Shader not working anymore in Unity5

I switched to Unity5 and after some issues with some animation controllers, i found a problem with a shader:

Code (csharp):

1. Shader "Custom/CameraNoise" {
2.     Properties {
3.         _MainTex ("Base (RGB)", 2D) = "white" {}
4.         _alpha ("Transparency", Range(-2, 1)) =0.2
5.     }
6.     SubShader {
7.         Tags { "RenderType"="AlphaTest " }
8.         LOD 200
9.        
10.         CGPROGRAM
11.          #pragma surface surf Lambert alpha
12.  
13.         sampler2D _MainTex;
14.  
15.         struct Input {
16.             float2 uv_MainTex;
17.         };
18.         half _alpha;
19.         void surf (Input IN, inout SurfaceOutput o) {
20.             half4 c = tex2D (_MainTex, IN.uv_MainTex);
21.             o.Albedo = c.rgb;
22.             o.Alpha = _alpha;
23.             o.Emission = 0.1;
24.         }
25.         ENDCG
26.     }
27.     FallBack "Diffuse"
28. }
29.  

It has an alpha value and a MainTexture, both setted by code with an array of Texture2D and a float.

The problem is that back in Unity4 it had no problems, but now the Alpha value doesn't change anything.