hi Al,
The problem is when the comparator/relay switches the motor load from solar 'SUN' voltage to the 24V external supply, [at the low SUN limit of 4V], the offload voltage of the solar panel jumps up to about 17V.
This 17V jump causes the comp to switch the relay back to the SUN voltage source, as it exceeds the high limit voltage of 14V, which as soon as it goes on onload drops to about 4V! again.
Basically the circuit design is unstable.
I have run a number of sims in LTspice which highlight the problem.
Regards