Some Days Are Like a Bad Dream When You're Tracking Down a ShortPosted 1/14/2003
By Jeff Bach
Did you ever notice that when things start to go wrong, they seem to all happen at once?
It seems like the most unlikely circumstances have to come together sometimes to make you question your sanity when diagnosing electrical or driveability problems with automobiles. When you are trying to find the trail of the elusive intermittent trouble code, the last thing you need is a gremlin in the mix to throw you off the trail.
Every once in a while, I have a day that seems like I'm in a bad dream from which I can't wake up. It starts out with me running late for the shop and pulling out right after the school bus. I get to follow it for about three and a half miles down a curvy road with no place to pass and lots of stops. The first stop has no one waiting at the end of the driveway but the bus just sits there, wasting my precious time. I start thinking this is a temporary driver who doesn't know the stops and the kids are playing a joke on her, getting her to stop at the wrong houses. Then a front door bursts open and a kid comes running out with a backpack in one hand and one arm in a jacket on the other. Boy, does that take me back.
I start to realize that I'm not the only one who has these days and there's nothing I can do about it, so I might as well just sit back and enjoy the ride. After enduring several more of the bus driver's methodical stops, I finally pull into the lot and there are two customers sitting on the front porch; one is looking at me ... then his watch. I walk into the office and the recorder is taking a message from someone who is guessing that since the recorder is answering the phone, we're on vacation. Sometimes, I long for the simpler times when I worked at the dealership and all I had to do when I came in a few minutes late was to work a little faster during the morning and I could make up the time before the "Roach Coach" got there. Now the thought of "faster" doesn't even occur to me until after I've had my first cup of coffee.
With this morning's chaotic beginning behind me, I start to quiz the gentleman with the watch about the intermittent nature of his electrical problem. He starts talking about the good old days before computers when he could do all of his own car repairs and I think to myself, "The good old days are just getting here." I agree with the guy about how complicated cars have become and we agree about how many similarities there are between doctors and technicians. Once I've collected enough information from the fellow about his Jeep's problem, I send him on his way and begin to settle into my zone in the shop where I do my electrical work. The complaint was that the windows work fine most of the time, but once in a while they just all stop functioning. Because this problem initially appears to be an intermittent power feed to the windows, I connected a current probe to my lab scope and get ready to test drive it while working the windows in hopes of seeing the current level drop or occasionally disappear.
Notice in Figure 1 that I have a rubber band around the jaws of the current probe. This helps to avoid false readings due to jaw separation during radical maneuvering. I start operating each of the window motors while looking at the scope screen for a current pattern, making sure the scales I have chosen on the probe and scope put me in the ballpark for a decent image.
Since the window is working now, I start on a test drive - heading for the torture track. I run the windows again as I round a bumpy curve to the right (Figure 2).
The spikes in Figure 2 are normal noise from the switch. Nothing out of the ordinary takes place during the test drive over rumble strips, railroad tracks and chuckholes until I hit the bumpy curve again, going around it to the left (Figure 3).
I got this pattern without operating the window switch. I could not duplicate it during the test drive, except on the same left bumpy curve with "some" G force. Once more around the curve and the pattern shown in Figure 4 came into view.
I could never get the windows to stop operating. And other than the spikes showing up on the scope screen there were no other clues that a short was occurring while making that left bend. But I know now the problem with the windows intermittently not operating is due to the short - which when present would keep the breaker open. The battle is nearly over now except for finding the location of the short, which could be in any of the doors, the body harnesses feeding them, or in the dash harness that crosses over from the fuse panel. Since G force to the right is where I could get the shorting to occur, I raised the left side of the vehicle up to a pretty good angle and start thumping various places on the body while watching the scope and hoping for an image. "Bingo" on the right "B" pillar (Figure 5).
Seeing this waveform while thumping the "B" pillar makes me think what it must feel like poking a shovel down through soft dirt to hear a distinctive thud where "X" marks the spot on a treasure map. I peeled back the boot on the right rear door to expose the harness shown in Figure 6.
It's the satisfaction I get from images like this one that make memories of the day's bad events fade into obscurity.
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