Sorry, I missed this information from your earlier post. Since you are measuring -12V you have your test leads backwards with the red lead on ground (black wires).
Why you have a reading of 12V...
12V is the source voltage for the circuit, however there is a current limiting resistor on the opto board that drops the forward voltage going to the IR LED to roughly 1.4V (there are many variables); this is a good thing as the maximum rated voltage for this specific part is 1.7V.
There is only two things that can cause this reading...
1) The IR LED is blown open ... this is an extremely rare failure mode for a diode.
2) The diode is installed backwards or miswired placing the positive voltage on cathode and ground on the anode resulting in a reverse biased condition preventing any current flow through the diode.
I can see from your posted image that the second condition is true as you clearly have the black wires (ground) on the anode (A) and the source voltage (current limited 12V) on the cathode (K). Reverse the positions of these two wires and it should work fine.