Anatomy Of An Instrument Cluster

The modern instrument cluster might seem straightforward, but as you'll learn below, most of them are anything but simple. From elaborately printed circuit boards to high-grade stepper motors and displays to onboard diagnostic capabilities, the modern instrument cluster is anything but simple.

So, if you’ve ever wondered, "What parts go into an instrument cluster?" This post is for you.

What Do Instrument Clusters Really Do?

At a superficial level, most modern instrument clusters display vital metrics like speed, distance traveled, vehicle mileage, and so on. But when you dive into the list, you see that things can get complex pretty quickly:

  • Speed can be displayed in English or metric units and at a certain speed and gear combinations, indicators may show an upshift or downshift arrow, an "eco" symbol to inform the driver they're being efficient, etc.
  • Distance traveled (i.e. trip odometer) can be paired with fuel economy readings, engine hours, and there can be more than one trip odometer option. And these readings can be in different units as well.
  • Engine speed/RPM, fuel level, engine temperature, cruise control, and other indicators can all have "paired" functions. This means that when one gauge shows a specific reading, it triggers another indicator.

With modern digital dashes, this is all accomplished with programming. With older dashes, these functions are accomplished with a mixture of dedicated computer chips, printed circuit boards, and clever engineering.

Circuit board

Finally, it's important to note that instrument clusters aren't just the housing for various gauges and indicator lamps. On many vehicles, instrument clusters also store vital data like ignition key transponder codes, mileage and/or engine hours, maintenance and diagnostic messages, and even special diagnostic information that's available to repair shops.

Common - And Not So Common - Instrumentation

Most instrument clusters have at least a speedometer and a fuel gauge. Beyond that, the instruments you find in a cluster can vary greatly:

  • Most vehicles have an engine temperature gauge of some kind, only some newer vehicles replace this gauge with an indicator light.
  • Vehicles with a manual transmission all but require a tachometer, yet many vehicles with a manual transmission have been made without them.
  • Oil pressure and engine (coolant) temperature gauges are nice to have, yet many vehicles do not have them.
  • Vehicle electrical system voltage gauges are also a good idea, and also not necessarily easy to find.

On vehicles made before the 1980s, almost all of the gauges represented a direct reading from a sending unit of some kind. A fuel tank level gauge, for example, was directly connected to a device in the fuel tank with a float. By the late 1980s, almost all gauges received their information from the vehicle's main computer.

Old gauges

These gauges most certainly did not get information from a computer.

And while there were certainly some very clever analog gauges from days gone by - engine hours, exhaust gas temperature, turbo boost pressure, fuel flow, engine vacuum - most of the really interesting data has been possible since the advent of computers.

A Few Words About Odometers (And Odometer Fraud)

If you're old enough to remember the movie Ferris Bueller's Day Off, you're probably familiar with the "mechanical" odometer. Prior to the cheap microchip, vehicles used a clever system of gears to count overall miles traveled by the vehicle. In some cases, these gear systems could be run backward to roll miles off the odometer. Only (spoiler alert), it didn't work all the time. Including for Ferris Bueller.

Ferris Bueller

When odometers were mechanical, they could be tampered with to reduce miles. But driving backward didn't usually work.

And while these mechanical odometers were clever, they fell out of favor in the early 90s due to both high costs and rampant odometer fraud. At the time, it was believed that digitally stored odometer data would be much more difficult to alter than some simple mechanical device.

Today we know that's not true, of course. If anything, modern digital odometers are easier to hack than older mechanical odometers, as most of the data stored in a digital instrument cluster is unencrypted. According to a recent EU report, odometer fraud is frighteningly common in much of Europe, and very likely other countries as well.

The good news is that - in the United States and Canada at least - voluntary reporting of vehicle mileage by repair shops, dealerships, and body shops has made it much harder to misrepresent a vehicle's true mileage. And while NHTSA estimates about 450,000 vehicles are sold in the US each year with a fraudulent odometer, that's a tiny fraction of the 40 million-plus used vehicles sold each year in the US. (Read about odometer replacement laws in this article.)

The Essential Components Of A Modern Instrument Cluster

The Lens

Usually made from acrylic or polycarbonate (only glass is common on older vehicles), the lens protects the gauge face(s) from dust, accidental damage, and moisture. And if there are scratches or scuffs on the lens, they can often be polished and/or repaired away.

The Gauges And Gauge Faces

Various gauges

Gauge faces are typically designed to be high contrast, and very often feature white lettering and numbers on a black or dark gray background. This design works well during the daylight hours - even in direct sunlight. At night, backlighting behind the entire gauge face can let light (usually white light) through and illuminate the numbers and lettering.

However, there has been a large variety of gauges used in vehicles in the last 80 years or so, with wildly different approaches to instrumentation.

Now, modern digital clusters use LED displays to show a nearly infinite variety of colors, layouts, and information - often with clever animated effects.

 

Stepper Motors

For gauges with a needle, a small electric stepper motor is used to move the needle. These stepper motors use a clever bit of engineering (this video does a great job of explaining) to precisely control the position of the needle for each gauge. The stepper motors themselves must be automotive grade, as they're subjected to a steady current, vibration, and big variations in temperature.

When a gauge stops reading on a newer instrument cluster (1990s onwards), it's often because a stepper motor has burned out.

Lighting

Prior to the early 1990s, instrument clusters featured tiny little incandescent bulbs that were prone to burning out. Automakers would usually include "extra" bulbs behind the cluster, to try and reduce the odds that a burned-out bulb would render a gauge unreadable.

