I have old plumbing. The company or brand of the stem is standard. No allen wrench screw to adjust anything. The water will not shut off when you turn the shower on, the water flows through the shower head but still flows through the tub as well.How can I correct this problem.

I just had the same problem, some how the screw on the back of the diverter got caught on the valve seat. I wiggled it out with a pair of lock pliers but be careful because when it came out the screw was still wedge on the seat. What a nightmare it was for me to get out...