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Old 07-02-2016, 09:21 AM   #1
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Default Problem RPM signal MS2

I have received an MS2 from someone with a '92 1.6 NA MX5, which I'm trying to adapt for my '94 1.8 MX5. He gave it to me because there was a problem with it, it would get (almost) no RPM signal (apparently it would not get above 120 rpm on JimStim, no idea if it got any signal on the car). I am trying to check everything to see where the mistake might be, he used the MiataTurbo wiki page to build the MS2. So far, I have found 1 thing that looked interesting. The MS2 is a DIY unit from diyautotune, with a wiring harness to connect the DB37 on the MS2 to the ECU connector in the car. In this wiring harness is a protected pair used for the RPM signal, which includes a positive and negative lead, and shielding. On the wiki page I found this diagram:

However, the diyautotune harness uses this diagram, which shows a difference in the ground wires only:

Now my question. The negative of the CKP (or CPS or RPM signal) wires is connected on pin 1 in the DB37 connector, but it is not connected to the connector used to plug it into the cars harness. I would like to ask you if any of you know what should be done with this negative wire. Should it:
A ) not be connected at all since it is not used (which would mean the wiring is correct as it is).
B ) be connected to the same place as the other ground wires.
C ) be connected somewhere else.

I've searched just about everywhere but cannot find an answer. I'm hoping since it is your wiki that was used to construct the MS2 you might know what should be done with this wire.

Thank you for your support.
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Problem RPM signal MS2-90-93_miata_standalone_ms.jpg   Problem RPM signal MS2-mswiring.png  
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Old 07-05-2016, 02:33 PM   #2
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Pins 1, 2, and 7 through 19 are all grounds and all go to the same location on the PCB.
In the factory Miata wiring, the CKP / CMP "negative" signals are part of the sensor ground.
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Old 07-05-2016, 02:43 PM   #3
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So you are saying, even if I do not solder the wire coming from pin 1 to the ground, it will still be connected to the sensor ground through pin 7 which is soldered, because they connect to each other on the MS2 pcb?

Hmm, well, at least that eliminates one possibility, thank you.
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Old 07-05-2016, 03:30 PM   #4
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use the official DIYPNP documentation. Not random DIY MS2 documentation of the world's shittiest diagram or MS3 documentation.

This documents the assemble and difference between the connector board jumpers:
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Old 07-05-2016, 03:38 PM   #5
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Ok I will, thank you. Are there no differences between USDM and EUDM first gen MX5's/Miatas (concerning the ECU)?
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