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Old 11-20-2010, 02:31 AM   #1
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Default Lets Talk Miata Tach signals

Ok I have a 92 miata with a inline 6 cyl. I had a shift light hooked up in my car before with no problems and the tach worked before I did the swap. My problem now is trying to figure out what type of signal the miata tach uses and do I need a tach adapter.


My new engine has a distributor and uses a 12V tach signal.

I do not know what type of signal the miata stock tach uses. (92 Miata)

Do I need a tach converter or something to make my tach work with the new engine?

I know the NB has a serial link connector and the ecu converts the signal to an old style 12V before it goes to the tach but is it the same on a NA.


I just need to know what I should do to hook a 12V distributor tach signal to a stock miata tach.


Any help would be great I am really confused on this one.
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Old 11-20-2010, 02:55 AM   #2
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Old 11-20-2010, 03:29 AM   #3
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Ok Ill verify that the supra is the same and go from there. Thanks
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Old 11-20-2010, 08:22 PM   #4
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Ok so the toyota uses a 5v sqaure wave does anyone know if there is some sort of converter for this or should I just deal with it.
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Old 11-20-2010, 08:42 PM   #5
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Should be able to just use a FET. Someone else would have to verify for me, though. I'm not sure how much it would distort the signal.
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Old 11-20-2010, 11:01 PM   #6
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i had same problem adapting my nissan sr20det to aim dash. sr20 output 0-5v square. aim wants 0-12v. i used a smallish pullup resister (47k) to fix. i watched the waveform on the scope and 47k was enough to give nice 0-12v square wave. i didn't want to go too low ohms so as not to pull too much current through the sr20 ecu. i would guess same applies to the toyota ecu.
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Old 11-20-2010, 11:46 PM   #7
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Ok I just thought of something as well. The signal out of the ecu is a 5v but if I pick up my tach signal off the gound terminal of the coil and see if that works if not MSD makes a tach adapter I can pick up used for like $35 and I will go that route. I would try putting in a resistor but I do not have a scope I can look at the signal with. That would make this a whole lot easier.
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Old 11-21-2010, 12:20 AM   #8
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just put a 47k on it and see if it works. didn't mean to imply u had to check. i got 10 bux that says it will work.
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Old 11-21-2010, 12:22 PM   #9
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Ok I will try that first I imagine it will work. I will see if I can pick up a 47K pullup resistor next week and see if that works. Thanks for the input
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Old 11-22-2010, 10:29 AM   #10
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Ok so I had the guys at my work help me with the math and its seems to all check out right on the money. Like I said before I do not have a scope but with the math in order and two resistors in hand I think everything will be ok. So after the holidays I will wire it all in and see how it works.

jasonb- thank you for your help I think you just saved me $80 by not ordering the autometer adapter.
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Old 11-22-2010, 10:52 AM   #11
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OK last thing until I do it and see if it works here is a super basic drawing of what I am going to do to wire this resistor in can anyone verify that it seems correct or if I am going to do something wrong.

Name:  Miatatachwiring.jpg
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Old 11-22-2010, 11:17 AM   #12
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I wouldn't use that circuit without knowing more about how the ECU's tach output is implemented.
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Old 11-22-2010, 12:17 PM   #13
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What do I need to know about the ecu tach output I have access to a fatory wiring book and it can tell me almost anything I need to know.
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Old 11-22-2010, 12:23 PM   #14
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Quote:
Originally Posted by Sean View Post
OK last thing until I do it and see if it works here is a super basic drawing of what I am going to do to wire this resistor in can anyone verify that it seems correct or if I am going to do something wrong.

looks good to me. the only thing i would suggest is to use a switched 12v if you have it (i used 12v switched supplied by my dash itself). that way it floats if the ecu is off, and is 12v when the ecu/tach are powered.

Quote:
Originally Posted by kday
I wouldn't use that circuit without knowing more about how the ECU's tach output is implemented.
por que? if u run the numbers the current the ecu has to sink to pull the 47k to 0v is tiny.
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Old 11-22-2010, 12:48 PM   #15
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Ok that is what I though and according to what I got it is about .25mV or under 1 watt power returning to the ecu I don't see how this would hurt anything. I am going to check in my book to see the circuit that the tach signal is on but I think it is ok. Now mind you I am not really good on the whole computer internals stuff but I have some really good helpers that say it will be fine as well.
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