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1G Autometer Tachometer Install

Posted by DirtyBirdRacing, Jul 5, 2020

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  1. DirtyBirdRacing

    DirtyBirdRacing Previously 123bobby123

    760
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    Joined Nov 25, 2002
    Chicago, Illinois
    These past few months on my track car I have removed about two rubbermaid containers of wiring and saved 60+ pounds on unnecessary wiring, dash, instrument cluster, etc. However I have had one issue I haven't been able to fix and I have searched everywhere I could on how to fix it and none of those fixes worked for me. Hopefully someone here can help me.

    Car is a 1991 TSi AWD, running 300M COP with the 1991 PTU and running ECMLink V3 (so stock ECM/MPI relay setup). I have the Autometer 3904 Tachometer and a voltage gauge (16V system) in place of all the stock instrument cluster stuff.

    I first tried hooking up the tach signal wire (pin 4 PTU, pin 109 ecu) directly to the autometer tach signal wire. This didn't work as I think that output is a 5V square wave and this tachometer wants a 12V square wave. From what I read some people use a pull up resister to do this but I couldn't find the value for the resistor needed so I went down the route of getting a MSD 8918. From what I understand a MSD 8918 and Autometer 9117 will do what I need, but the MSD 8918 doesn't require splicing (clamps around wire) and I wanted that as I couldn't find consensus on what wire to tap into.

    I have tried the wire mentioned above, CAS sensor (ECU pins 21 and 22), MPI relay pin 3, ECU pin 54 and 55 (coils on PTU), and even a fuel injector wire as a last gasp, LOL. All these hook ups were at or near the ECM.

    I didn't get anything on any of these, any thoughts? The ECM engine speed reads just fine in logs.

    Two interesting things I noticed:
    -When I tapped my screwdriver up against the pick up on some wires the tach needle would bounce with my tapping.
    -When hooked up to the MPI relay pin 3 (black-white wire) the tachometer showed 1000 RPM during cranking and when the engine transitioned to start it dropped to zero, so I think I got everything else wired up right in terms of good power and good ground.

    Thanks in advance.
     

    Drag Race Build 193  1

    1991 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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  2. 1990TSIAWDTALON

    1990TSIAWDTALON Moderator

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    Independence, Kansas
    Most are able to use Pin 21 for aftermarket tachometers. I run a stock dash and tap into the back of my dash at the "tach" spot on the back of the gauge cluster. I am not positive where that comes from but I use it for my Smartshift 100.1 shift box and it reads rpm's and reads correctly.
     

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    13.620 @ 108.460 · 1G DSM
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  3. DirtyBirdRacing

    DirtyBirdRacing Previously 123bobby123

    760
    10
    Joined Nov 25, 2002
    Chicago, Illinois
    Thanks for the info. I tried that pin 21 (black wire from the CAS) while running this weekend and that is where it is currently hooked up. Didn't work for me. I'm starting to think this 5" tach I got with an old car that I parted out isn't working correctly.

    I looked at the back of the gauge cluster I took out and it looks like D-05 connector pin 12 (white wire) connects to the tachometer circuitry signal input via that "tach" screw you mentioned. I had the tachometer hooked up to this white wire the first time around as I read this works for some people, but didn't work for me. That was before I tried with the MSD pick up. So if you tapped in there (or via the white wire like i did) you should get a 5V square wave signal as far as I know. I looked up that shift box and I think this aligns with the tachometer input signal that box needs. Can you confirm that by chance?

    Also, any idea if it matters that I hook up that MSD 8918 pick up near the ECM versus near the CAS? I figured that since its all the same wire it would be okay but I don't know if it being far form the source (CAS, PTU) would affect the signal or being around all the other wires by the ECM would affect it.

    I'll reach out to autometer again and probably try to pick up a cheap tachometer that takes a 12V square signal wave to test if its just a bad tachometer as that will probably be a fraction of the cost of a good oscilloscope, LOL.
     

    Drag Race Build 193  1

    1991 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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  4. DSMPT

    DSMPT Proven Member

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    Joined Jun 12, 2014
    Japan / Mexico, Arizona
    Do you still have the stock tachometer in the cluster? If it's still working, try to cut or remove the signal wire to the stock tachometer and see if the Autometer tachometer work by taking the signal from ECU pin #109 or PTU #4. I have an experience that more than 2 devices couldn't share the tach signal.
    Also you can try ECU pin #51. Some RPM gauge works with that.
     

    Drag Race Build 2K  3

    1990 Mitsubishi Eclipse GSX
    awd · manual · 1G DSM

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    1998 Mitsubishi Eclipse GSX
    awd · automatic · 2G DSM
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  5. DirtyBirdRacing

    DirtyBirdRacing Previously 123bobby123

    760
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    Joined Nov 25, 2002
    Chicago, Illinois
    Thanks for the input. I don't run the stock gauge cluster. I tried to route the tach signal that goes into the stock gauge cluster into the autometer gauge but that didn't work. ECU pin #109, PTU pin #4, the tach signal wire, and the tach signal screw on the back of the gauge cluster are all the same circuit. I did have PTU pin #4, ECU pin #109, and autometer gauge on the same circuit before so maybe you are right about two devices being on the same circuit being an issue. I had an issue like this with the 2g electronic VSS I swapped into my car. I couldn't get the speed logged in the ECM until I cut the wire to the stock instrument panel (if memory serves regarding that it was VSS, I definitely run into this before though). I wanted to still run the PTU pin #4 input into the ECU for misfire detection in case I needed that in the future. I also found pretty strong evidence that the PTU pin #4 circuit is a 5V square wave setup and the autometer gauge I have needs a 12V square wave setup (hence why I needed the MSD pick up to convert the signal).

    Anyway, I got it figured out. Attached are the wiring notes for the 300M COP setup that I have on my car. I am not sure how others wire up their COP setups so I wanted to include that so you can compare.

    5141EA9E-0A9C-45C4-9482-D3ACEDEEB069.jpeg A0EB30D3-611F-41FB-B5DD-B3359D5B3A53_1_201_a.jpeg

    I installed my MSD 8918 pick up on the yellow-green wire going to pin B on my COP plug. This is the signal wire for the cylinder 4 & 1 ignition coil if you were running that stock setup. This gave me an RPM signal on my autometer gauge, but it was about half of what ecmlink said. So this was an improvement and it made sense that I was reading off by a factor of 2 as my tachometer is only registering half of the cylinder firing events.

    I can not adjust pips per revolution on this gauge to fix this issue so I needed to find a way to double this pip per revolution signal I was reading to get the correct RPM on my gauge. After looking at the factory wiring diagrams I saw that both transistors in the PTU for both stock ignition coils (or in my case both sets of COP) grounded through the same PTU pin, pin #3 with a black wire. It turns out that these transistors flicker the power flowing to ground with each spark plug firing, with all 4 cylinders ignition events grounding through that one wire, so hooking up my pick up their worked great. The autometer gauge now matches the ECM link engine speed channel within less than 100 rpm or so all over the rev range.
     

    Drag Race Build 193  1

    1991 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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    DSMPT and 1990TSIAWDTALON like this.

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