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7A B2 Coupe GT swap - rpm signal

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  • 7A B2 Coupe GT swap - rpm signal

    Hi all,

    I have one question regarding 7A swap in my B2 Coupe GT.

    Does anybody know, what to use as RPM signal for tacho?

    Here you can see two pages from 7A wiring diagram. I see that on Audi B3 7A Gn/Bl wire goes to tachometer, but can't find where is that wire in engine compartment. If I follow diagram, that is connection 52 and on another page it leads to ignition distributor, but there is no wire color (or it is maybe Br/Ws on Hall sender?)

    page8.png

    page6.png

    Thanks all in advance.








    Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

  • #2
    It is not in the engine compartment as the signal is generated by the ECU
    Panthero Coupé quattro 20vt
    Indigo ABY coupé
    Imola B6 S4 Avant

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    • #3
      So, ECU generates RPM signal and send it to tachometer gauge?!?!

      B2 tacho used Ignition coil for rpm signal, so how can I solve this and what to use as rpm signal?
      Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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      • #4
        Connect it to your new coil - ve, assuming you are using the stock 7A it is likely it will just work.

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        • #5
          Yes, but where to connect on 7A coil? 7A ignition coil.jpg
          Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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          • #6
            I gave you that info in last post, the negative terminal.

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            • #7
              I just tried the negative terminal, nothing happened, but I found gn/bl wire that originally goes to B3 7A rpm gauge and connect it to my rpm gauge. It works but just to 1300rpm. If I put the gas and rev the engine, as soon as it gets about 1300rpm it drops to zero. When engine rpms fall below 1300rpm it works again. So at idle it works, but something "cut off" the signal above about 1300rpm?!?
              Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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              • #8
                The ecu generates the pulse for the tachometer and depends on the which ecu your using the early 2 pin or the later 4 pin. but you you'll need to use a pull up or pull down resister 10k. The early one uses the tachometer as fault and the later has a light with 2+2 fault pins

                Sent from my SM-M317F using Tapatalk

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                • #9
                  Great info

                  I have early one with 2 connectors.

                  So, what is the solution for this one I have?

                  Thanks.
                  Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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                  • #10
                    Have a look at this and have a go.

                    Sent from my SM-M317F using Tapatalk

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                    • #11
                      I just measured AC volts between pins 1 and 10 and it is 10V, as in service manual. Also, I tried to connect 9k resistor I have, between pin 10 and tachometer and everything is the same, rpm cut off after 1300rpm. Also, I noticed when I connected multimeter to measure volts between pins 1 and 10, that rpm works until 2000rpm and when I change on multimeter to measure DC that tachometer works until 3000rpm.

                      So, do you @trebormc have suggestion what to do next, you mentioned two ways to connect resistor?

                      Thanks
                      Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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                      • #12
                        I just found on some old forum, theme with Audi Coupe GT 7A swap, and the same problem with tachometer.

                        This is how he solved the problem:

                        "Used the tach signal from the 7A ecu, with a 220 Ohm voltage divider"

                        Does anybody know how to connect 220 Ohm voltage divider correctly?
                        Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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                        • #13
                          If you look on one of my old post I had the same problem when I fitted a aby system to my ur quattro that the tachometer hot to about 3000rpms then dropped to zero. I found info regarding a 10k resistor a cross 12v live and the the tachometer signal wire. This stops the signal wire floating. And this worked for me and still does.

                          Sent from my SM-M317F using Tapatalk

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                          • #14
                            Could you just explain how did you connect 10k resistor?
                            Audi URquattro 4.2l, Dodge Charger 7.3l, Golf 4 TDI 4Motion

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                            • #15
                              It's a good few years ago now but it I'll will of done it behind the clocks. With out looking it mite be the black/blue 12v+ one end of the resister and red/black/yellow tachometer signal. I think?

                              Sent from my SM-M317F using Tapatalk

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