Shower keeps turning on by itself

[drippy]

I have a single handle shower and the handle very recently broke. The molded plastic cracked so that it would no longer grasp the copper to turn it off and on or hot to cold. I have been using pliers to operate the shower (until I can replace it) -- the other evening I showered and went to bed -- in the morning I discovered that the shower had turned itself on. I turned it off 3 more times before finally it stayed off. What is wrong with it? Just prior to this (previous day) I had a horrible time getting the water to even shut off. I have to remember which direction and how far I turned to get the water on so that I can turn it back off. I have a feeling that I am not going to be able to just put a new handle on it to fix this. Does it have something to do with the alignment of the valve, such as turning it one way or another so that it lines up with what should be hot on one side and cold on the other? Did I just not catch the threads when I tried to turn it off?
Helpless in AZ [8]

[AaronJS]

Hi Drippy. Well, single handle fixtures are kinda simple. They
turn off clockwise, and on counterclockwise. There should be an
adjuster thing behind where the handle was that determines how much
hot water it'll let through the open valve, but it's set up so the
cold water comes first and hot mixes with it as it opens counter clock-wise. If you're able to take apart that valve from the front
you may be able to buy replacement parts to fix the valve, but
you may also have to get a new valve fixture entirely.
You really should be able to dismantel the vavle from the front and
get replacement parts for that whole inner valve assembly.
Hope that makes sense to you.

[drippy]

Thanks Aaron. If I am unable to repair the valve assembly, is replacing it a pain?

[AaronJS]

Well, the level of agrevation that would come with replacing the
valve depends on how it was installed. If there are union connectors
from the water lines to the assembly then that makes it a lot easier.
If everything is hard connected with solder copper and no unions then
the copper would have to be cut and then unions be needed to
reconnect to the new valve. Also it depends on how accessable it
is. It's probably better to tear out the wall from behind it and
not from in the shower, but that's probably obvious. It really
depends on if your shower is tiled, or if it's a shower surround.
It's pretty much a one day job for a do-it-yourselfer. I just did
one a couple months ago at my mom's house.