I could not find any schematic of the module, so if you could link the one you find i will appreciate, thanks.
I'd like to remind you that, notwithstanding what's written in the tinker kit site, the transistor in my module it's a IRFS803 (
http://www.alldatasheet.com/datasheet-pdf/pdf/227583/IRF/IRFS3806PBF.html).I'm quite sure now that the problem was an insufficient gate voltage.
Did you measure Vgs while the mosfet was on?
add: that is the voltage from the gate pin to the source pin (ground) on the mosfet.
it was too difficult to measure the gate-source voltage, the circuit it's in a bad position and with only two hands i can't measure it

I presume the voltages you posted were supply voltage and voltage across the load. I can get the Vds from those.
14.8 - 14.3 = 0.5v
That is much higher than it should be at full on. That will generate some heat, even at 5 amps.
yeah it's too high, that makes 0,1 ohm of resistance, while it should be 0,015 if fully on. That was with 9 volte at gate, today I'll try 10 V (don't know how) and we will see.
The fact is that now it looks quite useless driving a 15 volt with 10 volt...if i want to control it with arduino i'll have to do multiple fets switch? looks boring

Thanks to everyone.