Digital instrumentation
9.11.2 Digital instrumentation
the magnetic flux produced by each coil. As the number of turns remains constant the current is
The block diagram shown in Figure 9.41 is typical the key factor. Figure 9.40 shows the principle of
of a digital instrumentation system. All signal the air cored gauge together with the circuit for
conditioning and logic functions are carried out in
228 Advanced automotive fault diagnosis
Reset signal
Figure 9.42 Trip computer display and a vehicle ‘map’
● Service or inspection interval warning lights
can be used The warning lights are operated
Driver
broadly as a function of time but, for example, the service interval is reduced if the engine experiences high speeds and/or high tempera- tures. Oil condition sensors are also used to
help determine service intervals. ● Alternator warning light Works as normal
Display
but the same or an extra light can be made to operate if the output is reduced or if the drive belt slips. This is achieved by a wire from one
Figure 9.41 Digital instrumentation
the ECU. This will often form part of the dashboard phase of the alternator providing a pulsed sig- assembly. Standard sensors provide information
nal, which is compared to a pulsed signal from to the ECU, which in turn will drive suitable dis-
the ignition. If the ratio of the pulses changed plays. The ECU contains a ROM (read only mem-
this would indicate a slipping belt. ory) section, which allows it to be programmed to
a specific vehicle. The gauges used are as
9.11.3 Vehicle condition
described in the above sections. Some of the extra
monitoring
functions available with this system are described briefly as follows.
VCM or vehicle condition monitoring is a form of instrumentation. It has now become difficult to
● Low fuel warning light Can be made to separate it from the more normal instrumentation illuminate at a particular resistance reading
system discussed in the first part of this chapter. from the fuel tank sender unit.
Figure 9.42 shows a typical display unit, which
● High engine temperature warning light
also incorporates the vehicle map (see next sec- Can be made to operate at a set resistance of
tion). The complete VCM system can include the thermistor.
driver information relating to the following list:
● Steady reading of the temperature gauge
● high engine temperature; To prevent the gauge fluctuating as the cooling
● low fuel;
system thermostat operates, the gauge can be
● low brake fluid;
made to read only at say five set figures. For
● worn brake pads;
example, if the input resistance varies from
● low coolant level;
240 to 200 ⍀ as the thermostat operates, the
● low oil level;
ECU will output just one reading correspond- ● low screen washer fluid; ing to ‘normal’ on the gauge. If the resistance
● low outside temperature; is much higher or lower the gauge will read to
● bulb failure;
one of the five higher or lower positions. This ● doors, bonnet or boot open warning.
gives a low resolution but high readability for the driver.
A circuit is shown in Figure 9.43, which can
● Oil pressure or other warning lights can be
be used to operate bulb failure warning lights made to flash This is more likely to catch
for whatever particular circuit it is monitoring. the driver’s attention.
The simple principle is that the reed relay is only
Electrical systems 229
Vehicle speed
Fuel used
Fuel level
ECU Display
Driver input
Time
Figure 9.43 Bulb failure warning circuit
Panel lights
(to change display)
Figure 9.45 Trip computer block diagram Output
High S 1 open
Low S 1 closed R 1 Out of range if
looks as if the door of the vehicle map (plan view
wire breaks or shorts
of the car) is open. Low outside temperature or ice warning is often a large snowflake!