Since fuse 15 will blow when the car is being driven on rough roads, the way to get this problem to happen will be to cause some vibration to the dash. With all loads on the circuit turned off and the ignition key turned on, I started using my fist to bump the top of the dash. Within a few seconds, the labscope started recording amperage spikes as seen in Figure 8. At this point, I know the problem lies inside of the dash, but I’m not sure exactly where.
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The dash thumping was increased in intensity and the location was narrowed down to a place between the right end of the instrument cluster and the radio. Figure 9 shows the intensity of the amperage spikes increased as the vibration of the dash got closer to the place where the harness was shorting to ground. At this point, the 15 amp fuse burned and it is time to pull the instrument cluster to gain access to the wiring harness that lies behind the dash.
With the instrument cluster removed, the wiring in the dash can be seen easily. There is only one large wiring harness that runs across the dash and the dash is all made from plastic. A headlight bulb was installed in place of fuse 15, the current probe was clipped around one of the headlight wires and the harness was moved very gently. A few seconds of gently moving the harness made the headlight flash and the waveform in Figure 10 was recorded. At this point, I can make the electrical short happen and with the headlight limiting the current flow in the circuit, there is no possibility of doing harm to the harness, or setting the vehicle on fire.
Back in the dark dungeon in the interior of the dash is one small steel brace for the radio. This wiring harness had been rubbing on that one little black steel brace for 200,000 miles and finally had rubbed a very small hole in the insulation. When examining the harness, the hole in the insulation was about the size of a straight pin head. There was just one little tiny copper speck showing through the insulation, which was on the back side of the harness and I had to use a mirror to see the problem.