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2. Compare the length of the two single wires to ensure it matches the wiring
diagram’s “Cut Length” for the wires or a best-as-possible measurement of
the installed harness start to end. Remember, if one of the two wires is
shorter than the other, the shorter wire is the electrical end of the pair.
3. When the pin numbers are located for both ends of the single wires,
measure the pair open at the far end, then short the two pins at the far
end. The short should appear on the trace at the same distance as the
open to ensure:
A. The TDR is measuring the single wires you desire.
B. To be sure the wires at the far end are making good connection at
those pins.
The following are some examples of single wires in a harness and the type of
traces you can expect.
Figure 3-44 is an example of impedance (Z) variations over the length of two
single wires in the harness. Cursor 2 marks the end of the test leads (50 Ohm
coax with a short twisted pair to connectors lead at the end). Between Cursors 1
and 2, 19ft 4 in (5.9m) are the two single wires being measured as a pair with an
open at the far end. At about 9 feet the impedance rises for the remainder of the
run. This could indicate either the wires separated in the same bundle and never
came close over the last 10ft (3m), or that section of the harness contains less
total wires.
Figure 3-44
Figures 3-45 and 3-46 are examples of more wide variations in the two single
wire’s impedance. The same test lead is marked (CRSR2) as used in Figure 3-44
and the far end of the single wires is open as marked by Cursor 1. These are
also good examples of measuring two single wires in vehicles. The harnesses in
vehicles tend to be looser and cause wider variations in the impedance.
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