cek old password pada codeigniter

permisi gan saya mau bertanya untuk mengecek old password bagaimana di codeigniter saya coba gagal terus. kode my model untuk check password:

 public function pwd()
    {
        $nik=$this->session->userdata('id_doesn');

        $password=$this->input->post('pwd');
        $pwd = md5($password);

        $this->db->where('pass_dosen', $pwd);
        $this->db->where('id_dosen',$nik);
        $query = $this->db->get('dosen');
        if( $query->num_rows() > 0 )
            {
                return TRUE;
            } else
            {
                return FALSE;
            }
    }

my controller:

  public function check_password()
    {

        $result=$this->profile_model->pwd();
        if($result)
        {
            echo "true";
        }else{
            echo "false";
        }
    }

my view:

 <form class="cmxform form-horizontal tasi-form" id="DosenForm" method="post" action="<?php echo site_url('profile/updatepwd'); ?>">




                                      <input type="hidden" name="id_dosen" value="<?php echo $edit[0]->id_dosen; ?>">
                                      <div class="form-group ">
                                          <label for="password" class="control-label col-lg-2">Password Lama</label>
                                          <div class="col-lg-6">
                                              <input class="form-control " id="pwd" name="pwd" type="password" />
                                          </div>
                                      </div>
                                      <div class="form-group ">
                                          <label for="password" class="control-label col-lg-2">Password Baru</label>
                                          <div class="col-lg-6">
                                              <input class="form-control " id="password" name="password" type="password" />
                                          </div>
                                      </div>
                                      <div class="form-group ">
                                          <label for="confirm_password" class="control-label col-lg-2">Konfirm Password</label>
                                          <div class="col-lg-6">
                                              <input class="form-control "  name="confirm_password" type="password" />
                                          </div>
                                      </div>



                                      <div class="form-group">
                                          <div class="col-lg-offset-2 col-lg-10">
                                              <button class="btn btn-primary" type="submit">Update</button>
                                              <button class="btn btn-default" type="reset">Cancel</button>
                                          </div>
                                      </div>
                                  </form>

my javascript:

 $("#DosenForm").validate({
            rules: {
                nik: {
                    required: true,
                    minlength: 2
                },
                pwd: {
                    required: true,
                    minlength: 5,

                    remote: {
                            type: "post",
                            url: "<?php echo site_url('profile/check_password'); ?>",

                              data: {
                               pwd: function(){ return $("#pwd").val(); }

                                         }
                                   }

                },

salah dimana ya gan.