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Old 01-28-2013, 02:57 PM
irrig8r irrig8r is offline
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Join Date: Dec 2004
Location: Silicon Valley, CA
Posts: 4,539
Originally Posted by 1idejim View Post
Quite a few times i see a post where the OP states that he has 24 volts but the solenoid won't work.

I always ask the same few questions, what is the resistance of the wire path and solenoid and what short to earth is.

I took a call from a friend in Texas this week, 24 vac, but he also had 180 ohms resistance from the controller.

Most troubleshooters will begin fooling with the splices because that's what we're taught.

Before I look at anything i will take a short to earth measurement to test the wire insulation. If the insulation is good i look at the splices, if the insulation is bad i grab the A-frame.

Most solenoids require 300mA - 500mA inrush to lift the plunger. That's 7 - 12vA.

Using Ohms law you can see why the valve won't open.


There are 24vA in one Amp. 1A/24vac = 0.041667mA or 1vA.

0.133333/0.041667 = 3vA, less than half the current required to magnatize the coil and lift the plunger.

Electrical troubleshooting can be made a lot easier by doing a little more testing and a lot less guessing.

Hope this helps someone.
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I've never measured resistance "from the controller" itself... or do you mean he measured the resistance of the solenoid via the wire path to/ from the controller?

A reading of 180 Ω would make me suspect a partial connection or badly corroded wire connector (as you mentioned, checking the splices).

You know what they say about "when you hear hoofbeats, think of horses not zebras"?

Anyway, though it makes sense to suspect the insulation, please explain how you test the "short to earth" because it's outside of my limited knowledge and experience.
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