hey i'm writing to anyone who might be able to help me i upgraded my kids car to use an 18v milwaukee battery and every time i try to use it in reverse i trips the fuse but in forward mode the wheels spin no issue forward or backwards when reversing the polarity it's only when the controller try's to go in reverse it's a weelye controller in the car i've added a dc speed controller a low voltage cut off to protect the battery and a buck transformer to make sure the cars only getting 12v i've trouble shoot everything and have narrowed it down to the controller for some reason when going in reverse the wheels start to spin but the amperage jumps over 30 amps blowing the fuse and when running properly in forward the wheels draw about 3 amps combined any idea on why this would happen and any idea on a part i could replace it with to stop it from happening thank you

 

obviously wrong wiring.

to reverse direction of DC brushed motor, you need to reverse polarity. this can be done different ways but tpo make it really robust and safe, you want to use pair of relays with form-C contacts (inverting contacts). the commons go to motor. and NO/NC go to power source.
To choose direction you can use three-pos toggle switch. advantage of this setup is that even if both relays are energized at the same time, there is no short circuit.

 

Really, the cause could be a zener diode inside the controller intended to limit the reverse speed. I would certainly include such a circuit in any kids car control I designed.
To the TS: try driving YOUR CAR at 30MPH in reverse! How rapidly control is lost!!!

 

Did the car originally have a reverse? Are you sure you don't have a back EMF protection diode across the motor?

 

IF the controller is feeding that device used to limit the voltage to 12volts, that device is not able to pass the reversed polarity and is reverse polarity protected.