well if you use a servo with those r/c cars some members mentioned that they can't get the servo to work at all or that it jitters. i encountered the same problems. that's because i assumed i would have to use the wires coming from the servo, that was the main problem. i opened the servo to gain access to the little motor inside of it, soldered two wires to it, same way as if it would be the motor in the r/c car. and bingo now the servo travels its full way in both directions smooth and without any problems. so i basically do not use the board inside the servo. i do not have full auto mode, but the servo will be installed in a way that the stalk can't go past the up position and going down you know when to stop, cause you'll see it going down.
the servo did not work because none of the r/c cars shown gave "pulses" to the servo, and the pulse wire "tells" the servo to move to the left or the right (up/down). And using the (+) and (-) wires of the servo only let the servo move in one direction.
so eliminating the supplied wires solved the problem. Hope this explains it well enough.
Besides that, I used a board from a r/c car that was advertised at 4.8 volts but it can easily take 6 volts. So the board and servo in my setup run of 6 volts, the sending unit on 9 volts.
