Don’t Blow Your Fuse Finding Short Circuits

Tracking down the offending circuit, whether a fuse blows on a constant or intermittent basis, is never easy. With the proliferation of low current automotive electronic devises today, fuses typically feed multiple circuits. Manually separating harness connection points or cutting into harness splice packs one by one in order to install your Ammeter and find the offending leg of a circuit can turn into a game of blind ditch digging in a hurry.

What is needed is a better way to “see” the offending circuit while using a map to enable us to dig a minimum of holes. A low current probe combined with a graphing meter or oscilloscope and a schematic provides just such tools.

First we must understand some facts. Contrary to what is popularly believed fuses do not simply blow “instantaneously” once the current draw of the circuit reaches an amp or so above the rating stamped on the fuse. Fuses are thermal devises meaning they require heat to melt the fuse element. That required amount of heat energy is created by excessive current flowing for a period of time. This means that a fuse will survive current flow, in excess of its rating, for a longer period with a lower amount of excess current. At higher current draws the fuse will last less time before it blows open. Bottom line, the amount of time the fuse takes to blow can vary.

Why is this fact relevant to our excavating adventures? Because many times an excessive current draw or short circuit draw will happen multiple times before the required amount of amperage and time has had the chance to create enough thermal energy to actually melt the fuse or trip the circuit breaker on our favor short finder tool. This means that we can have many chances to catch and track down the offending current draw long before the energy level exceeds the point at which the heat generated opens the fuse. Look at the multiple excessive current draw events traced by current probe and graphing meter shown in Figure 1.

The 30 amp fuse that fed this main relay circuit was exceeded by 23 amps nine times in just 30 seconds. Yet the fuse never blew! That is nine more chances I had to locate the short compared to someone who was simply waiting for a fuse to blow or a circuit breaker to trip.

Using the low amps current probe and scope or graphing meter graph the circuit output over while operating circuits or doing a wiggle test. Start by pulling the schematic, or map, of all the circuits that the offending fuse protects. Then clamp your current probe around the feed circuit to the offending fuse. Then one by one operate each devise on that circuit until you see the current draw exceed the fuse rating

If the current draw exceeds the fuse rating as seen on the graph it is time to chase down the circuit regardless if the fuse blew or not. How exactly do we chase it down or what if the excessive current draw is not relative to a devise being operated and the draw occurs simply by turning on the key? No problem. Find a splice pack(s) and simply wrap your current probe around half the wires branching off of the splice, then retest the circuit. If the first bunch of wires displays the excessive draw then further divide the bunch of wires by half until you have narrowed down the offending wire. If the first bunch does not display the excessive draw move your current probe to the second bunch and again keep dividing till you find the highest current draw wire. Once you find the offending wire recheck your schematic to see if that splice pack feeds yet another splice pack. The interior lights circuit on my plain-Jane 1998 GMC Safari van has no less than 3 splice packs totaling 16 connections! Once you have singled out the offending wire cut it from the splice pack and retest your remaining circuits for excessive draws.

I say cut the wire of the offending circuit because many splice packs typically do not label the branches of the splice by circuit number. Cutting the feed to the circuit is a way of identifying the circuit in question. If the feed to the offending circuit is dead the devise it feeds will also be dead, thereby identifying the defective circuit. Next we need to see if the devise or the harness to it is causing the excessive current draw. To determine this disconnect the power feed harness wire from the devise connector. Then run your own fused jumper wire from battery power to the devise and recheck the current draw again using your current probe. If the draw is excessive you have a defective devise. If the draw is not defective you must now test the devise feed harness for a short. Do this by removing the battery feed fused jumper wire from the devise and attach it to the power feed wire previously removed from the devise connector and recheck your current draw. Now that you have located the offending wire it is time to dig just that one hole in order to find that harness short.