(your pic shows up at
C30 differential heating controller » Renewable Rochester 
)
It looks like only two sensors affect the controller: storage and collector.
Hot water is not flowing from
the solar tank to the water heater tank. Right?
If the sensors are the same type but voltages are different, I'd suspect the controller thinks the tank water is still cold, or the flow control valve is not opening for some reason.
There's a million brands of C-30 out there.
I'd get the mftr and model and search the internet for "manuals/troubleshooting".