Trust conner....

Or a short to ground. <img src="/forums/images/graemlins/kewl.gif" alt="" /> [/quote
If there was a short to ground, the coil would be grounded and the engine would not run.

Ah, so it's actual pulse that drives the coil also drives the tach? I thought there would be some sort of isolation circuit between the coil and the tach signal feed.

...and if you end up needing the gauge cluster, I have some available.

See what I mean. Trust conner.

Ray, sort of. The coil receives plus voltage at the coil + from the ignition switch constantly while on. The coil minus is connected to switched ground. The switch is the power transistor or ignitor in this case in the dizzy. The dizzy uses the ignitor and the reluctor/detector set to emulate an old set of contact points and break the ground path when the timing system calls for spark event. This interrupts the current flow in the coil, and as the magnetic field collapses, generates the high spark secondary voltage in the coil windings.

That takes care of spark. As conner said, if the tach wire is grounded, you get no spark and the truck does not run.

Tach operation works on the same circuit. At 3000rpms you have 50 spark events per second. At 1500 rpms, you have 25 sparks per second. Per unit time, you have twice as much on time in the needle actuating coil in the tach at 3000 as at 1500, so the needle swings twice as far. The tach gets power from the ign switch (via some other stuff, but that's the source) and is grounded via the coil -, so the ignition make/break ground also makes/breaks the tach needle coil ground path at a rate exactly proportional to rpm. There are diodes and transistors and stuff in there so the 'lectrons don't get notional and do stuff you don't want.

All clear?

Once I rethunk this, my original directions will fry your tach. BE WARNED. Disco the tach wire at the coil, and use a piercing jumper wire to ground to test along the tach wire from tach to round gauge connector to cluserl, looking for a single needle jump every time you connect to ground. Make the ground connection quickly, then break it. A case of St Vitus Dance will make a more easily read indication on the tach. Ign on for tests.

[quote]See what I mean. Trust conner.

Ray, sort of. The coil receives plus voltage at the coil + from the ignition switch constantly while on. The coil minus is connected to switched ground. The switch is the power transistor or ignitor in this case in the dizzy. The dizzy uses the ignitor and the reluctor/detector set to emulate an old set of contact points and break the ground path when the timing system calls for spark event. This interrupts the current flow in the coil, and as the magnetic field collapses, generates the high spark secondary voltage in the coil windings.

That takes care of spark. As conner said, if the tach wire is grounded, you get no spark and the truck does not run.

Tach operation works on the same circuit. At 3000rpms you have 50 spark events per second. At 1500 rpms, you have 25 sparks per second. Per unit time, you have twice as much on time in the needle actuating coil in the tach at 3000 as at 1500, so the needle swings twice as far. The tach gets power from the ign switch (via some other stuff, but that's the source) and is grounded via the coil -, so the ignition make/break ground also makes/breaks the tach needle coil ground path at a rate exactly proportional to rpm. There are diodes and transistors and stuff in there so the 'lectrons don't get notional and do stuff you don't want.

All clear?

Once I rethunk this, my original directions will fry your tach. BE WARNED. Disco the tach wire at the coil, and use a piercing jumper wire to ground to test along the tach wire from tach to round gauge connector to cluserl, looking for a single needle jump every time you connect to ground. Make the ground connection quickly, then break it. A case of St Vitus Dance will make a more easily read indication on the tach. Ign on for tests. [/quote/

Well written
Conner

One further think.

If the coil wire is grounded, the tach should peg full scale, if I have the little 'lectrons right this time....

On recollection, I have had the short initial bounce of the tach on startup when I had a disconnected tach wire. Look for a wire break. I have found that the insulation on some mitsu wiring will get very brittle over time if overheated, and crack. The wire them flexes at the crack and eventually fatigues and breaks one strand at a time until all are broken. This is the common failure of the LC filter on the v6 tach.

Thank you for all your help and information. I now have a working tach and am ashamed to say that it was much more simple that i thought. The tach wire was on the + side of the coil. Thanks again for all your help.

Glad to hear that the tach is fixed.

For others that have a tach problem, one thing to keep in mind is that the signal that the tach is reading is a high voltage spike.

When the ignition module interrupts the current flow through the coil, the coils magnetic field collapses. This induces a voltage in the secondary windings, providing a spark.

It also induces a voltage in the primary windings. With the motor running, if you put a scope on the negative terminal of your coil you will see a peak AC voltage of at least 300 volts (350 - 600 typical). It will also have a high frequency component ( AC wave) that decays to zero.

This is the signal that the tach needs to register a firing event.

Kevin

Is that decay the "ringing" of the coil? Never understood that term...

RK, glad you have known revs now. Mistakes aren't stupid. They are learning experiences. If I had somebody working for me that never made a mistake of commission, I'd can them for not trying hard enough. Think on that...