Using Arduino Mega between motor & circuit

Please forgive me for not being able to explain my question in proper scientific words. Also I assume this is the correct thread.

I am planning on using an Arduino mega with a motor control shield between the motor & the original control board of a cheap remote control car.

The original control board receives the signal from the remote control(RF) & on the it passes the current to motor, I am going to bypass it with Arduino to use as a ESC to control the motor speed. It is a brushed DC motor.

The RC car works with a 8.4V battery.

So far I’ve learnt to write the program to control the motors using Arduino & motor shield, but I don’t see any way to apply a “if” condition when the RC control board power the wires to motor.

Is there any way to achieve this?

Attached a drawing of what I need.

Thanks in advance

That is very unclear. Are you saying that you want a DC motor to be controlled EITHER by the existing RC control board OR by a Mega and a motor shield?

If so, what condition is it that should decide which of those two things controls the motor at any particular time?

What is controlling the Mega and shield? Or is that just running a fixed program?

Please post the program that you have written to control the motor here.

Steve

Thank you for the reply. I attached a drawing of what I need. May be I am asking something stupid.
Sorry if I’m wasting your time

Are you saying you want to use the 8.4V from the RC receiver to power the motor but Arduino and shield to control the motor?

Yes.
The motor must be powered from 8.4V but the speed control should be done through Arduino. When I push the front knob of the remote, the original control panel in the car send power to the motor.
All I want is to add Arduino between that control panel and motor as a speed controller.

I know adding an ESC is much cheaper & easier but all I have is this Arduino & motor shield. So trying to figure out something with these.