However, the advent of cheap LEDs (light-emitting diodes) made it cost-effective to use them to replace incandescent bulbs. And because LEDs can last decades before they fail, they quickly became popular with consumers tired of replacing bulbs.

Still, many vehicles on the road today use incandescent bulbs as instrument cluster backlights. Many people find that upgrading older bulbs to replacement LEDs is an easy way to make an older cluster look new again.

LED Screens Or Dot-Matrix Displays

On most instrument clusters produced since the 1990s, there's a small monitor that is some sort of LCD or LED screen or dot-matrix display. This monitor/display will be used to show:

  • Odometers and vehicle mileage
  • Outside temperatures
  • Oil lifespan remaining, tire pressures, airbag status, etc.

Many of these instrument clusters with a screen also come with a button near the monitor. When you press the button, that triggers the monitor to display a different set of information. While this might seem like a simple feature, it represents a significant evolution in vehicle instrumentation: Once a display could be changed, it was only a matter of time before we had animated digital readouts with elaborate layouts.

Discrete Indicators

Sometimes when you turn something on or off, the instrument cluster will display the change. For example, if you turn on the high beam headlights, a blue high beam light indicator may illuminate on the instrument cluster. The same goes for turn signals, emergency brake status, 4wd, and other systems that turn on and off.

And who can forget the famous "idiot lights"? The little symbols that appear on your dash to warn you about something like low fuel, low oil pressure, missing seat belts, low tire pressure, a warning to check the engine or the ABS, check wiper fluid, close that open door!, etc. Sometimes, the idiot is the person who needs a light to tell them something they should already know (like the "low fuel" indicator). Other times, the idiot is the person who came up with a confusing, hard-to-decipher symbol.

Dash warning lights

Before the advent of the cheap microchip, a lot of these indicators were difficult to include in a cluster. Indeed, even in the 80s and 90s, it was difficult to print all the necessary circuit paths on the instrument cluster "motherboard" to support all of these different indicators. To say nothing of configuring the computer chips so that they would "know" when to turn lights on and off.

Printed Circuit Board

Most instrument clusters made in the last 60 years or so included a printed circuit board, as it's the best way to run wires to a cluster of gauges. However, the density of the paths on these circuit boards has improved dramatically since they first saw automotive use.

Circuit boards

And since the late 1980s and early 1990s, the printed circuit boards inside an instrument have mounted microprocessors (a/k/a computer chips). These microprocessors interpret data and signals from the vehicle's engine computer and then display the output on the dash. These boards also usually have the connector (or connectors) for data coming from the engine or other vital sensors.

As printed circuit boards age, they begin to overheat. As they overheat, individual solder connections on the PCB can degrade. This leads to disconnected or flickering lights, intermittent failures of stepper motors, and other problems. If you need to repair an older instrument cluster, it's a good idea to hire someone who will check every single circuit pathway for resistance and continuity.

Backing

Last but not least, there is a plastic housing or backing that holds everything together. In the more modern instrument clusters, the lens often snaps onto the backing. This way, the lens and the backing protect the cluster from damage and also hold everything together. Typically, the re-assembled unit is installed right in the dash and then connected by a cable (or cables) to the engine computer via a wiring harness.

Wiring Harness

Depending on the age of your vehicle, the wiring system is set up in two different ways:

  1. On older cars (40+ years old), the instrument cluster comes with a bunch of wiring harnesses. Each wire runs from a certain gauge to the sensor in a certain component. A few examples:
    1. The speed sensor directly connects to the speedometer.
    2. The engine coolant temperature sensor directly connects to the engine temperature gauge.
    3. The oil pressure sensor directly connects to the oil pressure gauge.
  2. On modern cars, there's one wiring harness running from the instrument cluster to the engine control module (ECM) or body control module (BCM). The ECM or BCM has a bunch of wires running to sensors and lights all over the vehicle. The way this setup works is the ECM or BCM gets information from the sensors or lights all over the vehicle. It then sends the information to the instrument cluster, which displays the information to the driver.

The wiring system in modern cars isn't really that simple, though. The wire doesn't only run from the instrument cluster to the ECM, BCM, or sensor. Rather, a typical wiring path is this:

Instrument cluster > fuse > controller area network (CAN) module > microprocessor > ECM/BCM/sensor

This complicated wiring system makes it possible for all the information to be displayed in an instrument cluster. Pretty neat, isn't it? (By the way, if you are having trouble with your cluster and suspect it's the wiring harness, check out this blog post from ISS Automotive.)

Honorable Mention: Fuses

Fuses are not a part of the instrument cluster. Yet, fuses are a part of the wiring system. The fuses sit between the wire (or set of wires) and the sensor or ECU to which it's connected. There's usually a fuse box underneath the dash on the driver's side. Some cars come with one fuse dedicated to the instrument cluster. In other cars, there's one fuse dedicated to each gauge.

The purpose of having a fuse in the wiring system is to protect the instrument cluster and the sensors or ECU. If there's a short somewhere in the wiring, the fuse blows. In doing that, the fuse prevents the part on the other end from getting damaged. In other words, the fuse cuts power to the other part - and that means your cluster won't illuminate.

Summing Up

There have been a lot of different instrument cluster designs over the years, but the 1990s were definitely a big moment in instrument cluster history. The advent of inexpensive computers and microchip controllers made it easy for automakers to both cut costs and add features.

Today, it's possible to buy several brand new cars with advanced LED displays that include animations on start-up, near-infinite configuration options, and even 3D imagery.