Power Probe ECT 2000 - Manual (Page 24)

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 fl oating 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 fl oating. It is important to be aware that
the open circuit signal will capacitive couple
into fl oating 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 SMART receiver from
locating the break in the wire because all the
wires are transmitting signals. You can be eas-
ily led down the wrong circuit if you are not
aware of this.
To correct this problem, you need to tie all
parallel fl oating 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.
+Voltage
+Voltage
Open Circuit Signal
Open Circuit Signal
Fig. A. Top and bottom wires are fl oating
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.
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
Not connected
Not connected
Connected
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.