When I was a boy, about 9 or 10, I remember my dad (left) talking to my brother and me about his teenage escapades trapping for furs. My dad's father was killed in an auto accident when my dad was 15. He learned to trap on his own and used his fur money to buy his first car (a '36 Ford with a continental kit) when he was 16.

I remember when Greg (my older brother) and I set out on our own to try to catch our first muskrat using the knowledge gleaned from dad and by reading some of his trapping books. We were making our way along a local creek one cold, frosty morning before dawn. We were looking for some of the signs that dad had told us about, such as muskrat slides (long, slick mudslides leading into the water from atop a steep bank). We were also looking for well-worn holes in the bank at the water's edge where they made their dens, trails leading to the surrounding fields, and the best one of all -the sighting of a muskrat house, in the water, made from cattails.

We headed up a small tributary of the creek that went behind Brown's old apple orchard where we saw every sign dad had mentioned except the muskrat house. With the help of a flashlight and the approaching dawn, we made our way up the little creek, following it and marking places where we spotted muskrat signs as it wound through the orchard. The creek had carved around trees, leaving their bare twisted roots hanging over the bank, making dark, cavernous, hollowed-out crevices.

We made our way to the creek's origin just as the sun was peeking over the horizon from the east and we crested the hill of the dam from the west to behold a pond that was loaded with muskrat activity. A large muskrat house stood amidst a clearing in a thicket of dried cattails that had their plumes exposed and were wafting in the wind. Fields of soybeans surrounded the pond on three sides with trails leading to and from the muskrat house. This trip we had embarked upon was a scouting trip so we would know how many traps to bring back that night (forgiveness, we figured, was easier to obtain than permission).

After school we loaded into our packs all the traps that we thought we would need, along with our tools, stakes, tags and wire. Under cover of darkness, with the dim glow of the moon and our flashlight, we headed up the creek once more, this time stopping along the way to set traps in the places we had previously marked on the map that Greg had drawn earlier. After about three hours, we ran out of traps and started back down the creek toward home. About halfway back, there in one of the holes where we had set a trap, was a furry ball partially protruding from the murky water. Oh, the excitement was building as the adrenaline pumped through our veins! With cautious optimism we approached the set. Greg found the trap chain and gave it a slight tug. The fur ball followed. Yes! We had our first muskrat! On the way back to the house we found that we had caught three more. We could hardly contain ourselves as we came through the door to tell dad what we caught.

That was quite a long time ago but the excitement is linked to a place in my brain that comes close to the way I felt when I captured this first short circuit with my new current probe (Figure 1).

What you have to appreciate is the fact that most shorts we see are not evident when the customer gets the car to the shop. Many times they don't act up while we are checking and driving the cars. The trick is to anticipate a problem that you can't see and set the scope like a trap to catch your prey. Or, as dad would say, "think like a short." The obvious sign that a short has been visiting the vehicle is the blown fuse. You have to get familiar with your scope's trigger function if you want to catch the elusive problems that get by the average technician (the fact that you are reading AutoInc. puts you in the above-average category already).

In the past, if the fuse didn't blow, or the short detector's breaker didn't kick, then it was an intermittent short and typically you were pretty much left hanging until the next time it happened.

Not anymore.

With the CRT current probe you can set the scope trigger to catch the slightest increase in current that occurs above the normal level.

If your scope doesn't have an adjustable trigger level, set it for "wait for signal" and zero the current probe with the circuit active so it will only show a trace if above normal current appears. If your scope doesn't have any trigger options I would like to recommend a good one to you.

Once the trap is set you just need to figure out what it takes to activate the short. Try a simple thump test first. Gently hit the fenders and doors with your fist while monitoring the scope for any signs of abnormal activity.

The picture in Figure 2 is one from a very intermittent short that was caused by a bald spot in the harness rubbing a rusty bumper bracket. The fuse stayed intact during the thump test, making it virtually invisible to a short circuit detector compass due to the fact that the short only occurs for 70mS and is only 2.5 amps above normal. In some of my short testing I have clocked 20 amp fuses over 80 amps for more than 10mS before they blew (Figure 3).

If thump testing proves futile, drive the car over some bumps while trying some evasive maneuvering and watching the scope screen to see when the trigger trips.

This Mustang (Figure 4) was only blowing fuses about once a month but the short was captured using my current probe while coming to a stop on some rumble strips and traced to a wire running through the trunk hinge.

Note that during this occurrence, the fuse remained intact also. The 80 amp spike may have shown up with the compass, but I wouldn't have wanted to be in the trunk holding the compass up to the circuit with a flashlight in my mouth while another technician -let's call him Eric -drove the torture track. (Not that we've resorted to such techniques in the past or anything!)

The next problem was its intermittently dying at stops and being hard to start at times. The fuel pump current would look fine and then, for no reason, would get real nasty and the vehicle would stumble, sometimes stalling (Figure 5). This was captured using the current probe by setting the scope trigger about 20 percent over normal fuel pump current and recording the min/max current level. The fuel pump current looked too good during the normal period for me to suspect it causing the noisy pattern. It turned out to be the pass-through connection on the sending unit itself. The fuel pump vibration moving the wire was causing the short to be in sync with the rotation of the armature.

Figure 6 is another fuel pump circuit shorting. This is one of my favorites. This guy (Bob) spent "beaucoup" bucks trying to tune this problem out of his engine. When he finally threw in the towel and brought it to us, he made the comment that the thing was possessed by gremlins. You can imagine the fun I had with the image (Figure 6). I captured this short occurring using the overcurrent triggering technique to get the gremlin to rear its head in the center of the screen.

I love handing these pictures to people; I feel like the trapper showing a farmer the picture of the fox I caught that had been killing his chickens.

What do you think? Do you suppose my customer, Bob, showed his picture to anyone?

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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