I have a a former plow truck with a solenoid operated hoist. Truck is using 3 batteries and can use 4. The 2 batteries are fairly new and the truck started right up after sitting for about a year. The other battery is bad. The hoist wouldn't go up and unplugging the the connector to the solenoid valve showed zero volts in up but 9.74 in lower position. I put a battery charger on and put power to the 2 solenoid wires are could hear the solenoid work but no movement. I then plugged the connector back together and the control in the cab lifted the hoist. Played a bit and got the box to come back down but couldn't get it to lift again. Can a bad battery be what's causing all the problems? I think I read low voltage will cause a solenoid valve not to work. Couldn't even get 12 volts on the bad battery with the engine running.
If the bad battery won't hold 12 volts, take it out and run on two batteries. It won't start in January, but you can figure that out then. I'd guess your issue is more with bad connections in the solenoid wiring, but remove the bad battery, then see if you have close to 14v running, then see what volts you have at the solenoid. Don't forget to check to a good ground to see if the ground side is the problem.
There's a complete electronic system with only the hoist still operational and I'm wondering if the bad battery powers it and the 2 good batteries are just used for (24 volt) starting? I'm going to get a new battery and hope that the problem is just too low of voltage. It used to act up and sometimes you had to try a few times to get it to work. Maybe the battery was never that good and now after sitting for awhile is just barely holding a charge. I think the bad battery also powers the ECM (Cummins M11) because the truck wouldn't start when I had the battery disconnected. Battery gauge in truck showed just over 10 volts.
You could post a picture of the batteries and the cables hooking them up. Otherwise, I'm guessing. Don't trust the gauge in the truck, check the batteries voltages with the engine running. They should be all the same voltage. It would be extremely unusual to have only one battery powering a load like this, on purpose.
When running last year the truck gauge showed higher volts. I never checked the voltage on the good batteries but with engine running the bad one was 11.75 volts. Shut off was less than 7 volts but the volts jumped all over the place until settling at around 6.95 volts. I don't have a pic. but the battery tray with the bad battery has 2 red wires (10 gauge I think) hooked together on the positive post in addition to the battery cable. I'm guessing having these disconnected is why the truck wouldn't start. Interestingly both battery trays have a battery disconnect switch (neg. side) but if the switch for the (bad) battery tray is turned off makes a light come on on the dash so I leave it on. I shut the other switch for the 2 batteries off which works properly. It's kind of weird. Someone suggested maybe the ECM still holds some residual power but I'm not sure. I'm hoping a new battery will resolve the issue. It needs it anyway. If not maybe I'll see if it can be changed to non solenoid valve. It's frustrating when sometimes the hoist works great and other times takes several attempts.
You got some wiring issues, doesn't sound like that battery is getting any charge. A new battery won't fix it for long.
I think the battery is shot but is getting something because volts do go up with the engine running. I just realized I need to check the other solenoid on the valve. There's a bunch of other wires in there and it slipped my mind there's another set of wires for the lift solenoid. The pump also needs pilot pressure but I don't know how to adjust that. First I'll see if I have consistent power to the lift solenoid.
Figured some things out today. Picked up a reconditioned battery for $50 so not a big deal. For some reason I think the ECM controls the alternator because sometimes it charges and sometimes it doesn't appear to be charging. Almost like once the batteries are charged it stops charging until there's some load and then charges again. The problem with the hoist is wiring related but I'm not sure what's wrong. I needed the truck so made a jumper with one of the non used connectors that I plugged into the solenoids for up or down. It worked flawlessly other than I had to get out of the truck and manually hold the wires on the battery terminals. I took the side cover off the plow console to see if there might be an easy way to hook up the up/down joystick for the hoist. It was a mass of wires and electronics and the joystick even had some kind of circuit board with electronics on it. I didn't have much time to investigate it further but think it would be easier to just hook up a rocker switch or 2 to operate the hoist. Just run wires direct from the battery terminals.