Hello and thank you!
I have a Harbor Freight shallow well pump that draws water from a 55 gal barrell through a garden hose to a portable exterior propane shower. When the pressure drops, it is not high enough to activate the heater. Either the swith is bad or needs to be adjusted to kick on at a lower pressure. Took cover off and saw 2 adjustments.Does anyone know how to set this??

Turn the large nut one turn clockwise. Try it out and see if that is enough. If that's not enough, then give it one more. So on and so forth. Turn off power before you turn nut.

Just be aware that this will increase both the cut in and cut out pressures. If you find that the cut out pressure becomes too high, then turn the smaller nut CCW a full turn.

It is a trial and error kind of thing to find the right points. Just have to stick with it.

What's the control box setting? How much pressure does it take to activate the heater? Doe you have a bladder tank or this a on demand pump that turns on every time you draw water? Let's have more details about your system. Back to you, Tom