These are pretty much what my initial troubleshooting steps were – to completely disconnect the valves and test only the output on the controller.
I’ve retested the VIN voltage, and I was mistaken: It’s fairly stable in the 4.5-4.9V range. Must have been my shaky hands causing the 3.5-5V that I initially reported.
For each zone, though there is definitely something happening, it’s barely registering on my multimeter (back on DC voltage!). Your comment that the voltage will drop after the initial operation made me go back and recheck it incase I was too slow testing the zone output, but by my research that’s still going to be above 5V and not in the mV ranges I’m seeing.
So, still a problem with output voltage on the zones.. any other suggestions?
That being said, I’ve had a look at the solenoids and I’m pretty sure they ARE a latching type. I’m seeing ~40ohm resistance when measured from the end of the control wires (at the controller, which is ~20m from the solenoid) but they are Irritrol 2400MT. I was sure I checked this before I ordered the parts but obviously got it wrong anyway. Looks like I’m in for another unit!