is relatively easy. It starts by reviewing the wiring distribution diagram applicable to the side of the circuit (power or ground) you are troubleshooting. These diagrams show what components share power or ground sources, and can greatly reduce the amount of time you spend searching the car by first starting your search on paper.
Let's use an open ground as an example. Identify all the components that share the same ground point as your failed component. Notice that these components are wired in parallel to one another, with each individual ground meeting in a common splice. The final ground point typically is a hard point on the body or engine, but this is not the end of the ground path. Remember, though, it all starts and ends at the battery.If all the devices that meet at the splice are working, other than the one, then the fault has to be in the section of wiring unique to the failed component. If none of the components are working, then focus your efforts on that section of the wiring shared by them. Think of a tree with multiple branches. If one branch breaks (unique), the rest of the tree is still working. However, if the trunk is cut through (shared), all the branches will die.
Narrowing down the exact location is next, and still uses the voltage drop method. If the open is on the power side, check the fuse first by measuring voltage on each side. I've had a few that were blown electrically, but not visually, so don't depend on a look alone to tell.If the fuse is OK, then work your way back to the battery from the failed part at the most convenient test points using your wiring diagram as a guide. As soon as you see system voltage again, you know you've isolated the problem between that test point and your last. Simply narrow it down from there.
Locating a ground side fault is done the same way, but the numbers you're looking for are different. You'll measure system voltage all the way to the point you pass the break. There, voltage should read a perfect 0.0 volt.
Short?
You found the blower motor fuse blown and replaced it. But what caused the fuse to fail in the first place? The fuse is a circuit protection device, designed to fail before the excessive current flow can damage the wiring harness. A fuse will blow only when the power feed is in contact with an alternate ground path before the load of the circuit it protects. Remember, the load is what determines current flow, and if current can find a way back to the battery before passing through it, current flow is limitless (more in the January 2009 issue of Motor Age).Any circuit protection device, whether it is a fuse, circuit breaker or fusible link, is a thermal device. That is, it fails due to the heat created by the amount of current passing through it. High current flow means high heat, and the fuse will melt down if that heat load is consistent and exceeds the fuse's rating.
Locating a short that is constant is similar to locating an open in the circuit path. The first step is the same: using the Power Distribution schematic to isolate what circuits the blown fuse feeds. The fuse is wired in parallel to all of these components, and provides power to all of them. The short could be located in any of these paths, before the individual loads.To narrow down your search, take the time to make some observations. Does the fuse blow with the ignition off or on? Does the fuse blow only when a specific component is operated? Is that component controlled by a relay?
Pinpointing the actual location might take some effort, because power branches can and do run from one end of the car to the other. Continually replacing fuses is time consuming and an unnecessary cost. One alternative is to wire a headlight (or other bulb) in place of the blown fuse. This applies a load to the circuit before the short and protects the wiring from overload. The shorted section of wire now acts as the ground path to the new load (the light). The light will illuminate as long as the short exists and provide a visual indicator that will aid in your troubleshooting.
I strongly recommend that you DO NOT use a higher rated fuse or circuit breaker to locate shorts, as you might inadvertently damage the wiring harness or related components, adding to the cost of the repair.
The headlight trick also provides current flow you can follow with a tool designed for the purpose: the low amp clamp. First, measure the current flow through the light you are using and note this reading. Now locate the first harness splice point indicated in the Power Distribution schematic (there might be more than one). Place your amp clamp around half of the wires downstream (the load side) of the splice. If you see the same current flow, you are on the right track.
Split the wiring in half again, continuing to look for the current flow until you've narrowed it down to one. You can then follow that one wire to its load, using the amp clamp as you go. When the current flow drops, you've passed the location of the short.
The components that share the fuse can be shorted internally. Disconnect them one at a time while watching the headlight. If the light goes out, suspect the component you last unplugged. If the light stays on, leave the loads disconnected to prevent them from inadvertently affecting your testing.Last scenario: What if the replacement fuse doesn't blow, at least right away? Sometimes fuses fail with age and vibration, but more likely it died from an intermittent or momentary problem. Harnesses at rest can be just fine, but under certain conditions they can be stretched into contact with a body or engine component that acts as the ground.
Motors draw more current to get moving than they do with the motor running, and these start/stop cycles can cause current spikes that eventually lead to fuse failure. Other components, like the compressor clutch coil, can increase in current draw as they heat up. All are possible causes of intermittent fuse failures. Remember, they fail because of heat applied over time and will often withstand current peaks well above their rating for short periods.
Last Thoughts
You still can use the headlight as an aid in locating intermittent shorts. Some techs have told me that they even add a horn or buzzer as an audible alert. But these techniques might miss that intermittent, while your DSO or graphing multimeter coupled with a low amp probe won't. Look for harnesses that are routed incorrectly or stretched where they shouldn't be. Wiggle the harness as you go while watching your meter for current spikes that will help you locate these momentary faults.
Cycle motor circuits on and off to see what peak current draw is. A rule of thumb is no more than two to three times their normal running current. Let components run, like the compressor clutch coil, until heated while monitoring their current flow to make sure it's not excessive.
No matter which method you choose, the key to locating wiring faults is to do your initial search by ripping into the circuit on paper before ripping into the harness on the car.
