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Since the coil is in the distributor, simply hook the tach 'trigger' wire to the out put on the side of the distributor cap marked 'tach'.
The tach works off of the distributor.
I'm trying to install a factory tach my self. the guage cluster harness has a connector for the tach but I'll have to find the other end. this much I know there is a tach filter that connects to the distributer the other end attaches to the tach. try www.elcaminocentral.com they have a lot of good info on the process & yes it looks like you may need to run a wire from the distributer to the tach.
try a tach
The Tach's connection could have become loose over time. Also, if the tach is original to the vehicle, it may just be damaged from age and need replaced.
there will be a spot on back of distributor cap it will say tach run wire from there to gauge. You probably need to run a switched 12v lead as well to illuminate the tach at night. The tach slot on the distributor will take care of 'timing' the pulse to the tach. Often times the tach will have a switch setting for 4-6-8 cylinder. Read the manufacturer instructions carefully.
negative side of the coil
Your Tach Signal wire is Gray and Dark Blue (Gray/Dark Blue). Same wire, marked with both colors. This means the wire is GRAY with a dark blue line through it. It is located at the Coil pack on top of valve cover. --- This wire above is a 2 Cylinder Signal. 3.0L Tach Wire is GRAY/RED at Powertrain Control Module (PCM). 3.3L & 3.8L Tach Wire is GRAY/RED, DARK BLUE/TAN And RED/YELLOW at PCM. (Each wire is a 2 cylinder signal.)
I had the same problem with a Japanese mini-truck. I found the Sunpro CP2010 model handles the 3 cylinder issue by an internal switch. 4 Wire hookup - hot, ground, light and coil.
http://www.fieros.de/en/articles/tach.html
Tachometers count the number of times the ignition fires the cylinders, so a tach from a four cylinder will work on a four cylinder motorcycle. on a two cylinder it will read 50% low.
Tach signal? To install a Tachometer? I did it 2 days ago! finnaly got a Tach on my 1998 Nissan Frontier By concept 1 4 cylinder 2.4 right?....Yep if you would still light to know Email me at x31diablo31x@aol.com or myspace is myspace.com/diablo31 peace!