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Dash Gauges

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The value and use of gauges
See Figures 1 and 2

Most engine problems develop slowly and telegraph their warning signs clearly, if you are equipped to read them. About 25 years ago, the auto industry began a trend to eliminate dash gauges. The oil pressure, ammeter and coolant temperature gauges were replaced with small warning lights-quickly and aptly named "idiot" lights. The difference is that the idiot light tells you when something has already happened; the gauge will tell you when it's starting to happen, and will indicate a trend.

Fortunately, automakers offer gauges as optional equipment, sometimes in addition to the standard warning lights. Equipping your car or truck with gauges in addition to the warning lights can indicate a pattern that will point out irregularities in plenty of time to correct them and save the expense of more serious problems.

As an example, watching the coolant temperature gauge climb slightly above normal over time can indicate slipping belts, low coolant level, worn hoses or incorrect ignition timing. Any of these problems are easily corrected before they cause the warning light to come on, when the engine has already overheated.

As the age of electronics increases the sophistication of the automobile, increased use is being made of the electronic control module and even a separate body control module. These modules are small on-board computers that monitor hundreds of inputs from various sensors on the vehicle. Based on the information they receive, these modules make decisions to alter ignition timing or alter the fuel metering system.

Manufacturers have also found that the module is capable of handling even more functions than there is a practical use. As a result, more equipment is being offered to feed the driver a constant stream of information-everything from tire pressure to fuel economy to elapsed time or mileage.

Figure 1 The modern dashboard is filled with gauges, controls and other convenient features.
Modern dashboard
  1. Side Window Demist Outlet
  2. Air Outlet
  3. Instrument Cluster
  4. Passenger Air Bag
  1. Glove Compartment
  2. Glove Compartment Lock
  3. Radio
  4. Climate Control
  1. Power Source
  2. Cup holders/Ashtray
  3. Storage Tray/CD Changer
  4. Storage Bin

Figure 2 Electronic sensors have replaced mechanical gauges in many applications. A computer with a single logic chip in conjunction with sensors can easily monitor every system of the vehicle, providing warning lights and instant read out of information from tire pressure to fuel economy to elapsed time to your destination. Click on picture to enlarge view

Types of gauges

There are two types of gauges: mechanical and electrical.

Mechanical gauges

Mechanical gauges measure speed or pressure at the source and send the information to the gauge mechanically. The speedometer and Bourdon tube oil pressure gauges are examples of this type.

Bourdon tube oil pressure gauges are connected directly to a small tube in the main engine oil passage, by a plastic or copper line. The gauge consists of a flattened tube bent in the form of a curve that tends to straighten under engine oil pressure. The curved tube is linked to a needle that registers on a calibrated scale. They are easily distinguished by the copper or nylon line running from the engine to the gauge.

Electrical gauges

Electrical gauges monitor functions at the source and send the information to the gauge electrically.

Thermal (bi-metallic) electric gauges are activated by the difference in the expansion rate of a bi-metal bar. A sending unit controls the flow of current to a heating element coiled around a bimetal bar in the gauge. These gauges can be recognized by a pointer that moves slowly to its position when the ignition is turned ON.

Magnetic electric gauges move the indicator needle by changing the balance between the magnetic pull of two coils built into the gauge. When the ignition is OFF, the needle may rest anywhere. Balance is controlled by the action of a sending unit that will vary current flow, depending on temperature, pressure or movement of a float arm. A magnetic gauge can be recognized by a needle that jumps to its position when the ignition is turned ON. A 90 scale is also the maximum that can be used, since the needle must swing between the poles of a magnet.

Many electric gauges use an instrument voltage regulator to control the supply of voltage to the gauge. This prevents fluctuations in the gauge due to varying voltage.

How to read gauges

The problem with gauges is in knowing how to read them; it doesn't do much good to have gauges, if you can't interpret the reading.

Most gauges are marked with green (OK) or red (danger) areas or with calibrated faces. No gauge should be considered totally accurate; an indication of change is far more important than a totally accurate reading.

You should be able to quickly familiarize yourself with the normal readings on the gauges, and to easily spot a sudden or developing change in the readings.

Coolant temperature
See Figure 3

These gauges monitor the temperature of the engine coolant. As the engine warms up, the temperature will probably rise to somewhere around the 180-200F (83-94C) range. If you're stuck in traffic, the temperature will rise slightly. It will also rise slightly immediately after shutting the engine off, because the coolant is not being cooled, but will return to normal when the engine is started.

Variations in temperature as shown on the gauge are not normal, unless you are suddenly caught in heavy traffic or some other conditions cause the temperature to change. Too cool temperatures indicate a faulty thermostat. Too hot readings indicate low coolant level, worn hoses, defective radiator cap, incorrect ignition timing or slipping belts. If the normal operating temperature rises over the course of time, and the above factors are OK, suspect a worn water pump or a clogged system.

