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intermittient short finding with a current probe
Posted to Technical Tips Forum on 5/21/2009 58 Replies

From time to time I get vehicle in that will intermittently blow a fuse. There are several different ways to approach this problem, and since there is no one way that is the best, here is one way to approach the task.

This vehicle was a 1996 Subaru Legacy that spends a lot of it's time on gravel roads that are sort of rough. The fuse #15 blows, which shuts off the heater, radio, dash lights, and a few other things. Checking a wiring diagram of this circuit found many different branches in the circuit. I used a fused jumper that was hooked in place of the fuse, my current probe was hooked to the jumper, and the current probe hooked to my scope. After studying the wiring diagram, I decided to start looking for the problem under the dash. I find that many times, the best way to look for problems like this, that are burried under things is to use my fist, and start pounding things. A few light whacks to the dash, around the instrument cluster found this happening on the scope [current probe finding an intermittient short] Now I know the general direction of the problem. Many times when you start on a project like this and you move a wiring harness, the short to ground is gone, and is real hard to find again. This one reason that I like to use this process. when I started to remove the instrument cluster, the fuse blew. Now it's on to step 2 of the process. I keep an old glass sealed beam headlight with jumper wires soldered to it for just such occasions. The headlight will limit the current to the circuit, and if the circuit is shorted, it will light the light bulb, so, with the headlight hooked in place of the fuse, [headlight bulb limiting the current] with my current probe on one of the leads, I started moving things, and in a few minutes I had the cluster out, and had found where a harness had rubbed on a steel brace that was inside of the dash. The only visible imperfection I could see on the harness was where the outer covering of the harness was a little but rough. Moving the covering around I found a very tiny spot rubbed in one wire which was the culprit. A short piece of heater hose and a few plastic tie wraps fixed the problem.

Albin from Washington

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