Corner Wrench: Blown a fuse? Don’t blow a gasket!

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Many DIY home techs are pretty comfortable with most repairs on their vehicles — that is, until it comes to electrical. Despite the relative simplicity of classic systems, wiring and fuse problems seem to be a no-go zone for many driveway. Fortunately, classics aren’t really that hard to diagnose with some patience and a good tracking process. So when a fuse blows, don’t blow your stack — fetch an electrical multi-meter tester unit and get busy.

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Tracing faulty components

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Corner Wrench: Blown a fuse? Don’t blow a gasket! Back to video

Avoid the use of sharp-prod type test lights. Every hole you put in a wire’s protective coating is an entry point for corrosion. Instead, consider a tester with an inductive pickup clamp that can give you the readings you need without damaging the wire. When dealing with a blown fuse on a circuit that immediately blows the replacement, determine which circuits and components are protected by that single fuse. Unplug all of the components on that circuit and if a new fuse blows immediately or plugging the component back in and turning it on snaps it, you’re likely dealing with a wiring problem. If the fuse doesn’t blow until the component is in use for a few moments, you might be dealing with its internal fault. Testing the component for excessive resistance at its connector should confirm this.

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Test lights

If you’re dealing with a possible wiring short, try using a test light (no wire probing needed) with one of its clips hooked onto the positive battery post and the other on the load side of the fuse. If the wiring is good and all the components on the fuse have been disconnected, there should be no light. The stronger the light glows the bigger the short is. Tracing a short might be daunting, but it’s not impossible with the help of a credible wiring diagram.

Visual check

Run a visual check first, looking for any breaks or obvious damage. Check common areas such as where harnesses travel through bulkheads or are exposed to the elements, (such as under the hood or floor). Anywhere a harness is required to move, such as at a door hinge post or running from the body to the engine can also be a popular spot for internal fraying or damage. If nothing obvious shows up, then you’re in for the divide and conquer routine. Divide the circuit in half, take your readings with probes at each end of the section, and when you find the short, keep dividing the circuit until you narrow it down to the problem point.

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Resistance

When testing components, you’ll need to know their normal resistance values. They can vary wildly. For example a fuel pump might have 20-30 ohms resistance when good, while solenoids might only measure a few ohms. Again, this is where a manual will come in handy. How much difference can a heavy resistance value mean on a defective component? Blower motors can fail to the point they not only quit spinning but can melt the connector on the resistor unit. If you’ve ever had to replace a blower resistor twice in short period of time, measure the motor’s resistance before plugging things back in.