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I don't see what the problem could be. All my other motors work, extruder and hotend + hotend thermistor ... But my head bed LED never lights up and it never gets warm (looking at it from Repetier and touching it). What should I do?

Does the light come on on the Rambo board?
Does it read a reasonable temperature at room temperature?

You also have to have the heated bed enabled in the firmware. Which firmware and where did you get it?

I'm using the firmware from https://github.com/johnoly99/Marlin-for ... kmax-rambo

Here's some of my config...

#define TEMP_SENSOR_0 5
#define TEMP_SENSOR_1 0
#define TEMP_SENSOR_2 0
#define TEMP_SENSOR_BED 5
#define HEATER_0_MINTEMP 3
#define HEATER_1_MINTEMP 3
#define HEATER_2_MINTEMP 3
#define BED_MINTEMP 3
#define HEATER_0_MAXTEMP 275
#define HEATER_1_MAXTEMP 275
#define HEATER_2_MAXTEMP 275
#define BED_MAXTEMP 140

I'm not sure if I need to mention that I have the RAMBo board, but I do.

I never get a light, my temp never goes above 0.

untitled86 wrote:I never get a light, my temp never goes above 0.

If the temperature doesn't read correctly, you need to resolve that first.
Most likely you have a wiring problem for the thermister, which socket do you have it plugged into?
It should be the 3rd one from the top.
If that is correct, disconnect it from the board and measure the resistance with a meter, it should be about 110KOhms, if it's not you likely have a short somewhere. It is extremely easy to short the thermister legs on the inside of the hole in the heated bed.

I am having a similar issue with my Onyx board not working. I recently bought a 24v power supply for the heatbed. When I plug the Onyx into the heatbed connector on the Rambo board, the red light on the Rambo board turns off and no power goes to the heatbed. I tested the connector on the Rambo board to see if it was providing power at the connector, and it was. Could I have possibly reversed the polarity on the heatbed connector and blown a fuse or something???

Polarity is not important, only for the LED.

What fuse are you using?