24
Bench Tracing a Wire Harness
There are cases where you may have a wire
harness removed from the vehicle, sitting on
the bench, and tracing an open circuit. Wire
harnesses that are removed from the vehicle’s
electrical system have only floating wires in
them. The open connectors of the harness are
connected neither to positive nor negative
therefore all of the harness’s circuits are open
and floating. It is important to be aware that
the open circuit signal will capacitive couple
into floating circuits that run parallel and next
to the transmitting signal wire. (See Figure A).
Floating circuits that couple the open circuit
signal also transmit the signal too and will even
couple back to the wire you want to trace. This
prevents the receiver from locating the break in
the wire because all the wires are transmitting
signals. You can be easily led down the wrong
circuit if you are not aware of this.
To correct this problem, you need to tie all
parallel floating open circuits to either ground
or a positive voltage (see Figure B).
All neighboring wires and circuits must have
some potential of ground or positive on them to
prevent capacitive coupling from occurring.
It is recommended to trace OPEN circuits while the IGNITION is turned ON. This will supply a positive
voltage on certain circuits that can potentially capacitive couple. It is also a good idea to keep all of the
vehicle’s electrical loads (light bulbs, relays, motors, etc.) CONNECTED while tracing OPEN circuits.
This keeps certain neighboring circuits grounded, which also prevents them from capacitive coupling.
+Voltage
Not
connected
Not
connected
Open Circuit Signal
Fig. A.
Top and bottom wires are floating
because they are open. As you can see there are
signals coupling into them from the original cir-
cuit with the open circuit signal injected into it.
+Voltage
Open Circuit Signal
Connecte
d
Connecte
d
Fig. B.
Top and bottom wires are now tied to
positive or negative and there is no capacitive
coupling. The open circuit signal now stops at
the break in the wire.
Fig. A
Top and bottom wires are floating because
they are open. As you can see there are signals
coupling into them from the original circuit with
the open circuit signal injected into it.
Fig. B
Top and bottom wires are now tied to
positive or negative and there is no capacitive
coupling. The open circuit signal now stops at
the break in the wire.
Connected
Connected
Not connected
Not connected
