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Tech Corner

Fault Diagnosis Service Manual, Lighting – submitted by Rich Weiss
Edited from the original Lucas Manual

Fault diagnosis is the method of locating faults while the electrical equipment is installed and active. Ideally,
we strive for tests that are both accurate that can be carried out in the shortest possible time using the minimum
amount of equipment. The purpose of this article is to present a logical sequence of tests that meet these
objectives and can be carried out on the various components of an originally equipped Triumph engine.
1 . The majority of procedures involve circuit testing and the principle used will be that of checking for “voltage
drop" where a voltmeter is connected in parallel with the particular circuit to be tested.
2 . As voltage drop exists only when current is flowing and varies according to the amount of current. The
circuit, therefore must be checked “under load”, i.e. while active under its normal current. In certain instances,
this current will be measured using a test ammeter.
3 . The acceptable volt drop figure for most circuits is 10% of system voltage (1.2 volts on a 12-volt system)
but there are exceptions to this rule as in the case of the starter circuit where the maximum voltage drop allowed
is 0.5 volts.
4 . Throughout the procedures wherever an exception applies this figure will be clearly stated.
5 . The following is the minimum equipment necessary to carry out this fault diagnosis:

6 . Battery Tester
7 . Multimeter
LIGHTING
INTRODUCTION
Quoting from the document, ‘in order to comply with legal requirements, the motor vehicle must be fitted with certain
forms of lighting which must be maintained in working condition. Although the requirements may vary slightly with
different types of vehicles, in general, the present requirements are as follows:
1. Headlights (two minimum).
2. Side, rear and number plate lamps.
3. Direction indicator lamps.
4. Stop lamps.’

Apart from the legal aspect, efficient lighting will enable the driver to “see and be seen”, and a driver using defective or
incorrectly aligned lamps is not only a danger to himself but also to other road users. It must be remembered that lights
and lamp alignment are part of the testing from those states that require it. Additional lamps may be fitted to a vehicle, but
they must comply with the regulations governing those particular lamps. For example, any additional lamps fitted to the
front of a vehicle where the mounting height exceeds 24' from the ground to the center of the lamps automatically become
headlights unless they are used in conditions of fog or falling snow. As such, they must comply with requirements
applicable to headlights. When testing lighting circuits, the “volt drop” principle will be used. This will involve checking
the “supply voltage available" under load (at the battery) then the voltage available at the actual lamp involved. Where the
difference in voltage exceeds the permitted limits, the voltmeter will be connected in parallel with both the supply and
return sides of the circuit to determine where the fault exists. The exact procedure will be stated in this section. In the case
of direction indicator circuits, it will be necessary to use an ammeter in addition to the voltmeter, as the operation of the
“flasher" unit is dependent on current consumption.