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Inside the rear diff lock switch

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  • Inside the rear diff lock switch

    The diff lock switch bezel in my 94 S4 V8 avant was not in the best of shape. I happended to find a good one from a 93 100q at a breaking yard during the week and I decided to buy it and install it in the 94 S4 (looks much better). As I did so, I got curious about the components inside the switch and I decided to see how easy it was to take the old one apart. Pretty easy if you go slow. The diff lock bezel map is just glued into the housing. You can start it from the back and then use something thin from the upper side to work it off slowly (don't force it, be gentle, but firm).

    The actual diff lock switch has four small tabs that lock it into the housing - push from the top and pull from the back.

    Here is what the diff lock switch looks like in Phase 1 of the parts breakdown:



    The second photo shows the black speck of crud that was bugging me yesterday. It's some kind of stray material that got stuck under the spray on glue that they must have used at the factory (and apparently Hans and Franz didn't catch it in quality control).

    Third photo shows the crud removed (gentle removal with the tip of a kitchen paring knife)





    Not one to let obsession for detail stand in the way of leisure time, I then decided to pull things apart further to find out what was really going on inside the switch assembly.

    First thing was to review what the electrical schematics say about the diff lock switch. This diagram was taken from my Rear Diff Lock 101 post



    So here is the resulting bulb inventory: 4 incandescents and one LED:



    Now a series of shots showing what you see from the top and what is going on below when you get in and start your rear diff lock equipped UrS (S2, RS2, UrS4 and 95 UrS6).

    First a shot of the actual diff lock switch. The interesting part (to me) is the upper "Diff" switch is just a spring loaded plunger that you push down and then release (not a toggle on/toggle off switch per se). As you push the "DIFF" plunger down, a tang on the inner side of the plunger moves a copper conductor spring over, resulting in a temporary completion of the Diff lock circuit which then activates the Diff lock and the Diff lock indicator and warning lights.



    When you first get in the car, the Diff lock switch will be completely dark. When you start the car, the red LED "DIFF" indictor is turned on, presumably so you can find it in the dark (??). It is the only lamp that is on. I presume that an LED was chosen for the illumination because it is on all the time the engine is on.



    When you turn the headlights (or just the Park lights) on, the ambient lighting bulbs come on:



    This is what is going on underneath the indicator map. (NOTE: I had to use two different lamp holders to show that there are two ambient light bulbs. In my case, each of the two lamp holders had one ambient lighting bulb burned out).

    Lamp Holder 1:



    Lamp Holder 2:



    Finally, when you push the DIFF lock switch plunger down, the switch contact is temporarily made and the diff lock control module locks the rear diff and powers the Diff lock engaged and Diff lock warning lights:



    And underneath, using Lamp Holder 2 with its burned out ambient light bulb, to show the diff lock indicator and warning bulbs being powered.



    Now to try EdiGreg's Plastidip trick on the switch bezel and seek out replacement bulbs for the burned out ambient bulbs.
    Last edited by UrS4boy; 26 May 2014, 02:41.
    RS2'd 93 UrS4 5 spd sedan
    94 UrS4 V8 6 spd manual avant

  • #2
    Another great write up!!!
    Audi Nut!!!

    Comment


    • #3
      Never ceases to amaze me such are his talents!
      96 URS6 plus speck saloon
      96 URS6 plus speck estate
      94 2.6 80 Avant
      99 2.8 final edition Cabriolet

      Comment


      • #4
        Originally posted by Vorsprung durch Technik View Post
        ... his talents...
        Yeh, good one.

        More like unhealthy obsession to document even the smallest minutia of C4 UrS existence using available resources.

        RS2'd 93 UrS4 5 spd sedan
        94 UrS4 V8 6 spd manual avant

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        • #5
          Perhaps you were a German scientist in another life such is your forensic endeavour. That's to be taken as a compliment as we all know they are very focused and technically gifted bunch!!
          96 URS6 plus speck saloon
          96 URS6 plus speck estate
          94 2.6 80 Avant
          99 2.8 final edition Cabriolet

          Comment


          • #6
            Jas11n included this in his list posts of "how tos" for bulb and LED substitution.

            Worth checking them out.

            John

            Comment


            • #7
              Originally posted by johne View Post
              Jas11n included this in his list posts of "how tos" for bulb and LED substitution. Worth checking them out. John
              Thanks for the lead. I did the search and found his thread, then I searched within the thread to find the diff lock bulb replacement.

              Found it on Pg. 10 of the thread Posts 148 and 149

              http://www.s2forum.com/forum/showthr...iff#post362619
              RS2'd 93 UrS4 5 spd sedan
              94 UrS4 V8 6 spd manual avant

              Comment


              • #8
                Any idea of a suitable replacement for bulb number 1 in your diagram? Is it a 12V bulb?

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                • #9
                  Originally posted by nesshead6 View Post
                  Any idea of a suitable replacement for bulb number 1 in your diagram? Is it a 12V bulb?
                  It's a 12V bulb. Not sure why it is flat topped in that location.

                  It's the one that is "on":

                  RS2'd 93 UrS4 5 spd sedan
                  94 UrS4 V8 6 spd manual avant

                  Comment


                  • #10
                    Diff lock

                    Hello, Great info here,thanks!

                    Can i ask pls?, I have just bought an s2 3b ,, My diff lock light does not light up when activated, any chance of pointing me in the right direction as to finding out if my diff lock does not work, or if it just the bulb i need to replace ? How to test the system pls?, Thanks.

                    Comment


                    • #11
                      Lockable Rear Differential Controls, the whys/hows for the 92-94 UrS4, 95 UrS6, S2 an

                      Originally posted by Mark s42 View Post
                      Hello, Great info here,thanks!Can i ask pls?, I have just bought an s2 3b ,, My diff lock light does not light up when activated, any chance of pointing me in the right direction as to finding out if my diff lock does not work, or if it just the bulb i need to replace ? How to test the system pls?, Thanks.
                      You will need to get the car up on a hoist and a friend to either push the button or observe the action of the diff lock actuator while you do the other.

                      Some reading:

                      Lockable Rear Differential Controls, the whys/hows for the 92-94 UrS4, 95 UrS6, S2 and RS2:
                      http://forums.quattroworld.com/s4s6/msgs/170940.phtml

                      RS2'd 93 UrS4 5 spd sedan
                      94 UrS4 V8 6 spd manual avant

                      Comment


                      • #12
                        Thanks.

                        Thankyou for the reply and the link to the further information, I should be able to get to the bottom of the problem now,regards .
                        Last edited by Mark s42; 26 June 2016, 22:11. Reason: no need to put my name

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                        • #13
                          Thanks for your help. I'm finally getting round to replacing a few blown dash lighting bulbs..

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