Change of motors

OK so I built the Mendel using 17HS19-1684S stepper motors for all 3 axis and extruder. I then purchased the tricolour upgrade kit which came with different smaller motors (JK42HS34-1334A).
I decided to buy an extra motor to replace the original extruder motor so all 3 would be the same.
Now when I switch on the Mendel the extruder motor clicks away merrily to itself (which it shouldn't do) so I turned down the current to it until it stops but when I try to extrude the motor just vibrates!
Am I missing something? Is there a difference between the 2 motors?

Oh hang on I think the wiring is different! OOPS!