Dashboard Gauges - Needles and Numbers

Phil Ramsey

If you’ve ever piloted a plane, one of the first comments you’ll hear from a new passenger is “Gee whiz, look at all those gauges.” The information on your plane’s engine condition and other valuable data is a little more important when you cannot simply pull over on the side of the road.

Gentlemen, Start Your Engines

While attending college in Indiana, I was introduced to the Indianapolis 500. It was a marvelous dawn of change - the first year Jimmy Clark and the Lotus team showed up with small lightweight, rear engine cars. “They will never last the hard 500-mile grind” was the informed consensus. I was privileged to be in the garages during the time trials and I marveled at the exotic and beautiful engines and their custom-made chassis and body shells.

Looking at the instrument panel of one race car, I noticed some of the gauges were “crooked.” A commercial gauge was not placed in its traditional upright position. I caught the attention of one of the mechanics and he asked, “Wanna hear that Offenhauser run?”

When he fired up the engine, I saw all of the large red needles on the gauges jump to life. He then shouted, answering my question, “See how all the needles point straight up when the engine is tuned and running as it should.” He continued to explain how a driver - briefly taking his eyes off the track - can glance at the gauges to see if anything is wrong while he is driving at speed.

If the needle isn’t straight up, then something is wrong, and it is time to make a closer evaluation of the information on the gauge. “We tune, then turn the gauges with the needles straight up.”

These were analog gauges, by virtue of their needle indicators. They collected information using physical changes and transmitted the data to the operator who read the relative position of a needle on a gauge surface. The relative and approximate value of an analog gauge was read from the needle’s position on the instrument, on a scale from zero to an upper limit.

The simplest analog gauge on a car is the oldest … the oil dipstick. It reads the upper limit, the full mark and gradients below that using the oil meniscus as a needle. How do you suppose it acquired the name “stick”? The first gas and oil gauges were wooden sticks. I measured the gas level in my grandfather’s ancient tractor with a stick for years after his “newfangled” electric gauge broke.

The gauges used most in cars today are for oil pressure, gas level and water temperature. Because they are the most critical for engine life, these gauges are on every car I can think of manufactured in the last 60 years. (OK, not a water gauge on air-cooled Volkswagen Beetles and the like.)

Are the often-called “idiot lights” considered gauges? They are in a way, but they measure only two things: either on or off. Like needles, they are red for a reason, to draw attention to what is a serious deviation from normal range. I do remember a few gauges that had needle indicators as well as a little red light that glowed when some critical point was reached. I liked that arrangement. It was the best of analog solutions to give drivers the information needed about temperature, pressure and fuel.

Analog gauges were driven by several methods. Oil pressure was often read from a small line that ran from the engine block oil gallery directly to the gauge. Water temperature was measured by the effect of heat on a small gas-filled bulb. The gas expanded or contracted according to the water temperature, and the pressure was once again transmitted to the dashboard gauge by a small tube. These tubes were frequently less than 1/8-inch in diameter.

How They Are Measured

Oil pressure, water, oil and transmission temperatures were all measured by the direct expansion of liquid in a line. Speed is measured by rotating a cable within a housing connected to the output of the transmission, wheel or differential, then connected to the speedometer. The generator, or alternator, output is measured by wiring an ammeter or voltmeter to the dashboard. The gas gauge uses a variable resister, connected to a float within the gas tank.

At one time, all gauges read the information transmitted to them by analog measurements. We have now entered the digital age, in the way the information is measured, transmitted and displayed. The water temperature in my car registered 203ºF the last time I checked. On an analog gauge used in the past, that would have been just a bit past the one-third mark. Digital gauges are precise. They don’t merely get close to the actual number, rather they display the actual number.

Role of the ECM

The engine control computer, or ECM, has numerous sensors reporting in a digital stream. The list is long and getting longer. The amount of information the ECM uses is growing. The dashboards increasingly display pertinent information in digital numbers rather than analog needle arrangements. The number of conditions that could be displayed are beyond the driver’s ability to read them at a glance, so dashboards generally display only the original information on water, oil, speed and alternator.

When I plug my computer into the ECM I can directly read, in real time, a long list of sensor reports. This includes information on the water temperature, oil pressure and temperature, air intake temperature, air mass or volume used by the engine, transmission oil temperature, exhaust, O2 content, throttle opening percentage, manifold vacuum, injector pulse opening, ignition timing, advance, air-conditioning pressures and temperatures, and antilock brake operation.

These factors are measured by reading sensor results generated by electrical resistance and pulses generated by a rotating part located next to a magnet. In short, most all digital input is measured from sensors using electrical resistance or alternating current magnetic pulses.

All of this information is displayed digitally in alphanumeric form on my handheld computer. If this information was displayed in needle-and-gauge form in a car, the gauge console would be larger than that of the Beech Queen Air I used to fly. The average driver does not need all this information visibly displayed to drive his car.

The ECM uses it to calculate the maximum power and fuel efficiency setting for the engine. What the driver does need to know are the factors that indicate an engine is operating within safe limits. Those limits will provide transportation for him and longevity for the engine. That brings us back to the big four: water temperature, oil pressure, battery and gas level.

Why do gauges read from left to right? My theory is that the first gauge was merely a stick in the ground used to measure time. As the sun rose in the East and traveled across the sky, the stick traced out an arc from left to right, and soon hours were marked on the shadow pattern. The pattern was clockwise, or left to right. Our written language and our gauges were affected by this pattern.

I like gauges with needles. They are quick to convey the information needed and less confusing than numbers. Numbers must be mentally compared to a scale of good values in your head. This interpretation requires time and thought, where needles visually indicate conditions or levels in one quick glance - whether you are driving 200 mph around the Indianapolis racetrack, or 70 mph on the highway heading home.

One famous cartoon moron said, “I am not good at math because there are too many numbers.”

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