My factory tach is way off and I need to do some tuning. I found a tach outside in the tool shed and was wondering if and how I could hook it up to my car. I have a 91 RS with a 305 TBI. It has an external coil. The tach I found has 2 short (about 12") black wires, and 2 other long wires (about 4 feet), one bing red and the other green. I did a search but found no information that really helped. Any help is appreciated. Thanks.

 

You can hook up an aftermarket tach to your car, but without know what wire does what on your tach, it's going to be hard to know the proper way to wire it up.

Pretty much all aftermarket tachs have 4 wires, and they are for:

12V ignition- (use the IGN spot in the fusebox)
12V lighting- (tap into the INST LPS fuse wire (gray) under the fuse box)
Tach signal- (tap into the white wire off the gray connector of the ignition coil)
Ground- (tap into an existing ground or use your own, your choice)

If I had to guess, I'd say the 2 black wires are for 12v ignition and ground, the red wire is for the tach signal, and the green wire is for the 12v lighting, but don't take my word for it. It's impossible to know for sure unless you can get a copy of the install directions. Who's the manufacturer of the aftermarket tach?

 

Thanks for the info. The tach says "SW Stage 1". It is a pretty old tach. My dad probably used it to tune his '70 GTO

 

i think these are typical tach wire colors

green - coil ground
red - 12v
white - light
black - chassis ground