Figure 1 shows the plastic convoluted tubing removed from the wiring harness that runs across the front of this engine. It will be reinstalled later to dress it up like it was before and to hide the wires you're about to add.
On this car, 12.35 volts was found on the blue wire coming from the ignition switch when it was in the "run" position. That battery voltage comes from the automatic shutdown (ASD) relay on newer models, and other than for one second after turning on the ignition switch, that relay is only turned on during engine rotation, (cranking or running). That means the engine has to be running to verify you have the correct wire.
Figure 2. Cut wire
The red arrows show the two ends of the cut wire. The green arrows show your two new wires that will go to the switch. Notice there is a short piece of heat-shrink tubing on those wires already.
Figure 3. Wires are spliced.
The new wires are spliced and they've been sealed with the heat-shrink tubing. By using the convoluted tubing, it will look just like any other part of the car's wiring harness so those new wires won't provide undesired clues. You can buy new tubing from any auto parts store by the foot or by the box, or to really save your school's money, you can find used tubing and wires from any car in the salvage yard.
As a standard part of this exercise, students must be told they are not to pull wires from the tubing. There is no secret that wires have been added to the car, but finding them is not going to help them learn to diagnose electrical problems. The goal of this bug is to determine there is in fact an open circuit, it's in the blue feed wire, and to narrow it down to the closest pair of accessible test points. In this case, one of those test points is the terminal on the back of the alternator. The splice A142-2 is a good suspect for this open circuit, but rather than having anyone peel the tape from it, they can use the blue wire in the ignition coil connector as a test point. We know the circuit is good to that point because the engine runs.
Figure 4. Hole through firewall.
Figure 4 is a little out-of-context due to being such a close-up shot, but it shows the hole in the firewall for running wires into the passenger compartment. Use a 1" hole saw. Rubber grommets are readily available for a neat appearance and they'll hold the plastic tubing in place. This hole was drilled just in front of the glove box and will be covered by the carpet. Once the tubing is in place, it's rather easy to feed additional wires through later. Bend the end over so it doesn't catch on the ribs of the tubing. If you have a long stretch of tubing with many curves, a pull wire might be useful. If, for example, your tube is three feet long, use a piece of wire seven feet long with a loop tied in the middle. Tie the wires you want to run onto that loop to pull them through. That will leave you with the other end of the wire to pull the loop back or to pull wires the other way.
On this car I was using up two spools of wire so every bug had a blue and a red wire. This will get real confusing if you try to run wires for multiple bugs at once and add the switches later. At the very least, you'll want to label where each wire goes to for future reference. Masking tape works okay but if you're going to write a description, you'll want to put the switches in a lock box to avoid snooping. Students are going to get a kick out of your handiwork, and given the chance to see what you did, you can be sure they'll try. I found lock boxes to be unnecessary so I began labeling all wires with nothing more than numbers. Those are the numbers shown in the call-out boxes on the diagram.
If you want to add a little bit of order to your bugged cars, think about using the same wire numbers and / or colors, and the same switch numbers for the same bugs on different cars. As an example, switch # 3 might be for a worn alternator brush on every car. Knowing that will help you guide students to the source of the problem without having to constantly page through your notes. Wasted time means lost attention for some students.
Building the Switch Box
_For the first attempt, fifteen very small switches were placed in one row making it possible to have up to 60 bugs in four rows. This is quite impractical as there is not that great a variety of good learning exercises per vehicle and not enough class time to cover them all. In addition, the small 6 amp switches were purchased in bulk at .25 each which was a good deal, until they started to fail. Terminals melted out of the plastic housings from the heat of soldering, and some overheated from only two to three amps of current flow. Your results will be longer lasting and more reliable if you purchase high quality switches. Many good bulk deals can be found on eBay at reasonable prices. In way too many instances students tracked defects in the circuits to my switches rather than what they were supposed to find on the car. Those cheap switches also caused multiple problems to show up at once. That often caused students to think they were all related leading to inaccurate clues.
An aluminum sheet was used for my first eight switch boxes. Bend the sides to form a pair of edges to accept mounting screw holes. After bending, the plate should be about 1/4" narrower than the inside of the box to facilitate installation and removal.
Figure 5. Switch Plate Layout
The template shown in Figure 5 is easy to lay out in MS Word by just copying and pasting numerous circles. The holes were center punched and drilled to 1/2" to accept the better switches. The intent of four rows was to put all charging and starting system bugs in one row, sensors in two rows, and lighting and other systems in the last row.
Figure 6. Center punched and ready to drill
Not a whole lot of explanation is necessary in figure 7. The holes have been drilled and one switch has been mounted. For this bug a single pole single throw (spst) switch is all that's needed, but a double pole double throw (dpdt) switch is shown here. The price was right for a large quantity, and the extra switch section allows some flexibility, especially when it comes to some sensor bugs. To provide the desired operation of your bug, it might be necessary to apply a voltage to some point or to turn off a different circuit. The second half of the switch gives you that option.
Figure 8. The box.
Ahh. The box. This is just an inexpensive cash box that can be found at any office supply store. Here's another 1" hole with the rubber grommet. These boxes come with a lock on the front to keep snooping noses out.
Drill two holes on each side, slightly larger in the box and smaller in the plate, to accept self-tapping screws. Before you actually build this type of locking box, read through the previous section, "Bugged Cars" where I found out it wasn't really necessary. You may find a simple switch plate mounted inside the glove box is a satisfactory alternative.