My tub/shower has a simple, two-handle faucet with a diverter on the filler spout -- no mixing valve. After adjusting the water coming from the filler spout to a comfortable temperature, the water from the shower head turns scalding hot when I pull the diverter to turn on the shower. What is happening to my cold water?

About vthe only way this could happen is if the retaining screw holding the cold side washer came loose allowing the washer to float free,(see image, This is a three knob valve but almost the same as a two knob one). You could still regulate hot and cold but when you pulled up on the diverter but the water would be diverted up the shower raiser creating a vacuum that could suck the cold washer back to the seat blocking the flow of cold water. The solution to that would be to cut the water off and open up the cold water side and check the washer. Hey! Ya got my best shot. We don't see too many of these complaints. Let me know what you find. Good luck, Tom