Troubleshooting a linear current booster

Good day, all;

I have a Dankoff DL-8 linear current booster my brother removed from his solar water pumping system (removed because it was not working right).  With my basic electronics knowledge, I am trying to troubleshoot and repair it, but am not sure exactly what behavior I should see from it regarding input and output voltage (and current).

From what I understand, the output voltage should remain roughly equal to the input voltage when the input is below the motor voltage.  As the input increases, the output voltage should increase, as well, until the motor voltage is reached.  After this point, the output voltage should remain relatively steady (12 volts in my case), with the available current increasing.

Is this a correct assumption?

However, I see a one-to-one match between input and output voltages -- when the input voltage is up to 17-18 volts, the output is exactly the same.  

Inspecting the board, it appears as if some components (the MOSFET, power transistor, and power rectifier) have overheated at some point.  Also, a 15 Amp fuse is used, when the specs call for a max of 10 A (came this way from the installer, according to my brother).  Since it appears that an out-of-spec fuse could have cause these components to overheat and fail, I've ordered parts to replace them.

Once I have the components replaced, can anyone provide thoughts as to what testing I can do to check if I've fixed the problem?

Thanks in advance!

Cheers!