Two Wire Troubleshooting Guide

2-wire Troubleshooting Overview

If you have an SLM48DM 2-wire system and the module becomes damaged or has blown the fuse of the SL1600 controller, there is a high likelihood that the wire path has been damaged or is not properly waterproofed.  Replacing the SLM48DM module alone will not correct this problem and will almost certainly lead to a repeat failure.  A clamp on meter is the most useful tool for determining if a short exists; determining where the short is located and isolating the problem for repair.

When the 2-wire system is cut or damaged, it behaves very similarly to a broken mainline. Water, or in this case electrical current, flows to the direction of the break. A clamp on meter acts like an electrical flow meter that measures the flow of electricity towards that location of the broken wire.  Milliamps of current flow towards the break, and when the break is passed the current stops flowing.  If the system is split in two or more directions, the flow will also act in the same way as water, with high current flowing toward the break and lower, normal current that flows the direction of the good wire path.

Basic clamp meter instructions for 2-wire:

1. Use the ~ A position on the clamp meter to measure current with the clamp ring coupled over one wire of the two wire path.
2. Use the probe wires to perform a continuity test between wires or earth ground in the C W position.

Troubleshooting 2-wire shorts with a clamp on meter:

1. The normal ~ A current when no stations are operating should = 0.00
2. Normal ~ A current for one operating zone = 0.015 amps + ~0.003 amps per decoder on the system.
3. Total ~ A current draw for a typical system should not exceed 140-150 mA with a single zone operating. Not to exceed 250 mA with master valve or zones doubled up.
4. If the current draw exceeds these limits the SLM48DM there is a problem with the wiring.  The SLM48DM will continue to operate with a short load total of up to 500 mA without showing the problem even though there may be a partial short on cut or damaged wires. A system with this level of short will likely eventually fault out or fail once the wires develop enough corrosion and the ground moisture is high enough to become conductive.
5. After the ~ A current exceeds 650-900 mA the SLM48DM module will likely become damaged and fail or eventually blow the fuse of the SL1600.
6. When the short load exceeds 1.0 amp the slow-blow fuse of the SL1600 transformer will blow, but the damage to the module is probably already done. Earlier models of the SL1600, ver. 1.10, and prior have no fuse and may damage the transformer as well.

Locating the short

If the current on the 2-wire system is greater than 150 mA, you must service the wire of the system to locate the damage to prevent repeat failures.  Here are the easiest steps to locating and repairing a broken or damaged 2-wire path.

1. Disconnect the wire path form the controller and run a continuity test on the 2-wire path to identify the possibility of a dead short.
2. Verify the amp load exceeds 150 mA or greater to positively identify a partial short on the 2-wire path.
3. If the load is over 500 mA, wire a solenoid in series on the 2-wire leg as a resistor to draw current away from the controller and prevent damage to the module. If the mA reading is then becomes too low to get a current reading on the 2-wire path, a second or third solenoid can be wired in parallel with the first to increase the reading.
4. Determine the layout of the system and any splits in the wire path close to the controller.
5. From the first valve box and wire path split use the clamp meter on one leg of the wire path to trace out the current load and determine the direction it is coming from. Follow the path of greatest current towards the short.
6. Continue tracking from box to box until the current draw drops indicating that you have passed the short location.
7. Return to the prior valve box.
8. With the clamp meter check the lead wire of the decoder to make sure the cause is not a failed decoder. A failed decoder may read 500-600 mA over that decoders lead wires. If there is no problem with the decoder lead wires, then split the connections for the 2-wire path.
9. Trace and mark the 2-wire path using a 501 or 521 if necessary.
10. Connect the Pulser to the 2-wire path that is isolated from the controller. Make sure the path is isolated from the controller before powering up the Pulser!
11. After locating the short using the Pulser make the repairs to the damaged wires using SLCONN waterproof connectors.
12. Reconnect the wire paths using new DBR dri-splices from the SLCONN.
13. Operate a zone at the controller and verify with the clamp meter that ~ A current has dropped below 150 mA.

The SLGDT does dump to ground when hit by the Pulser, so yes, it will give a false indication of a ground short, or it is an excellent ground rod locator.