1 Determine the cable’s VOP using Table 3-1
and enter it into the TDR.
2 Connect the two leads to both conduc-
tors at one end of a duplex pair. (As
noted earlier, a TDR cannot trace a sin-
gle conductor.)
3 Turn off the power to the circuit being
traced. If you don’t, the Fault Mapper
TDR will give you an on-screen prompt
that voltage is present.
4 Push the “test” button on the unit, and
the unit will trace the circuit.The photo sequence shows the testing
capabilities, as well as some limitations, of the
TDR.
What’s the issue regarding lightbulbs? The
filament in the bulb completes the circuit by
connecting the positive and negative feed con-
ductors. The same thing happens if you twist
the bare ends of two conductors together. The
TDR’s signal bounces from one of the paired
conductors to the other, and in both cases the
bouncing stops where the circuit is completed,42 electrical systems troubleshooting
The TDR has identified an open circuit 21.6 feet (6.6 m) away. If you
know how the wiring harness in question is routed through the boat, you
can work your way right to the problem to make the necessary repair.
Does it seem too good to be true? It might be, if you don’t have a clear
understanding of what else might be implied by this reading.
DC – junction BDC +
batteryswitch open cabin lightcabin lightjunction A
TDR will pick up
junction A or B as
either a short or
open in the circuitThe TDR can “map” only as far as the first junction——a short circuit——in a cir-
cuit (the first limitation). To continue tracing a circuit beyond a junction, you
must find the junction (the TDR has told you how far away it is) and isolate
the next leg of the circuit by disconnecting the branch in question. For
example, if a cabin light circuit contains a tee to feed a light on the oppo-
site side of the boat, as shown here, you will have to disconnect that feed
wire from the main harness and trace each branch of the circuit separately.
The TDR indicates a short circuit 30.6 feet (9.3 m) away,
but this is a red herring. Another limitation with a TDR is
that lightbulbs show up as short circuits. In this case, there
is, indeed, a (perfectly functional) navigation light 30.6 feet
away from the TDR.234