I've got a simple question. Because the 94-95.5 use the 4-pin coil packs for the tach signal, when I switch to LS coils I imagine that I will no longer have that wired up at all (pwr, gnd, ecu gnd, signal). Is there a simple way to just get the tach on the instrument cluster to get its signal directly from the megasquirt instead of through the coilpack wiring?

EDIT: Should state my hardware. Running MSPNP2 and have done the resistor mod in the diagnostic box, not that I think that affects this.

Found this old post...
Quote:Originally Posted by Halcyon Ah, now it's starting to make sense. Sorry used to working on american cars with distributers.

I have a 94 so the tach reading comes from the coil unlike the 99.
Unfortunately, the LS coils don't have a tach signal.You need the tach circuit built on your MS and have it wired to the black/white wire in the main harness. Then you will get a tach signal.

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So do I have the tach circuit on my MSPNP2 is the real question?

The answer is yes, the MSPNP2 uses options port 4 for tach output.

 

could it be that even with the 6 pin ignition coil connector unplugged, that there is still a tach signal being sent to the instrument cluster and data link connector? it looks like that part of the wiring harness tees off beforehand, but it could be graphically misrepresented

i highlighted the circuit in red that i believe would be disconnected from the 6 pin ignition pre coil plug. I think the green circuit might still be intact?

 

I also have a 94, but I bought ls coils with a harness that’s just plug and play.

 

Can you elaborate on your PnP harness? Does it plug into the coil pack harness?

 

Yea plugs right into the old coil pack harness, I didn’t have to wire anythingbut that’s for wasted spark, for sequential you do have to run some wires to your ecu, I think 2. I have another harness I can take a picture of tomorrow

 

Yup wasted spark is okay. Main thing in question here is tach signal. Thanks, ill check back for the pics!!

 

Here’s a few pictures.

 

Wow thats a nicely made harness! Looks like it gets each signal and pwr from those 2 coilpack connectors.. But I didnt see the ecu gnd or regular gnd. Does it have another bit that grounds to the block? Was it supplied by a miata vendor?

 

All grounds are combined, here is a picture of the whole harness. I got it from aedynamix, but I got it when they were real cheap, now their kits expensive. They might just send you a harness if you ask.

 

And going back to the tach... You are also in a 94/95.5 and dont run a separate tach signal from options port of the MS to the tach?

 

Would grounding both the coil ground and ecu ground together cause any issues? I assumed they called for one pin grounding at the cylinder head and the other pin grounding on the ecu ground for a reason.

 

yea I have no nothing on my options port, everything seems fine, haven’t had a problem.

 

Awesome!


And skyline, you need separate grounds for each I am pretty sure

 

ATX, that's what I was assumed when I re-wired my ls coils so I did so. The picture GreyMiata posted seems like they omit separating the ground and instead, ground both ecu and coil ground together.

 

I’m not sure, mines all grounds the same, I’m not sure why it would matter, grounds a ground, but you could be right, I just haven’t had a problem.

 

Heres an update to my specific scenario, that is using LS coils on 94 connecting to 6 pin pre ignition plug, wired for wasted spark. I used 18 gauge wiring for all of the wires except power. The LS connector came with my used coils, so I cant say all LS harnesses are wired as such. But the 6 pin should be the same for all 94-95.5.

 

Just an update.

I am trying to get my car started for the first time after the rebuild and am no longer getting any tach signal on the gauge cluster. Any ideas on how to get one there besides going straight from the MS2?

 

I have a MS3PNP, but from this thread it sounds like it's just software options to get the MS2PNP to send the tach signal instead of the getting it from the coils:

https://forum.miata.net/vb/showthread.php?t=479809

I have a fuzzy memory that the stock Miata ECU was built to look for the incoming signal from the coils sort of as a backup to the CAS signal, so the wire in the 94/early 95 cars does indeed go from the coils to both the tach and the ECU as shown in the diagram. So you should just be able to just turn on the tach output option in TS.

Also, you say you are trying to get your car started. I believe the MS solely uses the CAS signal, and doesn't care about the potential incoming tach signal, so the lack of tach signal should not prevent starting and running, albeit without a functioning tach. TS should still show correct RPM. I think.

 

You are correct, TS still shows RPM.

I have tachout as the output signal in tunerstudio. I have had the 1k resistor in the car for years. Do I just use the tach out from the options port?