EUREKA!!
Problem is now (semi) solved.
It seems trhat the pressure switch was stuck. I put a flat head screwdriver between the stabilizer valve assembly and the cartridge assembly and applied light upward pressure and heard a click sounding like a microswitch.
I turned the shower and pump on and HEY PRESTO! hot water is now coming from the showerhead and the low pressure light is now OFF.
I hope this info will help many others.
TALBERT
October 2006