When things go wrong and you try to trouble shoot some of the problems, you have no access to the wiring that runs to the zone from the controller. (no exposed wiring) i.e. On a non working zone, I want to check the resistance of the solenoid as well as to see if it has 24Vac applied to it under load. You must disconnect the zone wire and the common to measure resistance, for voltage you must disconnect the wire, insert a test wire into the connector, splice the zone wire and the test wire together to measure the voltage. What a PITA. Older style, with terminal boards with screws that allow you to wrap the wire around it and tighten them provide access easily by just doing your measurements on the screw. On long runs to distant solenoids, often a bigger diameter wire is used to lesson the resistance. Sometimes this wire will not fit into the push to connect buttons. I have seen demo's from installers using a dremel tool to make the wire smaller so that it will fit. Just my typical experience. (-_-)
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