Trip computer

9.11.4 Trip computer

Figure 9.44 Equivalent circuit of a dual resistance self-testing system

The trip computer used on many top range vehicles is arguably an expensive novelty, but is popular none-the-less. The display and key pad of a typical

operated when the bulb being monitored is drawing trip computer is shown above. The functions avail- current. The fluid and temperature level monitor-

able on most systems are:

ing systems work in a similar way to the systems

● time and date;

described earlier but in some cases the level of a ● elapsed time or a stop watch; fluid is by a float and switch.

● estimated time of arrival; Oil level can be monitored by measuring the

● average fuel consumption; resistance of a heated wire on the end of the dip

● range on remaining fuel; stick. A small current is passed through the wire

● trip distance.

to heat it. How much of the wire is covered by oil will determine its temperature and therefore its

The above details can usually be displayed in resistance.

imperial, US or metric units as required. Figure Many of the circuits monitored use a dual

9.45 shows a block diagram of a trip computer resistance system so that the circuit itself is also

system. Note that several systems use the same checked. Figure 9.44 shows the equivalent circuit

inputs and that several systems ‘communicate’ for this technique. In effect it will produce one of

with each other. This makes the overall wiring three possible outputs: high resistance, low resist-

very bulky – if not complicated. ance or an out of range reading. The high or low

resistance readings are used to indicate say cor- rect fluid level and low fluid level. A figure out-

9.11.5 Displays

side these limits would indicate a circuit fault of If the junction of a diode is manufactured in a either a short or open circuit connection.

certain way, light will be emitted from the junc- The display is often just a collection of LEDs

tion when a current is made to pass in the for- or a back lit liquid crystal display (LCD). These

ward biased direction. This is an LED and will are arranged into suitable patterns and shapes such

produce red, yellow or green light with slight as to represent the circuit or system being moni-

changes in the manufacturing process. LEDs tored. A door open will illuminate a symbol which

are used extensively as indicators on electronic

230 Advanced automotive fault diagnosis

Figure 9.46 Display techniques

equipment and in digital displays. They last for a the light passing through it is no longer twisted by very long time (50 000 hours) and draw only a

90°. This means that the light polarised by the small current.

first polariser will not pass through the second, LED displays are tending to be replaced for

and will therefore not be reflected. This will show automobile use by the liquid crystal type, which

as a dark area on the display. These areas are con- can be back lit to make it easier to read in the

structed into suitable segments in much the same daylight. However, LEDs are still popular for

way as with LEDs to provide whatever type of many applications.

display is required. The size of each individual The actual display will normally consist of

area can be very small such as to form one pixel

a number of LEDs arranged into a suitable pat- of a TV or computer screen if appropriate. tern for the required output. This can range from

A vacuum fluorescent display works in much the standard seven segment display to show

the same way as a television tube and screen. It is numbers, to a custom designed speedometer

becoming increasingly popular for vehicle use display.

because it produces a bright light (which is Liquid crystals are substances that do not melt

adjustable), and a wider choice of colours than directly from a solid to the liquid phase, but first

LED or LCD displays. The glass front of the dis- pass through a paracrystalline stage in which the

play can be coloured to improve the readability molecules are partially ordered. In this stage a

and aesthetic value. This type of display has many liquid crystal is a cloudy or translucent fluid but

advantages but the main problem for automobile still has some of the optical properties of a solid

use is its susceptibility to shock and vibration. crystal.

This can be overcome, however, with suitable Mechanical stress, electric and magnetic

mountings. Figure 9.46 shows some instrument fields, pressure and temperature can alter the

and other displays using a number of the previ- molecular structure of liquid crystals. A liquid

ously discussed techniques. crystal also scatters light that shines on it. Because of these properties, liquid crystals are used to display letters and numbers on calcula-

9.12 Diagnosing

tors, digital watches and automobile instrument displays. LCDs are also used for portable com-

instruments system

puter screens and even television screens. The

faults

LCD has many more areas of potential use and developments are on going. In particular this