how to setup wheeljoint2D motor by code?

Hello all!

i'm trying to setup simple 2d car physics. i have a car with two WheelJoint2D component. each one of them hold one wheel. every thing is working fine beside the fact that i can't setup the motor by code (C#) here my simple script that should set the motor to -1000 and make the car jump with space key. (ignore that fact that it will jump once)

 using UnityEngine;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour {
 
     private bool isGrounded= true;
 
     public float Power;
     public float speed;
     public WheelJoint2D wheelj1; // draged to this one the front wheel WheelJoint2D
     public WheelJoint2D wheelj2; // draged to this one the back wheel WheelJoint2D
 
     public JointMotor2D wheelj1jm;
     public JointMotor2D wheelj2jm;
 
 
     // Use this for initialization
     void Start () {
         speed = -1000f;
         wheelj1jm = wheelj1.motor;
         wheelj2jm = wheelj2.motor;
 
         wheelj1.useMotor=true;
         wheelj1jm.motorSpeed = speed; // try to setup the motor speed - nothing happen!
     
     }
     
     // Update is called once per frame
     void FixedUpdate () {
 
 
         if(Input.GetKey(KeyCode.Space) && isGrounded ){
             this.rigidbody2D.AddForce(transform.up * Power);
             isGrounded=false;
         }
     
     }
 }
 

i tried to find some sample code for this but couldn't find any. i don't get any error but the motor don't get i value -1000. what am i doing wrong? thanks!