PID Autotuning with Heatbed On
When autotuning is started with M303 command, firmware disables all others extruders and heatbed.
That way autotune tunes up with only one resistance heater (witch makes only small deviation of voltage ~ 11.8V).
In real print, when I use two extruders and heatbed, voltage drops as low as ~10V. Autotuned PID parameters do not allow heater to get close to desired temperature.
If I move temp limit to higher value, PID allows heater to heat more then in last step (but less then set limit).
This means that Auto PID tuning can not find correct values in case of higher resistance.
If you can allow at firmware side for heatbed and other exturders not to be disabled on autotuning, maybe I could get correct PID values.