Tach wiring... [not the usual]

[Gerard]

A friend of mine bought the Miata powered '69 Sprite featured on BaT a few weeks ago. I was surprised how clean the install is and how nice the car drives, but a lot of things have not been finished.

It's going to be a present for his dad, who's b'day is later this month. One of the things that needs to be done is to wire the tach. Now, because the engine uses a coil pack for ignition, I'm not sure the correct method to wire the original Sprite tach. Any suggestion for articles or methods?

 

[Trevor Jessie]

Is it running the original Miata ECU?

 

[Gerard]

I believe so... stuffed under the dash somewhere. I believe the coil packs use two wires to supply the pulses.

 

[Trevor Jessie]

Hmmm.... I'm not good at electronics, but there has to be a signal wire. Most likely a square pulse, but I do not think it will drive an RVC tach.

 

[Trevor Jessie]

Might be easiest to get an MSD tach driver and be done with it.

 

[Gerard]

Thanks for the pointer to that Trevor. Looks like an easy solution.

 

[dklawson]

If the ECU can drive a "modern" aftermarket tachometer, it will most likely work with an RVC type Smiths tach. The earlier Smiths RVI tachs that wire in series with coil power do not generally work with any electronic ignitions.

 

[Morris]

I have a Ford edis system in my car. I was able to get my rvc tach working by running a leg off the wire that runs between the edis and ecu. I am guessing that on a miata the spark is driven directly from the ecu so this will not work. Other tricks I tried that did not work for me but have worked for other people:
1) Connect the tach out from the ecu to the tach using a pull up resistor (a resistor connecting the 12v in and the signal in on the tach ).
2) Connect a diode to the ground side of each coil pack, then connect the othet side of each diode to a single wire running to the tach.