The voltage reported (5VDC initially and then 1.8VDC) is unusually low. There are two things to check:
– make sure you measuring DC (not AC) voltage (it seems you are, just to double check).
– remove power from the controller, and use multimeter to measure the resistance between COM (common) wire and the zone that has problem. Most 24VAC solenoids should have a resistance of somewhere between 20 to 50 ohm. If it’s too low, it indicates a shorting, either in the solenoid or in the wiring.
Another possibility is that the internal boost converter on the controller is not functioning properly. But judging from the fact that the other zones work, this is unlikely to be the case. Still, if you want to isolate solenoid issues with controller issues, you can do the following:
– remove the COM (common) wire from the terminal block: this will disconnect all solenoids from the controller circuit.
– Turn on the problematic zone for 1 minute, and while it’s running, measure the DC voltage between COM terminal and the zone port. This should give you a voltage that’s somewhat higher than the DC power adapter you are using. For example, if your power adapter is 7.5VDC, the voltage you measured here should be higher than 7.5, indicating the internal voltage booster functions correctly. Basically this is the voltage that the controller presents to the solenoid, before the solenoid is actually connected. Once the solenoid is wired into the circuit, and if the solenoid has a shorting, that will lower the measured voltage significantly (which could explain why you are only getting 1.8VDC).