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RPM needle is dead!

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  • RPM needle is dead!

    The needle on the tachometer of my 90 quattro 20v just died, I bought the car a year ago with only 50Kmiles and since I got it the tachometer needle usualy gave 500rpm higher reading for the first 2/3 miles and then worked ok but today when I started the car after standing for 2 weeks the needle just didn´t work, I just moved it from one place to another half a mile away and it might come alive if I drive it a little longer but if it doesn´t what should I check first?

    Thanks!

  • #2
    I have found this to be burnt/broken/corroded tracks on a very small potentiometer on the tacho head itself. I have fixed it for a while in the past with some electroclean, and wiggling it, but it failed again, I have no moved it to a different spot on the track, and seems to be holding up OK, it reads about 200rpm out at the top of the range (low) but it works, which is better than I have had for years....

    While in there I also hit any solder joints that looked ropey, including the clock power, which has fixed the clock that would turn off and on (resetting the time) going over a certain kind of bump...

    You do need to remove the clocks, remove the autocheck, and remove the circuit board from the binnacle, and the tacho from it's case to do this, but it is all straightforward

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    • #3
      Thanks for the reply, I´ll remove the clocks and have a look!

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      • #4
        Perhaps a more permanent fix would be to unsolder the pot, measure it's value and the tap point and replace with a cermet type, which is more stable and usually sealed against the environment. Carbon track pots are rubbish really and only used for cheapness. Edit: Silicon spray the whole board before putting it back, to guard against further corrosion...

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        • #5
          Please take some photos when you are in there to let us all see where the problem is.
          1996 S2 ABY Coupe silver
          2003 S3 8L dolphin grey
          2014 RS4 B8 prism silver

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          • #6
            Originally posted by Tridac View Post
            Perhaps a more permanent fix would be to unsolder the pot, measure it's value and the tap point and replace with a cermet type, which is more stable and usually sealed against the environment. Carbon track pots are rubbish really and only used for cheapness. Edit: Silicon spray the whole board before putting it back, to guard against further corrosion...
            Absolutely right, in fact mine is sitting at one end of the scale so may even be zero = a piece of wire(!)

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            • #7
              or just replace the rpm guage. get a spare set of clocks. worth it in the long run. user for parts or replacment

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              • #8
                Started the car today to put it in the garage to take the clocks out for repair and the tachometer is working again! nice!

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                • #9
                  Yeah, mine's done that for years! Working properly now

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                  • #10
                    Originally posted by Tridac View Post
                    Perhaps a more permanent fix would be to unsolder the pot, measure it's value and the tap point and replace with a cermet type, which is more stable and usually sealed against the environment. Carbon track pots are rubbish really and only used for cheapness. Edit: Silicon spray the whole board before putting it back, to guard against further corrosion...
                    The pot resistance value might well be stamped on it.

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                    • #11
                      Almost certainly, if I have it apart again, will report back

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