Thank you all for your helpful posts. I'm here two years later and found this thread to be useful and just what I needed to test the shut off solenoid. I tested my old solenoid and it worked on the primary circuit to pull the rod in but then failed to stay in when tested with the secondary circuit.
So, replace the solenoid.
I ordered a new one from Amazon, installed it and it did not solve the problem.
What?
So I pulled it out and went through the test procedure outlined above on page two.
Guess what! The new solenoid had the wires in the connector in the WRONG order. I simply rearranged the red and white wires to reflect the original and installed it and voila! All good.
Again, many thanks to all that have posted and provided advice and help and solutions!