Rain Bird MDC2, Instructions Manual, Page: 89 / 124

Troubleshooting,
continued
83
Problem Finding a break in the two-wire communications cable.
Solution Measure the line voltage on the L1 and L2 terminals inside the controller. If the voltage is correct,
then the fault is a result of a break in the two-wire communications cable. To locate the break,
you will need an as-built irrigation drawing showing the complete installation and tracking of the
two-wire communications cable, including each field decoder connection point.
If the break is in a ring cable, disconnect one end of the cable from the controller.
You can now find the approximate location of the break by testing each field decoder to
determine which decoders are not responding. Please see
Testing the operation of the
decoders
on page 37. If, during the test, a field decoder activates, you will know the break in the
cable must be beyond that decoder. Field decoders will not respond if there is a break in the
cable.
Before testing, you may want to depressurize any pressure-controlled pumps and deactivate
any pump decoders in your system. Please see
Entering the Pump/Master valve Information
on page 28. This will stop the pumps from activating during the test, conserving water.
Once you localize a break between two decoders, you then need to find the exact location of the
break manually by measuring its voltage.
A cable’s voltage after a break is not always zero (0). In fact, it may only be 10 V lower than the
reading before the break.
First, check connections at the two decoders and between the two decoders. Measure the
voltage on the cable from the last decoder that is working. Then measure the voltage on the
cable at the non-working decoder connection.
If you cannot find the fault in any of the field decoder connections and there is no specific hint
at to where the fault may be, you can try using a cable tracer to locate any irregularities along
the cable.
If you do not have access to a cable tracer, you can either replace the complete length of the
questionable cable or follow the systematic approach we will outline next.