It is a solenoid. We inherited a drip irrigation system when we moved into this house nine years ago. The system has been pretty good and we have only had to have one visit from professionals to fix a problem we had when an animal had chewed through some wires. This time the solenoid was the problem. Each of the solenoids is housed in a box and we haven’t looked into these boxes as there is really no need to.
In future we will make sure we do regular inspections of the boxes as there was a problem in one of the boxes that caused our system to stop working. When I checked the box I discovered that the solenoid was completely covered with roots from a neighbouring bush. After I had removed all the offending roots I discovered that the roots had damaged the solenoid.
I was able to go to the irrigation store and buy a new solenoid and my clever husband was able to put the new solenoid in.
I have just been looking at some old posts of mine and came across this one.