Fun with Fuse 15
As an example of an intermittent short circuit problem, I will use a case study from a 1997 Subaru Legacy/Outback. This Subaru has been ridden hard and put away wet many times. The vehicle lives on top of a mountain and is driven two miles each way on washboard road every time it comes to town. The odometer is showing 208,000 miles and still has the original automatic transmission. The customer concern is that intermittently the instrument cluster lights will go out, the high beam headlights will go out, numerous instrument cluster lights will come on and if driven long enough, the engine will finally stall.
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Looking into the driver’s compartment, the floor is littered with several blue 15-amp fuses. There is also a good supply of fuses in the ashtray, too. This vehicle has been driven for 25 miles without the fuse blowing. Now since driving it until it blows the fuse will bring us no closer to knowing when and why this problem occurs, the best thing to do is to first learn how the circuit works and then what the circuit powers.
There are several diagnostic paths that can be taken on a problem like this. Some things I have seen used by techs to troubleshoot a shorted circuit include installing a higher amperage fuse to power the circuit or installing a circuit breaker in place of the fuse so the circuit breaker will still protect the circuit. This will close and supply power to the circuit and after all, this is an intermittent problem and we all know intermittent problems are hard, if not impossible to find. I do not like any of these diagnostic approaches so let’s think out of the box a little and use a solid diagnostic approach to the problem.
Any time I work on intermittent electrical problems, whether they be short to ground problems or intermittent open circuit problems, I always am afraid I am going to move a wire or wire harness and the problem will go away, never to resurface no matter what I do. This always leaves an unfixed car leaving the shop and wasted labor that is unbillable.
Starting with a logical diagnostic process for this concern begins by examining a power distribution wiring diagram for fuse 15 (the location of the intermittent fuse failure), and it uncovers a lot of useful information. Some things I pick up on right away include learning that fuse 15 powers only small electrical loads like some relays and the instrument cluster, the fuse only has system voltage when the key is in the start or run position and everything fuse 15 powers is located under the dash. This gives me a good direction where to look for the intermittent short to ground. Keep in mind, I want this problem to come to me. I do not want to take this whole dash apart and start opening up wiring harnesses to find the problem.
To accomplish this task, the system needs to be tested dynamically, which means to be able to make the short to ground happen in the shop where I can easily capture some data and pinpoint the location of the problem without having to take the car apart. The first thing is to install a fuse loop in place of fuse 15. This allows a low amp current probe and lab scope to record any current flow on the circuit.