— written and illustrated by Ralph Birnbaum
Fuse-popping shorts in electrical circuits can be hard to locate. Younger techs have been known to incinerate a couple dozen fuses looking for a voltage short to ground, while the truly fearless among us simply install a fat jumper wire in place of the fuse, and follow the smoke from burning insulation. Neither approach will garner the "top troubleshooter" award.
Here's a fasty and easy way to test those shorts safely. The procedure is rock simple and fiendishly effective:
1. Remove the burned fuse.
2. Grab a single filament sealed beam and attach a couple of extension leads to the bulb terminals. Install wire ends the same size as the fuse blades at the other end of the test leads.
3. Plug the sealed beam leads into the fuse terminals in place of a new fuse.
4. Turn the circuit on and start looking for your short.
Our circuit test device uses a sealed beam, a couple of old coiled meter leads from the junk drawer, and an inline fuse holder (just to play it safe). One pair of wire ends fit snugly on the sealed beam terminals; those on the opposite end slide easily but snugly into the vehicle fuse receptacles.
A length of heat shrinkable tubing at one terminal prevents a short circuit at the sealed beam electrical connections.
If the short in the circuit is still present when you plug in the sealed beam, it will illuminate if the source side of the fuse is connected to voltage. (We'll illustrate that condition in a moment.) Some source contacts are hot at all times; others, only with the ignition switched on.
If the sealed beam lights, it tells us that there is a short to ground on the feed side of the circuit in the wire between the fuse and the normal load.
With the headlight plugged into the fuse holder, a dead short illuminates the headlight. One terminal at the fuse box supplies voltage, while the short provides ground.
We've connected a low amp probe to the circuit, and the DMM display tells us that the headlamp draw is approximately 5 amps (the bulb is rated at 65 watts). (Our amp probe is set to display 10 millivolts per amp, so we divide the milliamp reading on the DMM (50.8) by 10 to get 5.08 amps.)
Why This Works
Let's look at an automotive bulb circuit, in good working order. We've eliminated the light switch normally included in such a circuit to keep things simple. The battery feeds the load, and it lights.
Now let's look at the same circuit, only this time with a voltage short to ground.
The normal load (in this case a bulb) is off, and the fuse is blown. We could install another fuse, but it would blow immediately. Unless you are faster than a speeding bullet, that is far less time than it will take to find that short!
One of those circuit breakers that comes with a short-finder test kit isn't a lot better than a new fuse under these circumstances, since the circuit will toggle on and off as the circuit breaker opens and closes. What we need is some way to keep the circuit live while we test. That way, we'll get immediate feedback when we eliminate the short.
To keep our circuit turned on, let's wire a sealed beam into the circuit in place of the fuse.
The normally hot feed wire between the fuse and the small bulb has become a ground wire, due to the short. We've even colored it black instead of red.
With one terminal connected to source voltage and the other to ground through the short, the sealed beam lights up. This is a big diagnostic plus, since the circuit is live, and stays live as we start tracking the short by wiggling wires and wire harnesses. We'll know we've isolated or eliminated the short when the load starts working again.
We've shown a small bulb here as a load, but it could just as easily be a wiper motor or any other electrical load.
We are putting the short to work, helping us locate the problem. The sealed beam lights because of the short. As soon as we eliminate the short, the small bulb illuminates and the sealed beam will dim or even go out.
Using the Tester
To see how the headlight reacts in a normal circuit, we can plug it into any fuse box connector, and turn on the load normally powered by that fuse. Doing so gives us some idea how the sealed beam will react in a circuit when a short is successfully removed.
Here, our sealed beam is installed in place of the interior light(s) fuse. This is a pretty hungry circuit, feeding several interior lights. With the driver door opened, the dome and courtesy lights all illuminate and the headlight is on too, but dimmer than normal. Total current through the headlamp and interior light bulbs is 4 amps.
Next, let's move the headlamp tester over to the rear wiper circuit. This time, total current is much lower, about 1.3 amps.
The wiper works, but there isn't enough current in the circuit to light the sealed beam. That's why we said that the sealed beam may go out--or go dimmer--when the short in the circuit is removed and normal circuit current is restored.
Less Than 10 Bucks
Some won't take this tester seriously because it is homemade, has no tool company logo on it, and doesn't come in a blow molded case with a hundred dollar price tag. In fact, if you buy everything you need, this tester costs less than 10 bucks. Sorry to disappoint, but that's a ten spot well spent for a tool that does the job and prevents further circuit damage!