Figure 3 The coolant temperature gauge reads coolant temperature in degrees Fahrenheit or (Centigrade), or may merely be marked "Cold" and "Hot."
The coolant temperature gauge reads coolant temperature in degrees Fahrenheit or (Centigrade)

Oil pressure
See Figure 4

The oil pressure gauge will tell you if your engine is getting proper lubrication. At fast idle when the engine (and oil) are cold, the pressure will probably be at maximum on the gauge, around 60 psi (413 kPa). Depending on the vehicle, rpm and condition of the engine, oil pressure should be constant, somewhere around 30-40 psi (207-275 kPa) at cruising speed, and less at idle. Under load, you can expect the oil pressure to rise slightly and to fall off with deceleration. Low oil pressure can warn of low oil level, wrong viscosity oil, overheating, clogged oil filter or worn engine (many miles).

Figure 4 Oil pressure is monitored in pounds per square inch (PSI), (kPa in the metric system), or marked "Low" and "High."
Oil pressure is monitored in pounds per square inch (PSI), (kPa in the metric system)

Ammeter
See Figure 5

The ammeter will indicate the condition of the charging system. It will show charge (+) when the battery is being charged and discharge (-) when the battery is being used. Just after cranking the engine, the ammeter will show a charging condition if lights and accessories are OFF. As the energy spent in cranking is restored to the battery, the pointer will gradually move back toward the center, but should stay slightly on the charge (+) side. If the battery is low, it will show a charge condition for an indeterminate period.

At speeds above 30 mph (48 kph), with lights and accessories on, the ammeter should read on the charge side, depending on the condition of the battery. At road speeds, the ammeter should never show discharge. If it does, check the belts or charging system.

A battery that appears to charge rapidly, then discharge rapidly, is failing and replacement time is near. Slower than normal charging rates indicate a slipping belt or a problem in the alternator.

Figure 5 The ammeter monitors the rate of charge or discharge of the battery.
The ammeter monitors the rate of charge or discharge of the battery

Voltmeter
See Figure 6

Voltmeters are used on some vehicles in place of an ammeter because they give a more complete indication of battery condition. Although the vehicle uses a 12-volt system, the system operates at slightly over 13 volts. If the voltmeter reads under approximately 13 volts after the engine has been running a while, look for slipping belts or too low a voltage regulator setting on vehicles with adjustable regulators. Continuously high (above approximately 15 volts) or low (below 13 volts) voltage may also indicate a defective alternator or defective battery.

Figure 6 The voltmeter shows the battery condition at any given moment, more accurately than an ammeter.
The voltmeter shows the battery condition at any given moment, more accurately than an ammeter

Vacuum gauge
See Figures 7 and 8

Vacuum gauges are always mechanical types that measure manifold pressure (engine vacuum), which relates directly to fuel consumption. Engine vacuum varies inversely with engine speed, so you should also drive at the highest indicated vacuum. Try to maintain the highest vacuum under all conditions.

The vacuum gauge readings are also a good indication of the condition of your engine. Actually, the readings are not as important as a steady needle. At idle, the vacuum gauge should show a steady reading of anywhere from 8-16 in. Hg (27-54 kPa) on an engine in good tune and operating condition. A needle that twitches at idle indicates fouled plugs, stuck or worn valves. A low reading at idle that stays low usually means a leaking vacuum hose, incorrect ignition timing or worn valves or valve guides. As engine speed increases, erratic readings may mean a blown head gasket or worn valves.

After the fuel crises of the 70's, the public's interest in fuel economy has given birth to a variation of the vacuum gauge, called a "motor minder". This gauge is a vacuum gauge with words ("Poor", "Fair," "Good" and "Excellent") and color bands (red/yellow/green) replacing the numbers on the face of the vacuum gauge. Since there is a direct correlation between in./Hg (vacuum) and fuel economy, gauge manufacturers have already interpreted the numbers for the driver.

The latest variant to the vacuum gauge, or the "motor minder" mentioned above is the upshift light. The operating principle is basically the same, there is a sensor monitoring the intake manifold vacuum which then triggers the light telling the driver when to shift to obtain better fuel economy.

Figure 7 Vacuum gauges monitor engine vacuum in inches of mercury (in./Hg) or Kilopascals (kPa).
Vacuum gauges monitor engine vacuum in inches of mercury (in./Hg) or Kilopascals (kPa)

Figure 8 A normal vacuum gauge can be used to determine the most fuel efficient driving conditions.
A normal vacuum gauge

Tachometer
See Figures 9 and 10

Tachometers are among the most popular of gauges, possibly because of their identification with racing. They are useful while driving to keep the engine at its most efficient rpm.

Figure 9 The tachometer measures engine rpm (1000's of engine revolutions per minute).
The tachometer measures engine rpm (1000's of engine revolutions per minute).

Figure 10 Most tachometers have only four connections to make. One goes to ground, one to power (ignition switch), one for lights and one to the distributor side of the coil or the trigger side of the ignition module.
Most tachometers have only four connections to make. One goes to ground, one to power (ignition switch), one for lights and one to the distributor side of the coil or the trigger side of the ignition module

See also our special section: Troubleshooting Basic Dash Gauge Problems

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1998 W. G. Nichols - Chilton's Easy Car Care