Mavlink cmd rc_channels_override and range value [1400-1500]

I have an application which communicates with the ardurover board through mavlink.
And the command rc_channels_override works but I am sure about its usage.

Channel 3 seams to be controlling the movement forward/backward.
The range of the parameter value is between 1000 and 2000 pwm, value 1500 pwm supposedly being the standing still value.

The robot needs to move slow and clearly I see that 1400 pwm is slower than 1000 pwm.
But any value in the range [1400-1500] is not accepted. The rover won’t move.
And 1400 is too fast for my needs.

Why isn’t a value in the range of [1400-1500] accepted?
How to get the robot move slow?

ps: before sending a rc_channels_override command, a arm_manual command is sent.

Its the ESC on the rover most probably. The ESC usually has a dead zone around middle/neutral/1500pwm which means it won’t move in that zone. If you graph the RC3_OUT (throttle) and your forward speed you can slowly increase the throttle and you will see what minimum PWM value it required before the rover moves.
I’d suggest reading through all the parameters
I don’t think you want rc_channel_override. I think you want to correctly set CRUISE_SPEED and let the APM figure out what is the best throttle value.

Thanks, Grant