Technical Tach Wiring **Need Help**

Just picked up a spark-o-matic tach off ebay. Trying to figure the wiring before it arrives. Most all tachs ive delt with in the past had four wires. This one has three. Anyone have one and can shed some light on correct wiring for this tach??

 

As a guess, one wire is hot, one wire is ground, one wire is the feed from the HEI type distributor. Thats what it was on my small block Chevy with an HEI distributor. Bill

 

One ground, one hot, one feed from the coil and one for the light.

 

When you get the tach, take a few pics of where the wires actually connect inside.

 

That equals 4, not 3. That's why I was asking.

 

I will.get better pics once it arrives. As of now, this is all I have.

 

So the light inside would turn on when the key is rolled and powers the hot side of the tach?

 

Oops, I read it wrong. Sorry I was sleepy...

 

That would be a big help. The picture you have shows that the wires go underneath, but does not show where they connect.

It's usually possible to trace the circuit, and figure out which wire does what. It's not easy, but it's possible.

Finding the original instructions for a 50-60 year old tach, is pretty difficult, but it's another thing to keep trying to do.

 

Colored stripe seems to say black bands equal's powered source for coil and yellow band chassis ground?

 

Got it all figured out. Very simple. Thanks for the assist!!