Technical Wiring an Old School Tach

I have an old Knight 8,000 rpm tachometer. Its old, the ones that they used to sell in the catalogs. It has 4 wires out of the back. I have figured out-
Black- GND
Grey- LIGHT
Purple-???
White-???
Dont know what Purple and White get connected to. Which one is the AC/Switch and which one is the wire to the coil?? Any help would be much appreciated!!
Here is a link to the pic of what it is.

http://i63.tinypic.com/34dflvn.jpg

 

Purple usually goes to the coil, but don't quote me on that.

 

HERE IS A UNIVERSAL ONE
THE KNIGHT UNIT WAS A KIT, YOU WILL HAVE TO TRUST THE PERSON THAT PUT IT TOGETHER. CANNOT FIND A SCHEMATIC FOR ONE, ONLY THE AD YOU POSTED.

 

I have a defective Sun-Pro tachometer with the regular red/black/white/green wires. If you want me to, I can take it apart before I toss it and take a couple of pictures of where the wires connect internally on the circuit board. It's a long shot, but it may allow you to figure out which is which. Let me know if it'll help.

 

Might be of use.

eBay item number: 122532993170
sorry, can't post the link.
ciao,
mario

 

I know this is an old thread but it comes up when one googles the Kinghtkit Electronic Tachometer, so might be the best place to post the connection instructions:

Note: I believe there is a typo in the white wire, it should say attach to the positive terminal.

 

^ Instructions describe a positive ground hookup. Diagram shows a negative ground hookup.