im wondering wich wire you use for the tach. source on a 4.6L. ford, twin coil setup. i tapped into the negative wire, and at idle im at 500 rpm, but when you throtle up, the tach. goes to 0.any ideas?


Unfortunately, it isn’t that easy anymore. Distributorless ignition systems (which I think is what you’re looking at-- engine and make is good, but year and model would be nice too) don’t work in a way that you can just hook a regular tachometer up to them. Usually there’s a tachometer hook up on the ignition module or on the coils, which is where you need to hook it up, although I’m not sure if you can plug in just any tach.


I’ve got a pretty detailed 4.6 wiring schematic book. Will take a look at the book and post back later to see if this is feasible. Guess you’re adding an aftermarket tach to ???.


I don’t know what you’ve got in mind here or which negative wire you tapped into, but this may not work. Some aftermarket tachs use a power lead, ground lead, and a lead that goes to the negative side of the ignition coil. This is the one that provides a pulse to the tach.

The 4.6 schematic shows the factory tach receives a pulse from the PCM so I’m afraid I can’t be of much help on this, especially while not knowing exactly what you’re working with.
The schematic shows a tan wire/yellow tracer from the PCM so maybe the tach has to be tied into that one.


If your Ford (or, other Fords of that year) came with tachometer, then, it would be a simpler matter. You could be causing problems by adding a load (another circuit) to a circuit to the engine computer; but, it’s your truck.


Check with the manufacturer of the tach to see if they have an adapter/interface module for your truck’s DIS. I know Auto Meter has interface modules to connect their tachs to various DIS systems. I used one for a Buick I was working on.