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.
INSTALLATION VIDEOS:
CLICK HERE For videos on connecting tachs to various systems and scroll to the end of this page
GENERAL GUIDELINES FOR WIRING TACHOMETER SIGNALS:
- Know where to pick up your signal at. If you are using an aftermarket ignition system, the manufacturer's instructions will label where the tach signal will be coming from.
- Keep tach signal wires away from speed signal wires, ignition wires, fans, pulsed pumps, anything that may either interfere with the signal or the tach wire may actually resonate a signal to something else.
- Filtering: NVU tachometers have a built-in digital filter designed to optimize the signal coming in from the source. You can change it based on your needs.
- Know if you have a high energy ignition system before connecting the signal wire. Damage can result if installed improperly.
EFI, CDI, MSD, HOLLEY SYSTEMS:
ALWAYS have an external tach signal other than the coil. DO NOT CONNECT TO THE COIL ON ANY "BOX" SYSTEM.
WHAT IS THIS TACHOMETER CARD?
Tachometers operate on an extremely wide range of signals from high voltage to open collector (no voltage). The resistors included in your kit are there in case you need them as it used to be easy to run up to a Radio Shack and grab them.
WHEN TO USE THE 1K RESISTOR.
The 1K resistor is to cut down on high voltage damage. Use it on all coil systems when reading from the negative side of the coil. Its used when picking up the signal on high voltage systems/coils. Use on any high performance or Pertronix ignitions. If in doubt, install it you can always remove it later if the signal is weak. The diagram is on the card, the resistor is installed inline as the diagram shows on the card.
WHEN TO USE THE 10 K LS RESISTOR
Use this only when reading a tachometer signal from a GM PCM. It is a pull up resistor and not inline. The diagram is below and on the card.
TACHOMETER WIRING SCHEMES:
Even with modern computer controlled engines being used in our favorite classics, the most common setup uses an electronic module in the distributor (sometimes points) which sends a 12V pulsed signal to the negative side of the coil where the tach reads.
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. Use the 1K resistor provided on your TACH CARD. This will protect the tach from the potential for high-voltage feedback.
ELECTRONIC IGNITION COIL
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. Use the 1K resistor provided on your TACH CARD. This will protect the tach from the potential for high-voltage feedback.
"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.
COIL ON PLUG
Below, a coil on plug setup
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. We suggest using the 1K resistor just in case you need it to prevent damage to the tachometer.
GM PCMS
All NVU tachometers can read from any GM PCM with the addition of a pull up resistor. All GM PCMs regardless of the number of cylinders, gas or diesel, output a 4 cylinder open collector signal. The pull up resistor converts this to a square wave. 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.
Want to get even more confused? We looked up some different aftermarket ignition tach schematics and they are all over the place. Take a look HERE
Remember, if its a high energy or multiple spark output, make sure to NOT take the signal from the coil.
INTERFACING WITH HOLLEY TERMINATOR SYSTEMS
TACHOMETER SIGNALS, WIRING AND SIGNAL CONDITIONING:
HOW TO SET-UP TACHOMETER FILTERS AND SPEEDS:
DIESEL TACHOMETER SET UP AND FILTERS: