Thursday, January 19, 2017 Detailed Auto Topics
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Vehicle manufacturers provide dash gauges to give drivers important information about their vehicle. A dash gauge can show when a problem exists. If we know what they are telling us, dash gauges can also let us know before a problem occurs. Understanding our dash gauges can help prevent many problems.

typical dash gauges and what they mean

1. What is a tachometer and what are RPM

The tachometer gauge shows the speed at which the engine is rotating. We measure engine speed as revolutions per minute abbreviated RPM. Normally the numbers on the tachometer gauge stand for 100 RPM. For instance 25 actually shows 2,500 RPM.

A lower RPM on the tachometer gauge, shows the engine is turning slowly. This is the case when sitting still with the engine running. We call the lowest RPM idle speed. This will usually be between 500 and 800 RPM.

Tachometer gauge at an engine idle

It is also normal for the engine RPM to indicate a higher idle speed, when the engine is first started or when the engine is cold. The higher RPM allows for a faster engine warm-up and keeps the vehicle from stalling when cold. When the engine reaches full temperature, the idle speed should return to its normal range. An engine RPM lower or higher than normal indicates a problem. On later model vehicles, this will also cause the check-engine light to come on.

Our tachometer gauge also serves as a diagnostic tool. The relationship of engine RPM to vehicle speed, shown on the speedometer gauge is very important. For instance, when driving, as the RPM increases on our gauge, vehicle speed should also show an increase on our speedometer gauge. If the engine RPM rise without accelerating, we may have a transmission problem. Paying close attention to our tachometer gauge may easily prevent a transmission failure.

For better fuel mileage we need to keep the engine RPM as low as possible. Accelerating slowly will help keep engine RPM down and fuel mileage up. Our engine computer learns the way we drive. When we accelerate slowly, it causes the transmission to shift at a lower RPM. Accelerating slowly will greatly increase fuel mileage. Driving at a slower speed also reduces the engine speed and increases our fuel mileage.

2. The speedometer and odometer

Most people are quite familiar with the speedometer gauge. The speedometer gauge indicates how fast the vehicle travels in miles per hour, abbreviated MPH. Often, vehicle makers also provide a scale to indicate kilometers per hour or KPH. Learning the relationship between the engine RPM on the tachometer gauge and the MPH on the speedometer is important, as discussed above.  An odometer reads the cumulative miles a vehicle travels.  Higher miles suggest more wear and the odometer gauge often determines the value of a used vehicle.  Because of the economic bearing on vehicle value, the odometer reading is regulated by Federal law. 

The odometer gauge may also be handy for scheduling vehicle maintenance.  Combined with other factors, such as driving style, it shows when we need to change oil, service a transmission, replace a timing belt and many other things.  We also use the odometer gauge to help in calculating fuel mileage.

3. What does the voltmeter show?

A typical volt meter reads around 14 volts

The voltmeter indicates how many electrical volts our charging system has available. Most vehicles operate on  a 12 volt system. When running the system will generally produce slightly above that amount. Readings from 12.5 to 14.5 are about normal on most vehicles. Readings below 12 volts or above 15 volts generally indicate a problem in the charging system.  This could be a bad battery, alternator or other things.  Low system voltage may cause engine cranking problems and cause several warning lights to come on.

4. The fuel gauge and what it means

Our fuel gauge show how much fuel remains in the tank.  Refilling our tank, when the gauge reaches 1/4 remaining, can prevent many problems. Fuel in the tank cools the fuel pump.  A higher fuel level also provides head pressure to lessen the load on the fuel pump. Continually running the fuel tank below the one quarter mark may significantly shorten the life of the fuel pump.

5. What is the oil pressure gauge?

Many people think the oil pressure gauge checks the oil level or condition.  This is not so, though some vehicles have lights to monitor these things.  Oil pressure is more like blood pressure in our bodies. Oil pressure is a measure of the force pushing the engine oil through the engine. Normal oil pressure is 20 to 50 pounds per square inch abbreviate PSI.  Our oil pressure will normally rise and fall with the engine speed, shown on our tachometer gauge. Most vehicles require at least 10 PSI of oil pressure for each 1000 RPM of engine speed.

Oil pressure is critical to the engine.  If the oil pressure gauge drops, the engine must be turned off. Severe engine damage will occur very quickly if oil pressure is too low.

6. What the temperature gauge means

Over heating causes very quickly causes severe engine damage

The engine temperature gauge show the average temperature of the engine.  Many people mistaking think it checks coolant level or condition.  This is not the case, but it is extremely important. Most engines will operate between 195 and 215 degrees Fahrenheit. Temperatures above this will damage the engine very quickly.  Temperatures above 260 degrees Fahrenheit are likely to do permanent damage to the engine.

Ironically, temperatures below 195 are also bad. When an engine runs too cold, sludge tends to form and we lose lubrication. Low engine temperature is also a leading cause of poor fuel mileage.

7. What is an amp meter

They rarely use an amp meter in modern vehicles, but they are a very useful gauge. The amp meter shows the rate and direction of current flow in the vehicle. When the needle points toward the negative (-) sign, the battery is being discharged, or has a negative current flow. The amp meter gauge pointing toward the positive (+) shows the system is charging. When the engine is running, the amp meter should point slightly positive. Being too far from zero in either direction indicates a problem with the battery or charging system.

8. The turbo boost gauge

Only turbo-charged vehicles will have a turbo-boost gauge. This gauge shows the pressure or vacuum in the intake manifold. More pressure means more performance, but lower fuel mileage. It may also suggest a problem with the turbo-charger. If boost drops, relative to engine RPM, shown on the tachometer gauge, a problem may be developing.

Like any part of a vehicle dash gauges can malfunction

General Motors (GM) speedometer and tachometer failure

General Motors vehicles have had a tremendous problem with dash gauge malfunctions.  The photo above was taken with the engine not running.  The speedometer gauge tends to show too many MPH at times. The tachometer gauge may read erratic or not at all. Fuel, oil pressure and temperature gauges also can read erratically and stick. The problem is caused by substandard stepper motors used in manufacturing the gauges.

AGCO can replace the instrument cluster and reprogram it for you if needed. Being familiar with the proper reading of the dash gauges can help the driver spot early warning signs.  This can easily prevent a breakdown and save a great deal of money.  Paying attention to your gauges can pay big dividends.

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