Hi, can you draw a circuit diagram of what you have in the image you posted. A pic of a hand drawn circuit in jpg will be enough if you do not have a CAD.
It doesn't look as though you have the pinouts correct.
The protection diode is in the wrong place and wrong configuration.
They are darlington pair transistors and if you read the data sheets you will find that you will loose 2V collector to emitter when they are turned ON.
Tom...... 
https://www.fairchildsemi.com/datasheets/TI/TIP120.pdf
https://www.fairchildsemi.com/datasheets/TI/TIP125.pdf