differential temperature controller solar always on?

Solar differential controllers control the pump by turning the pump on when the solar panels reach a temperature 8-12 degrees above the temperature of the water in the storage tank. The pump pump continues to run until the temperature reaches the set high limit temp usually 130- 150 degrees.
Or on cloudy days it will turn the pump off if the collector temp is 3-5 degrees above the the storage temp, until the 8-12 degree above is reached then turn the pump back on.
If your pump is running without adding heat to the storage you probably have air in the system, insufficient pressure, or a defective pump.
The manual for the SR208C should be available online.

Or, another possibility although unlikely: A bad sensor or line break, or loose connection, or the insulation around one or both of the sensors has come loose or been removed causing the sensors' voltage to change because of the resulting change in the sensor temp. that then makes the controller "see" an incorrect temperature.

Thanks for the answers. I already have the manual/instructions. its typical chinese translated instructions. it gives me no information at all. just the settings. The system doesent appear to have any air in it. The pressure is fine at 2.5 and not dropping. the expansion tank is also set to 2.5. the pump turns on inj the morning and off in the evening. just not during the day. I thought the water in the header tank should be heated to a certain temp. once reached the pump turns on and curculated the fluid arund the system. once the temp had dropped it turns the pump off again. unfortunately i am in a rural area in Spain and there are no experts here at all. Thanks for your help anyway.

Thanks. i have checked the sensors and as far as I can tell. they all operate correctly. I have even changed them so now have a spare set. and a spare controller

Even though other conditions such as those Lucman wrote about are often more likely, I'd still check the insulation, wiring and integrity of the connections of the sensors - which means more than just checking the sensors themselves.

If the (necessary) insulation between the sensors and the surrounding environment is insufficient or deteriorated, or somehow greatly different from one sensor vs. the other, the temp. of the sensor(s) will be lower than if the insulation remains intact. Different levels of insulation among sensors will throw the temps. and so the differential temperatures off.

Also, if thermal contact between the sensors and their pressure barriers (the tubing/piping containing the working fluid) is not good through loosened strapping, pipe or sensor corrosion or (minor) shifting/creeping over time from thermal expansion, the sensor temp. will be incorrect or at least different from what it may have been when the system was new.

The whole idea is to keep the thermal resistance between any sensor and the working fluid as low as possible while keeping the thermal resistances between any sensor and the surrounding environment as high as possible, and also keeping those thermal resistances the same among all sensors.

Also, make sure the physical contact between the sensor and containing surfaces are clean and VERY tight. Use thermal grease between the sensors and the containment surface, but don't waste money on the stuff that has silver in it. That's a rip off. The copper stuff works about 99.5 % as good and is completely fit for purpose.

Thanks, they are new and I have checked them. I even have a themometer reading to se if I am getting the same similar reading. It all appears to be correct.

Does the water temperature in your storage tank continue to rise when the pump is running?