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help connecting TDC & rpm sensors

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  • help connecting TDC & rpm sensors

    there are 2 sensors on the flywheel housing, rpm & TDC which together with a 3rd one whose function I do not know are connected to the DME through 3 connectors above the brake booster

    Can someone help me which sensor goes to which connector?


  • #2
    One sensor has a red plastic band around it (at the connector end) and one has a gray plastic band around it. The third sensor doesn't have a plastic band.

    If you look at where the sensors connect, you'll see a connector that has a red plastic band around it. The other 2 connectors don't have a colored plastic band, but you'll notice that the plastic that the female end is made of is made of gray plastic (the other 2 are made from black plastic).

    The sensor with the red plastic band connects to the connector with the red plastic band. The sensor with the gray plastic band connects to the connector that is made from gray plastic.
    BTW, the "mystery" sensor is a TDC sensor that outputs to the diagnostic connector. You can even omit the sensor as it is not used by the ECU.


    • #3
      Colin, thanks for your reply.

      My problem is that the original sensors were removed to be replaced by an electrician who forgot to mark which was which and as the two main sensors are the same I am not sure how to connect them. I mean, i know which sensor goes to which connector but I do not know which sensor goes to the rpm and which to the TDC hole in the flywheel housing

      By looking them I see the one with the red band is the 'aux' one, so now i have only 2 possible ways to connect them (as opposed to 6 before reading your reply)

      Problem is I am having an electrical problem (no spark no injection) problem, so having one extra variable is not going to help me to troubleshoot it.

      thanx again


      • #4
        I bought new sensors, after seeing my friend go through a no-start problem with a Porsche 944 in my driveway. The cause was a faulty TDC sensor.

        Anyway, the new sensors have no bands on them, and I am wiring an engine essentially from scratch. I think I can narrow down which sensor goes where on the wiring harness by tracing it to the DME connector, but what I don't know is, which sensor goes where?

        It sort of makes common sense to me that the cylinder ID sensor is the one on the bottom and the TDC sensor is on the top of the boss provided on the bellhousing, but can anyone verify this for me? TIA.


        • #5
          The little sensor only goes to the diagnostic port - you can delete it and not worry about it.

          If you are not sure which of the black plugs the TDC and RPM sensors go to, there are only two possible combinations. The engine will start and run if they are connected properly. If you mix them up the engine won't run. Try it one way; if the engine runs, you got it right. If it doesn't, swap them.

          You also need to make sure they are plugged in all the way or else the engine won't run either, even if they are on the right plug.



          • #6
            Oops, I got a little mixed up on sensor functionality / terminology.

            Thanks for answering, marques.

            I meant cylinder ID and engine speed sensors, the ones that are paired together on the bellhousing as you deduced.

            I think the sensor positions might be the other way around than I originally posited. It would make sense to me that the engine speed can be sampled better if there are 3 pulses per crank revolution, and a cylinder ID would be easiest to determine if the pulse occurred once per crank revolution.


            • #7
              The reference sensor, or TDC sensor is triggered once per revolution. It lets the computer know when piston #1 is at TDC (minus some offset to allow timing advance). It sits a little further back on the bellhousing.

              The rpm sensor is triggered a lot more than 3 times/revolution. It actually counts every side of every tooth in the starter ring gear, so twice the number of teeth per revolution. It's the one closest to the engine.



              • #8
                ahhh ok. Thank you!