Ignition fuse keeps blowing at on position

  • 5.7L
  • V8
  • RWD
  • 150,000 MILES
Starter turns over. All interior lights work. No other electrical issues. But when key goes to on, fussed blows and started will not engage. I can find nothing online about this issue. Any suggestions are appreciated. I just do not know where to start!
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Tuesday, May 16th, 2017 AT 5:34 PM

1 Reply

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a twelve volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it is not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

Look for a wire harness that fell down onto hot exhaust parts, or one that is laying on a metal bracket and may have rubbed through. Move those harnesses around and see if the test light goes out. You may also be dealing with a circuit that only turns on for one second when you turn on the ignition switch, then not again until the engine is cranking. If that is the case, you either need to constantly crank the engine while doing the testing, which is not very practical, or look for a relay that is being turned on and off for this circuit. There are ways to bypass a relay to allow you to do the troubleshooting at your leisure.

The fuse you listed suggests this is a circuit that usually does not develop a short due to a failed part, other than a shorted oxygen sensor on rare occasions. Most commonly this circuit develops shorts due to bare wires. The fuel pump may be in this circuit too, but again, a grounded twelve volt feed wire to it is more common than a shorted fuel pump motor. On some GM models, that feed wire runs under the carpet, right under the driver's feet.
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Tuesday, May 16th, 2017 AT 6:20 PM

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