Tach signal wiring specifics

[matthewdesigns]

I'm currently not a DSM owner anymore, or at least a complete DSM . I gave up my 2g for a Miata, which is now turbo'd with an Evo3 16g using a GVR4 ecu/harness and ECMLink V3 controlling everything (the stock Miata ecu has no way to be modified/flashed). Thus far the car is running great and I've been able to knock out most everything electrical even with my limited knowledge. However, I'm lacking a tachometer signal and am currently digging into how the 1g/GVR4 signal works. I have searched quite a bit and have yet to come up with what I specifically need.

The Miata uses a wasted spark ignition that fires in 1/4 and 2/3 pairs like DSMs. The igniter is almost identical (Mitsubishi J702T for Miata vs J722T for DSM), with the addition of a tach signal pullup on the blank pin found in DSM igniters. I am not using the Miata igniter or coilpack, but rather both those items are from a DSM (not a '90). The Miata ecu and igniter are still installed in parallel at this point, and I could potentially get the pullup from that location if needed, but I don't see anywhere that the DSM tach signal uses one. The Miata has a single wire that runs from the igniter to the tach, with no diversion to the ecu. The 1g uses a signal wire from the igniter that splits to the tach and pin 109 on the ecu. Does this tach feed to the ecu have any interaction with the function of the tach itself? Or is it simply there as an added input monitored by the ecu (ie for logging)? Merely connecting the tach output from the 1g ingiter to the tach input wire in the Miata harness produces no effect, so I'm guessing that the split to the ecu may in fact be necessary. I have a 1g wiring diagram and can read where everything goes, but no explanation of why the tach signal splits.

This is from miataturbo.net and references a diagram for a Toyota COP setup with Megasquirt, but it contains relevant information:

The tach output provided by the COPs, if my memory is correct, is an open-collector. In non-geek terms, that means that it provides a momentary closure to ground for every ignition event. To cause something useful to happen with this, we need to add a pullup voltage. In the diagram that DXO posted, a 1k resistor is indicated between the +12 line and the combined tach signal line. So when none of the coils are conducting to ground, that line floats up to 12v. When a coil does conduct to ground, the line goes to 0v.

When the stock ECU is present, it provides this pullup on its IGf wire, which is why that note is there about "MS Only".

Any insight into this would be greatly appreciated. It's certainly not a deal breaker to drive with no tach, but I'd like to have it working. And at some point I'll post a build thread so you guys can see the car.

 

[brads]

There was another thread in here with a guy trying to adapt an S2000 dash into a DSM. Seems like yours is a similar issue. His turned out to be that the DSM tach output from the ignitor is 5V square wave, where the dash was looking for a 12V square wave. He used another tach adapter to fix it.

If you are comfortable wiring electronic components, its just a matter of using a few transistors to convert this 0-5V square wave to 0-12V, right at the dash. Then your ECU will still see the 0-5V signal it wants for whatever it uses it for. And the dash will get 0-12V

 

[matthewdesigns]

I did see an S2000 thread or two when I was searching but didn't read too far. I'll have another look at them. Thanks for the info on voltage conversion too. IDK what the requirement for the Miata tach is so I'll try to figure that out. While the igniter/coils look nearly identical they may certainly output different voltages.