E-revo problem

Hey guys..

I just bought an used e-revo, but now, after clean it a litle bit, I noticed that when the motor spins, each differential spins the wheels in opposite directions..

I mean... if the front wheels spins to the front, the rear one will be spinning to the rear..

I'm a noob, but I'd like to know where is tho problem, so i dont need to diassemble it completly... Any Idea?

PS. sorry about the bat english, but i'm Portuguese...

Thanks