My vehicle keep on reversing none stop even when i stop pressing S key with wheel collider.

hi i am new to unity and i have no idea how there code causes my vehicle in the game to non stop reversing when i press S key. is not entirely non stop, but it reverse for a very long time only stop. enter code here

 /*    
  * This Script Controls the car!
  */
 using System.Collections;
 using System;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class LPPV_CarController : MonoBehaviour
 {
 
 
     public enum WheelType
     {
         FrontLeft, FrontRight, RearLeft, RearRight
     }
     public enum SpeedUnit
     {
         Imperial, Metric
     }
     [Serializable]
     class Wheel
     {
         public WheelCollider collider;
         public GameObject mesh;
         public WheelType wheelType;
     }
 
     [SerializeField] private Wheel[] wheels = new Wheel[4];
     [SerializeField] private float maxTorque = 2000f, maxBrakeTorque = 500f, maxSteerAngle = 30f; //max Torque of Wheels, max Brake Torque and Maximum Steer Angle of steerable wheels
     [SerializeField] private static int NoOfGears = 5;
     [SerializeField] private float downForce = 100f; // The Force to apply downwards so that car stays on track!
     [SerializeField] private SpeedUnit speedUnit;   //Speed Unit - Imperial - Miles Per Hour, Metric - Kilometers Per Hour
     [SerializeField] private float topSpeed = 140;
     [SerializeField] private Transform centerOfMass;
     [SerializeField] private Text speedText;
 
 #if UNITY_ANDROID || UNITY_IOS
     [SerializeField] private LPPV_VButton accelerateButton, brakeButton, handBrakeButton;
 #endif
 
     [HideInInspector] public bool Accelerating = false, Deccelerating = false, HandBrake = false;
     private Rigidbody _rgbd;
     public float CurrentSpeed
     {
         get
         {
             float speed = _rgbd.velocity.magnitude;
             if (speedUnit == SpeedUnit.Imperial)
                 speed *= 2.23693629f;
             else
                 speed *= 3.6f;
             return speed;
         }
     }
 
     private void VisualizeWheel(Wheel wheel)
     {
         //This method copies the position and rotation of the wheel collider and pastes it to the corresponding mesh!
         if (wheel.mesh == null || wheel.collider == null)
             return;
 
         Vector3 position;
         Quaternion rotation;
         wheel.collider.GetWorldPose(out position, out rotation);    //Fetch the position and Rotation from the WheelCollider into temporary variables
 
         wheel.mesh.transform.position = position;
         wheel.mesh.transform.rotation = rotation;
     }
 
     private void Start()
     {
         _rgbd = GetComponent<Rigidbody>();
         if (centerOfMass != null && _rgbd != null)
             _rgbd.centerOfMass = centerOfMass.localPosition;
         for (int i = 0; i < wheels.Length; ++i)
             VisualizeWheel(wheels[i]);
     }
 
     private void Move(float motorInput, float steerInput, bool handBrake)
     {
         HandBrake = handBrake;
         motorInput = Mathf.Clamp(motorInput, -1f, 1f);
         if (motorInput > 0f)
         {
             //Accelerate vehicle!
             Accelerating = true;
             Deccelerating = false;
         }
         else if (motorInput < 0f)
         {
             //Brake Vehicle!
             Accelerating = false;
             Deccelerating = true;
         }
 
         steerInput = Mathf.Clamp(steerInput, -1f, 1f);
         float steer = steerInput * maxSteerAngle;
 
         for (int i = 0; i < wheels.Length; ++i)
         {
             if (wheels[i].collider == null)
                 break;
             if (wheels[i].wheelType == WheelType.FrontLeft || wheels[i].wheelType == WheelType.FrontRight)
             {
                 wheels[i].collider.steerAngle = steer;
 
             }
             if (!handBrake)
             {
                 wheels[i].collider.brakeTorque = 0f;
                 if (Accelerating)
                     wheels[i].collider.motorTorque = motorInput * maxTorque / 4f;
                 else if (Deccelerating)
                     wheels[i].collider.motorTorque = motorInput * maxBrakeTorque / 4f;
             }
             else
             {
                 if (wheels[i].wheelType == WheelType.RearLeft || wheels[i].wheelType == WheelType.RearRight)
                     wheels[i].collider.brakeTorque = maxBrakeTorque * 20f;
                 wheels[i].collider.motorTorque = 0f;
             }
             if (wheels[i].mesh != null)
                 VisualizeWheel(wheels[i]);
         }
 
         StickToTheGround();
         ManageSpeed();
     }
 
     //This Method Applies down force so that car grips the ground strongly
     private void StickToTheGround()
     {
         if (wheels[0].collider == null)
             return;
         wheels[0].collider.attachedRigidbody.AddForce(-transform.up * downForce * wheels[0].collider.attachedRigidbody.velocity.magnitude);
     }
 
 
     //used to keep speed withing Minimum and maximum Speed Limits
     private void ManageSpeed()
     {
         float speed = _rgbd.velocity.magnitude;
         switch (speedUnit)
         {
             case SpeedUnit.Imperial:
                 speed *= 2.23693629f;
                 if (speed > topSpeed)
                     _rgbd.velocity = (topSpeed / 2.23693629f) * _rgbd.velocity.normalized;
                 break;
 
             case SpeedUnit.Metric:
                 speed *= 3.6f;
                 if (speed > topSpeed)
                     _rgbd.velocity = (topSpeed / 3.6f) * _rgbd.velocity.normalized;
                 break;
         }
     }
 
     private string imp = " MPH", met = " KPH";
     private void Update()
     {
         if (speedText != null)
         {
             if (speedUnit == SpeedUnit.Imperial)
                 speedText.text = ((int)CurrentSpeed).ToString() + imp;
             else
                 speedText.text = ((int)CurrentSpeed).ToString() + met;
         }
     }
     private void FixedUpdate()
     {
         float motor = 0f, steering = 0f;
         bool handBrakeInput = false;
 #if UNITY_STANDALONE || UNITY_WEBGL
             motor = Input.GetAxis("Vertical");
             steering = Input.GetAxis("Horizontal");
             handBrakeInput = Input.GetButton ("Jump");
 #endif
 
 #if UNITY_ANDROID || UNITY_IOS
         steering = Input.acceleration.x;
         if(accelerateButton != null)
         {
             if(accelerateButton.value)
                 motor = 1f;
         }
         if(brakeButton != null)
         {
             if(brakeButton.value)
                 motor = -1f;
         }
         if(handBrakeButton != null)
         {
             handBrakeInput = handBrakeButton.value;
             if(handBrakeButton.value)
             {
                 motor = 0f;
             }
         }
 #endif
         Move(motor, steering, handBrakeInput);
     }
 
 }