Tach reads double the actual RPM
 

OK, I finally found out which wires in the instrument cluster harness needed to be spliced to hook up the tach in my V8 Miata. The problem is that the tach is reading exactly double what it should. I assume this has something to do with how the signal is generated on the V8.

Is there some way to cut the signal in half to make my tach read correctly? I'm using the same signal source on both the Maxigauges and the car's tach. MaxiGauges reads correctly, tach is double those readings at idle and when blipping the throttle.

I read about this once... found it:

Don't know if it will work on a miata, but:
Volvo's That Run - Chapter 9 - Speedometer and Tachometer - Turbobricks Forums

Ah, looks like there is a potentiometer to adjust, at least on the NA's. Hope it's on the NB cluster.

Help with Tach adjustment - MX-5 Miata Forum

OK, so there is no potentiometer on the NB's. There are screws on the backside labeled sin+ and sin - and cos+ and cos-.

https://www.miataturbo.net/attachmen...ine=1376280968

I opened up the tach and this is what I found

https://www.miataturbo.net/attachmen...ine=1376280968

When idling at 1000 RPM there is 1000 ohms resistance across sin+ and cos- and there are 800 ohms across cos+ and sin-. Those values do change when the throttle is blipped.

Any suggestions? Any additional readings that I could take that might help the electrical gurus here?

remove the tach output from half of the coils. you're sending 8 pulses per cycle instead of 4.

This is it. Had the same issue once, but the opposite. Read half the RPM, 2 pulse instead of 4.

How can I do that when there is only one coil? It's an old Ford V8.

Time to convert to MS3x!

Yeah, you're screwed. Can you convert to a multi-coil system? Or perhaps there is a way to run inductive or resistive detection on the individual spark plug wires?

Dakota Digital makes a converter that will do this all for you. It's about $80.

Believe MSD does one, too.

replace the factory tach with an aftermarket tach in the cluster. I messed with my tach for a few weeks before I said screw it and made one fit and works better than I could have imagined.

This is what I'm likely going to do tomorrow, I guess I was just hoping for a "get two xx resistors and a flip flop and put on a breadboard" instead of dropping the $85.

I did send an email to Hakan to see if there was any way to pull an output from the MaxiGauges. It obviously has a similar converter built in as those gauges were reading double actual RPM when I installed them but in the setup menu I was able to switch from 4 to 8 cylinders (or vice versa) and it started reading correctly on the digital display.

So you have a extra high revving V8, what's the problem;)

2 easy options, get a new aftermarket gauge or make a new rpm faceplate.

Perhaps hold the throttle at 2k and see what u get then?

Also can you measure voltages across the probes?

Hmm not too sure now lol. Reducing the value with resistors wont fix it since you'll still have the same offset. It will just lessen the control over the needle.

https://www.miataturbo.net/attachmen...ine=1376621532

I bought a Dakota Digital box the other day. For $80 I can forget about it. Still need to get a speedo healer and tie into the VSS but speedo can wait. That's all that is left.

Going for sand and buff next Saturday. It's never been buffed since paint.