Lawn Sprinkler's water pressure is low in one zone

Question

All my sprinklers worked about a month ago.  However, now one of my six zones are not producing enough pressure; all the other zones work just fine.  I checked the voltage for this problematic zone and it was around 25 VAC. The resistance is around 60 Ohms. So, it seems that the solenoid is just fine (please correct me if I am wrong).
Question:

Am I correct in my hunch/diagnostic that it is the zone-valve and
not the solenoid or sprinkler head that are the problem?
What is a good way (diagnostic) to find out which zone-valve is busted?
What should be my next steps?

I want to make sure that I am reasonable accurate since the zone-valves have not been touched in the past 20+ years and hence have a lot of dirt/mud on them; besides, they are each in 10-inch diameter plastic canisters under the ground (making access rather difficult).
Thanks.

Barry · Accepted Answer

According to your description, your system is over 20 years old. If a zone is not producing enough pressure to run the head, in my experience, the problem is likely to be a leak somewhere along the pipes connecting the zone valves to the heads. You need to carefully inspect the problem zone whele it is running. Look for signs of water bubbling out of the ground indicating the presence of a leak. Sometimes the fault in the pipe causes the water to go into the ground rather than up so leaks can be difficult to locate. My system is 40 years old and still running with the original zone valves. However I do get leaks in the pipes due to the age of plastic tubing.

Correct answer by Barry on September 3, 2021

George Anderson · Answer

Manually operate the valves until you find the zone that isn't working right. It might be a solenoid, but usually those are an "all or nothing" failure. Solenoids can fail electrically or mechanically. The coils can short out and/or the plunger itself can get stuck.  Once you find the failing zone you can further diagnose what's going on with it. And we can help better with more info.
Co-incidentally I have a zone valve right now that is stuck open.  So I had to shut down the supply. Probably fix it tomorrow.  Almost certainly a stuck solenoid.
Pictures and more information on the zone valves will enable us to help you better.
You are lucky, IMHO, to get 20 years out of zone valves without service.  They must have been pretty high quality to start with.

Lawn Sprinkler's water pressure is low in one zone

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