WPF combo box update source trigger

Question

I've added triggers for combobox, i.e.

• When combo box is 'Disabled' then 'Text' property should set to '1' and added updatesourcetrigger to property changed.

• When combo box is 'Enabled' then 'Text' property should be selected one and added update source trigger to property changed.

But the problem is I'm unable to call the update source trigger when combo box is Disabled and 'Text' property is set to '1'.

Below is the XAML code snippet:

<ComboBox Name="cmbIntervals"
                        Grid.Row="5"
                        Grid.Column="1"
                        Width="150"
                        HorizontalAlignment="Left"
                        VerticalAlignment="Top"
                        IsEnabled="{Binding ElementName=chkRollingInterval,
                                              Path=IsChecked}"
                        ItemsSource="{Binding Source={x:Static es:MasterParameters.Instance},
                                                Path=NoOfIntervals}" 
                        Tag="{Binding Path=[NoOfIntervals], Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                        >
                  <ComboBox.Style>
                    <Style TargetType="{x:Type ComboBox}">
                      <Style.Triggers>
                        <Trigger Property="IsEnabled" Value="False">
                        <Setter Property="Text" Value="1"/>
                        <Setter Property="Text" Value="{Binding RelativeSource={RelativeSource Self}, Path=Tag, Mode=TwoWay, UpdateSourceTrigger=Default}"/>
                      </Trigger>
                      <Trigger Property="IsEnabled" Value="True">
                        <Setter Property="Text" Value="{Binding RelativeSource={RelativeSource Self}, Path=Tag, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
                      </Trigger>
                      </Style.Triggers>
                   </Style>
                </ComboBox.Style>
              </ComboBox>

From above XAML code when ever i updated 'Text' property to '1' for combo box 'Disabeld' mode i need to update this '1' value to the source property i.e. 'Tag' and from there to 'NoOfIntervals' but it doesn't happening.

Thanks, Nag