Common Gauge Types

Bi-Metallic Strip

The bi-metallic gauge operates using heat. Inside the gauge is a small piece of bimetallic material that is connected to the needle. Wrapped around this strip is some thin wire which gets warm when electrical current flows through it. This material is created using two different metals bonded together. These metals expand at different rates when they are heated. This causes the material to bend as one metal expands to a greater length than the other.

Moving Coil

This type of gauge uses magnetism to move the needle. Inside the gauge is a coil which is connected to the supply terminals. When a voltage is applied to the terminals current flows through the coil creating a magnetic field. This field causes the needle to be moved by a varying amount. As the field increases the needle will move toward the maximum value indicated by the gauge. The field is linked directly to the voltage applied and the current flowing through the terminals. This value will depend upon the state of the sensor that is being used.

Tachometers

Inductively Coupled

These tachometers use a the magnetic field generated by the car ignition system to generate the pulses to drive the tachometer.

Direct Coupled

Direct coupled tachometers are connected to the ignition system itself. The pulses generated for turning the ignition coil on and off are used to drive the tacho.

The Gauge

The gauge itself simply measures the speed that the pulses are generated by the engine. There are two factors that will govern this which is the number of cylinders and the engine speed.

The tachometer itself operates by averaging out the pulses using a moving coil gauge. When the pulse rate (frequency) is low the average is low and the needle will be closer to zero RPM. As the engine speed rises the average will rise moving the needle away from zero RPM.

Speedometers

A speedometer is used to measure the speed of a travelling vehicle. The speed is measured at some point between the gearbox and the wheels. In most cases the measurement is made on the gearbox but could also be made at the wheel itself.

A speedometer is basically a tachometer that has been specially calibrated to read MPH/KPH rather than RPM. The calibration takes several factors into account such as the gear reduction from the tailshaft to the flexible cable, the final drive ratio in the differential unit, and the diameter of the driven tires

If the wheel/tyre size is changed this will result in an inaccurate reading. If the diameter is decreased the wheels will turn faster leading to a higher speed reading than before.

Fuel Gauge

The Sender Unit

The purpose of the sender unit is to measure the level of the fuel in the fuel. All sender units use a float that rises and falls with the level in the tank. This float is connected mechanically to an electrical device known as a variable resistor. Variable resistors can be found in many other electrical such as volume controls and household dimmer switches.

Operation

The fuel gauge itself operates as described in the bi-metallic gauge section.

If you think of the system in relation to a household dimmer switch you can begin to see how the whole system operates. When the dimmer switch is in a low position the light is very dim and not much heat is generated. When the bimetallic strip is cool the needle will be in the empty position. However as the light switch is adjusted the brightness will increase, generating more heat. In the fuel gauges this is causes the bimetalic strip to deflect to a greater extend and the needle to move further toward the full position.

The Voltage Regulator

When a car is sat still, without the engine running and just the ignition switch on, the voltage measured at the terminals of the battery will be approximately 12V. However when the engine is started this will rise to about 13.6 volts. If it weren't for the voltage regulator this would cause the needle to move since the current flowing through the fuel gauge circuit would rise causing the temperature of the bimetallic strip to rise.

Voltmeter

A voltmeter is used to monitor the state of the battery and its charging system. For normal operation the voltage should remain in the range 11.5 to 13.6 volts.

If the needle indicates a voltage higher than this it may indicate a problem with the charging system. This condition should be avoided quickly as it may cause damage to sensitive electrical equipment such as stereo head units or amplifiers.

A lower voltage would indicate that the charging system is not working or the battery needs to be replaced. Action should be taken as soon as possible because the car will become unreliable and difficult to start.

The gauge itself is a moving coil instrument and it is connected directly to the battery terminals

Ammeter

The pupose of an Ammeter is to ensure that the battery is not being continually discharged. It is a more accurate way of measuring the operation of the charging system.

This type of gauge is a moving coil meter and must be connected in such a way that both the feed to the iginition switch and the charging wire from the alternator are connected to it. This means that all current flows through the gauge for all of the cars needs. If the current is flowing from the battery to the cars equipment then the gauge will show a -discharge this will cause the battery voltage to decrease over a long period. When the engine is running the alternator or generator should create a charging current slightly higher than that being drawn by the cars equipment. This will be shown on the Ammeter as being slightly toward the + on the dial. If the meter moves excessively to the right this would indicate that the charging system is over charging continued operation in this state will damage the battery.

When combined with a Voltmeter it can help diagnose porblems causing flat batteries. For instance if the Ammeter shows that the battery is being charged and the Voltmeter is reading low then the battery is probably at fault. However if the battery voltage is low and the Ammeter show a continued discharge then the alternator/generator probably is at fault.

How stuff works
http://auto.howstuffworks.com/fuel-gauge.htm
Fundamentals of Motor Vehicle Technology
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