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After desoldering all the larger components, both sides of the board
were "photographed" with a digital scanner because it gives a "flat"
image, (a close-up with a camera distorts the outer edges of the
board), giving a 10MB file for each side of the 5-1/2" x 7-1/2" board.
Then the image was opened up in a CAD program and set as the bottom
layer, shown here with a 300% enlargement....
... a library of almost 200 symbols for every single component in the
IDM was created-- most of them custom-made symbols --every IC, every value and size of
resistor, capacitor, and diode, along with the solder pads they were
soldered to, and the various through holes in the board...
... and then they were laid out on top of the image, and another layer was overlaid to do the actual tracing on.
then the mouse was
used to draw the lines for every single circuit trace over the top of
the image...
This process was repeated for both sides of the board.
After the tracing was completed, the image was removed, leaving just
the tracing visible. Here, IC 19 and its related components on the
top side of the board are shown.
This is IC 19 related components on the bottom side of the board.
Next, the bottom tracing is "flipped" left-to-right so everything is oriented as if
the board was invisible and you were looking directly through the board.
Then, the top and bottom tracings are overlaid so one can get the big
picture of how each and every component relates to all the other
components. After doing the overlay, it was necessary to review the
entire trace to make sure that all the connections were complete.
In these tracings, the round green circles are "vias", holes through
the various layers of the PCB where a circuit trace crosses through
from one layer to another. If a trace was found to end at a via, and
didn't come through another via directly on the other side, that meant
that it went somewhere else through one of the two inner layers. Some of
these inner traces were only 1/2" long before they came up through to an
outer layer, but others were 5"-6" away from where they went
in. Wherever these "dead ends" were
found, it was necessary to take a multimeter, put one probe on the "dead
end" and with the other probe methodically test for continuity to
every single point on the board until the other end(s) of the
trace was found. As these inner traces were discovered, they were added onto
a new layer in the tracing; over 45 were found.
Finally, the combined image above was "disassembled" to produce a true
and correct electrical schematic so that that the actual function of the
circuit could be determined. An LM2903 is a dual (2 comparators in one
package) voltage comparator made by Texas Instruments. The arrows next
to it indicate whether that pin is an input or output.
The letter "C" seen at the upper right is the same "C" as shown in
the top and bottom tracings. The various letters were used to keep track
of various reference points during the disassembly & redrawing process.
This process was repeated for each "sub-circuit" in the IDM until we
had a complete schematic for it.
This particular circuit is part of how the IDM monitors injector
current draw to determine whether the injector solenoid or wiring is
shorted or open, so it can report problems to the PCM.
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