29.8.
Maintenance of Lighting Systems
On modern vehicles the graphical display on the instrument panel warns the driver of a lighting fault. Other than the usual visual checks for cable security and condition, maintenance works in lighting systems occur only after a fault is developed.
29.8.1.
Failure of a Lamp
The warning to the driver of bulb outage should be checked to ensure that the monitoring system is not giving a false signal. Incase of an incorrect indication, the circuit should be checked. Most lighting faults are due to the failure of a fuse or bulb.
Fuse. A blown fuse should be replaced with a fuse of the correct rating. If the new fuse blows immediately then the fault must be identified before installing another fuse.
Bulb Failure. The suspected bulb should be replaced with the recommended one. Glass surface should not be touched with the fingers, especially the quartz-halogen type, and hence a clean cloth should be used. Stains on a bulb can be removed by washing with a methylated spirit and drying with a lint-free cloth.
Wiring Faults. If a visual check of the cables does not reveal the defect, and the bulb and fuse are intact or serviced, then the circuit should be tested with a voltmeter. The principle of testing as applied to a simple lighting circuit is shown in Fig. 29.37.
Fig. 29.37. Voltammeter checks on simple lighting circuit.
29.8.2.
Tests
Test I. A voltmeter (Vi) is connected across the battery to measure the voltage under lighting load.
Test II. When the voltmeter (V2) is connected across the lamp the voltage should be similar to the voltage at Test I. If a resistance in the circuit causes the reading to differ by more than 10% of the battery voltage, the cause should be determined by using Tests III and IV.
‘ Test III. Voltemeter (V3) indicates the voltage drop on the insulatet line. If an excessive drop is observed, the location of the fault can be identified by moving one voltmeter lead along the connection points in the circuit path until a stage is reached where the change in voltage is considerable.
Test TV. Voltmeter (V4) shows the drop in the earth line. By using a similar technique to Test III, a high resistance can be pinpointed. The total voltage drop from Tests III and IV should be less than 10% of the battery voltage, i.e. less than 1.2 V for a 12V system.
A 12V test lamp can be used to identify quickly for an open-circuit (Fig. 29.38). A circuit break between points 1 and 4 is easily determined and with one side of the lamp connected in good earth, the break can be located.
Headlamp Alignment.
The headlamps must be correctly aligned to meet the regulations in respect to dazzle and also to achieve good illumination of the road. Headlamp alignment, as well as checking the condition of the lamp forms a part of the annual M.O.T. test.
Although the lamps may be checked manually by observing the illumination pattern on a vertical screen, most garages use special aligning equipment to achieve greater accuracy (refer section 29.3.6).
Fig. 29.38. Open-circuit test using a test lamp.
