I don't understand the phrase "the ENTIRE texture became ambient".

Look there is nothing special about "ambient" other than it is added to everything else the surface generates. Whatever colors you put in there are added directly to the render.

It's simple math: if the rest of the shader (diffuse, specular, etc.) produces color A, and the ambient value is color B, then the output is A + B.

You mentioned converting the texture to black and white. Why? That doesn't change the fact that the texture is non-zero somewhere.

If you can't follow the math (by the way this is 7th grade math, A + B, so you should be able to follow that) then you'll have to show me what you're trying to do and what you're using as inputs (maps) and I'll show you how to do it.

I suspect you're trying to make parts of a texture map glow, while making other parts not glow. That is more than simply A + B. 

That's where you should be using a mask. White or black, on or off, glowing or not glowing.

The mask goes into the Ambient_Value.

The color of the glow goes into the Ambient_Color.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)