Call to a member function get_col() on a non-object jQuery WordPress

I have a number of php files in my WordPress plugin the main file I am having issues with has a class in it and I can call one method from it as my display page loads and that works fine but when I try and do it from a third file from my jQuery post I get the error sated in the title of my question. The error is said to be in the file with the class but I can’t see an error. I have made sure that global $wpdb has been added to every function so and the first function works to get the track names but doesn’t if I call it again through my last code as a test. Any help would be appreciated.

Class file

<?php
class SelectList
{
public $tableName;
public $driverTableName;
public $classTableName;
public $posTableName;
public $trackTableName;

public function __construct()
{
    global $wpdb;
     $tableName = $wpdb->prefix . "raceresults";
     $driverTableName = $wpdb->prefix . "driverData";
     $classTableName = $wpdb->prefix . "classData";
     $posTableName = $wpdb->prefix . "posData";
     $trackTableName = $wpdb->prefix . "trackData";
}

public function ShowTrack()
{
    global $wpdb;
    $category = '<option value="0">choose...</option>';
    $postids = $wpdb->get_col("SELECT trackName FROM wp_trackData;");
    foreach ($postids as $value) 
    {
        $category .= '<option value="' . $value . '">' . $value . '</option>' ;
    }
    return $category;
}

public function ShowDate($track)
{
    global $wpdb;
    $type = '<option value="0">choose...</option>';
    $postids2 = $wpdb->get_col("SELECT DISTINCT raceDate FROM wp_raceresults WHERE trackName = '" . $track . "';");
    foreach ($postids2 as $value2) 
    {
        $type .= "<option>" . $value2 . "</option>"; 
    }
    return $type;
}
}

$opt = new SelectList();

jQuery Code

jQuery(document).ready(function()
{
    jQuery("select#kDate").attr("disabled","disabled");
    jQuery("select#kTrack").change(function()
    {
        jQuery("select#kDate").attr("disabled","disabled");
        jQuery("select#kDate").html("<option>wait...</option>");
        var id = jQuery("select#kTrack option:selected").attr('value');

        jQuery.post("<?php echo plugins_url("/race-results/getResults.php"); ?>", {id:id} , function(data)
        {
            jQuery("select#kDate").removeAttr("disabled");
            jQuery("select#kDate").html(data);
        })
        .success(function() { alert("success"); })
        .error(function(xhr, status, detail) { alert("error ("+status+") : " + detail); })
     });
  });

Get Result

<?php
include "popDrp.php";
echo $opt->ShowDate($_POST[id]);
?>