It sounds like something may be wrong with the throttle since its battery indicator lights do not come on and it only outputs 0.5 Volts at full throttle. The full throttle output Voltage should be around 4 Volts.
Output Voltages of +4.9 Volts should only be expected on the purple wire. The black wire is the negative wire and the blue wire is the throttle input signal wire.
The red, yellow, and green wires are for the throttle's LED indicator lights and it appears from your test that they switch on and off negative current which is interesting because we always presumed that they switched on and off positive current to control the illumination of the LED indicator lights.
We carry a replacement controller for 24 Volt scooters which is our item # SPD-SD500-6 on this page: https://electricscooterparts.com/speedcontrollers24volt.html
And if it is a 36 Volt scooter then the replacement controller is our item # SPD-36750B or SPD-SD1000-6 on this page: https://electricscooterparts.com/speedcontrollers36volt.html
Before replacing the controller we recommend load testing the battery pack to make sure it is in good condition. Testing the throttle's output signal Voltage to make sure that it is within specifications which is 1-4 Volts. And jumping the motor wires directly to the battery pack to make sure that the motor is working.
The throttle output signal test needs to be performed with a good battery pack, so test the battery pack first and if it needs replacement then do so before testing the throttle. To test the throttle signal plug the throttle into the controller and check the Voltage between the black and blue wires while moving the throttle from its zero to full throttle positions.
Hi Soura, Unless the axle bearings are extremely worn out, then the brake pads should not be able to contact the rotor by placing the go-kart on the ground - and even if they were that should not apply enough friction to stop the motor from spinning.
When the wheels turn in the air, however, will the wheels will not turn when placed on the ground that is usually caused by an old, worn out, or faulty battery pack - or possibly towards a problem with the battery charging system.
We recommend load testing the battery pack to verify its condition, and also testing the Voltage at the charger port and the battery charger's output plug to verify if these two parts of the charging system are working properly.
The charger port should have the same Voltage as the battery pack. However, if the charger port has any Voltage present at its leads then it can be presumed to be working. The battery charger should output around 28-29 Volts.
The battery pack should have around 27-28 Volts with no load and when under load its Voltage should not drop under 23 Volts.
We have a Battery pack Load Testing Guide to help with load testing the battery pack.