TACHS TACHS AND MORE TACHS!

NVU Tech Team on 26th Jun 2019

TACHOMETER INSTALLATIONS

Tachometers operate by reading a signal from your ignition system. This is a "pulsed" signal. Each pulse is counted and the microprocessor converts this frequency into the pointer movement you see on the dial. Gasoline engines generally count the number of times the coil fires. Diesel engines mainly use some sort of sensor. The same theory applies to both types. NVU tachometers can also read pulsed signals from some PCMs, check out our Tech blog on Interfacing the GM PCMs for more on that.

Scroll to the bottom to check out even more features of our tachometers! Auxiliary inputs, built-in indicators, digital filtering, street/sport speeds set-up and MORE!

WOW! as of 12/2024 we have updated our tachometers to incorporate a built-in high-energy protection AND LS pull-up circuit! No more inline resistors!

This is located on the #5 DIP switch. see page 12-1/2 in your booklet for more information, here is the page here as well:

TRADITIONAL COIL INSTALLATION:

This setup uses a traditional distributor/coil. The tach reads from the negative side of the coil. The green is shown for clarity and may be a different color in your application.

ELETCRONIC IGNITION DISTRIBUTOR/HEI PERTRONIX (SEE NOTE)

Most distributors built after 1976 use an electronic module instead of points and have a TACH OUTPUT right on the distributor. This output generally has a 12V square wave signal, It may be "high" or "low" voltage. Either way it is designed for a tachometer to see.

PERTRONIX SYETEMS: We recommend using a 1K resistor inline to prevent damage to the tachometer circuit.

"MSD" BOXES AND OTHER IGNITION CONTROLLERS AND ALL EFI SYSTEMS

This is probably the most popular ignition setup for aftermarket ignitions. The "MSD" box is a Multiple Spark Discharge system (MSD) which can generate up to 60,000 sparks (pulses) per spark plug firing. EVERY system like this has a tach output right on the box. It is either a 12V or 5V square wave signal designed to be clean for a tach to read. Notice the box has power and ground going to the coil. This creates a high voltage feedback that will damage the tachometer input. This is exactly why a separate output is used.

We looked up some different aftermarket ignition tach schematics and they are all over the place. Take a look HERE

ALL AFTERMARKET EFI SYSTEMS, EVERY SINGLE ONE

Every aftermarket EFI system has a dedicated tach output. The EFI works in conjunction with the distributor to control fuel and to some extent ignition. Consult your EFI manufacturer's instructions for connection point, wire color and pulse settings (#of cylinders). This is usually a hall effect signal which is no problem for NVU tachs, we have 1000s running on all sorts of systems.

DO NOT CONNECT DIRECTLY TO THE COIL.

COIL ON PLUG

In most popular setups you will be able to take the tach signal directly from the PCM. In those instances where you cannot, the signal can be picked up from the negative side of the coil. This is essentially reading one cylinder at a time so the tach should be set up for a one-cylinder engine. The PCM actually acts as the distributor for COP (Coil on Plug) systems.

GM PCMS

All NVU tachometers can read from any GM PCM with the 5th DIP switch ON (UP) This is our internal pull-up resistor and no additional pull-up is required. All GM PCMs regardless of the number of cylinders, gas or diesel, output a 4 cylinder open collector signal. For a more in-depth discussion take a look at our Interfacing with GM PCMs article.

That covers most of the basics. Still not sure? Give us or your ignition system manufacturer a call BEFORE you wire it up, it will definitely save you some tine and headaches down the road.

Remember, if its a high energy or multiple spark output, make sure to NOT take the signal from the coil.

In the event that your tachometer does not read any speed when connected properly and it HAS been "tuned", have your tuner make sure that the tach output is turned ON. We have seen this hundreds of times and is almost always the case with an unresponsive tachometer on a GM PCM.

If your tach seems "slow" remember to set it at 4 cylinders, if it's on 8, it will read 1/2 speed. You can also speed up the tach response by going into "Strip" mode on OLED screen tachometers. (Video below)