1 General information and
precautions
General information
The electrical system is of 12-volt negative
earth type. Power for the lights and all
electrical accessories is supplied by a
lead/acid battery which is charged by the
alternator.
This Chapter covers repair and service
procedures for the various electrical
components and systems not associated with
the engine. Information on the battery, ignition
system, alternator, and starter motor can be
found in Chapter 5.
Precautions
Warning: Before carrying out
any work on the electrical
system, read through the
precautions given in “Safety
first!” at the beginning of this manual and
in Chapter 5.
Caution: Prior to working on any
component in the electrical system, the
battery negative lead should first be
disconnected, to prevent the possibility of
electrical short-circuits and/or fires.
Disconnection of the battery will, however,
erase the audio unit anti-theft security
code and also erase the memories of many
of the electrical system electronic control
units. Refer to Chapter 5, Section 1 for
additional information on the effects of
battery disconnection.
2 Electrical fault finding -
general information
2
Note: Refer to the precautions given in
“Safety first!” and in Section 1 of this Chapter
before starting work. The following tests relate
to testing of the main electrical circuits, and
should not be used to test delicate electronic
circuits (such as engine management systems,
anti-lock braking systems, etc), particularly
where an electronic control module is used.
Also refer to the precautions given in Chap-
ter 5, Section 1.
General
1 A typical electrical circuit consists of an
electrical component, any switches, relays,
motors, fuses, fusible links or circuit breakers
related to that component, and the wiring and
connectors which link the component to both
the battery and the chassis. To help to
pinpoint a problem in an electrical circuit,
wiring diagrams are included at the end of this
manual.
2 Before attempting to diagnose an electrical
fault, first study the appropriate wiring diagram,
to obtain a complete understanding of the
components included in the particular circuit
concerned. The possible sources of a fault can
be narrowed down by noting if other
components related to the circuit are operating
properly. If several components or circuits fail
at one time, the problem is likely to be related
to a shared fuse or earth connection.
3 Electrical problems usually stem from
simple causes, such as loose or corroded
connections, a faulty earth connection, a
blown fuse, a melted fusible link, or a faulty
relay. Visually inspect the condition of all
fuses, wires and connections in a problem
circuit before testing the components. Use
the wiring diagrams to determine which
terminal connections will need to be checked
in order to pinpoint the trouble-spot.
4 The basic tools required for electrical fault-
finding include a circuit tester or voltmeter (a
12-volt bulb with a set of test leads can also
be used for certain tests); an ohmmeter (to
measure resistance and check for continuity);
a battery and set of test leads; and a jumper
wire, preferably with a circuit breaker or fuse
incorporated, which can be used to bypass
suspect wires or electrical components.
Before attempting to locate a problem with
test instruments, use the wiring diagram to
determine where to make the connections.
5 To find the source of an intermittent wiring
fault (usually due to a poor or dirty
connection, or damaged wiring insulation), a
“wiggle” test can be performed on the wiring.
This involves wiggling the wiring by hand to
see if the fault occurs as the wiring is moved.
It should be possible to narrow down the
source of the fault to a particular section of
wiring. This method of testing can be used in
conjunction with any of the tests described in
the following sub-Sections.
6 Apart from problems due to poor
connections, two basic types of fault can
occur in an electrical circuit - open-circuit, or
short-circuit.
7 Open-circuit faults are caused by a break
somewhere in the circuit, which prevents
current from flowing. An open-circuit fault will
prevent a component from working.
8 Short-circuit faults are caused by a “short”
somewhere in the circuit, which allows the
current flowing in the circuit to “escape” along
an alternative route, usually to earth. Short-
circuit faults are normally caused by a
breakdown in wiring insulation, which allows a
feed wire to touch either another wire, or an
earthed component such as the bodyshell. A
short-circuit fault will normally cause the
relevant circuit fuse to blow.
Finding an open-circuit
9 To check for an open-circuit, connect one
lead of a circuit tester or the negative lead of a
voltmeter either to the battery negative
terminal or to a known good earth.
10 Connect the other lead to a connector in
the circuit being tested, preferably nearest to
the battery or fuse. At this point, battery
voltage should be present, unless the lead
from the battery or the fuse itself is faulty
(bearing in mind that some circuits are live
only when the ignition switch is moved to a
particular position).
11 Switch on the circuit, then connect the
tester lead to the connector nearest the circuit
switch on the component side.
12 If voltage is present (indicated either by
the tester bulb lighting or a voltmeter reading,
as applicable), this means that the section of
the circuit between the relevant connector
and the switch is problem-free.
13 Continue to check the remainder of the
circuit in the same fashion.
14 When a point is reached at which no
voltage is present, the problem must lie
between that point and the previous test point
with voltage. Most problems can be traced to
a broken, corroded or loose connection.
Finding a short-circuit
15 To check for a short-circuit, first
disconnect the load(s) from the circuit (loads
are the components which draw current from
a circuit, such as bulbs, motors, heating
elements, etc).
16 Remove the relevant fuse from the circuit,
and connect a circuit tester or voltmeter to the
fuse connections.
17 Switch on the circuit, bearing in mind that
some circuits are live only when the ignition
switch is moved to a particular position.
18 If voltage is present (indicated either by
the test bulb lighting or a voltmeter reading,
as applicable), this means that there is a
short-circuit.
19 If no voltage is present during this test,
but the fuse still blows with the load(s)
reconnected, this indicates an internal fault in
the load(s).
Finding an earth fault
20 The battery negative terminal is connected
to “earth” - the metal of the engine/
transmission unit and the vehicle body - and
many systems are wired so that they only
receive a positive feed, the current returning via
the metal of the car body. This means that the
component mounting and the body form part
of that circuit. Loose or corroded mountings
can therefore cause a range of electrical faults,
ranging from total failure of a circuit, to a
puzzling partial failure. In particular, lights may
shine dimly (especially when another circuit
sharing the same earth point is in operation),
motors (eg wiper motors or the radiator cooling
fan motor) may run slowly, and the operation of
one circuit may have an apparently-unrelated
effect on another. Note that on many vehicles,
earth straps are used between certain
components, such as the engine/transmission
and the body, usually where there is no metal-
to-metal contact between components, due to
flexible rubber mountings, etc.
21 To check whether a component is
properly earthed, disconnect the battery (refer
to Chapter 5, Section 1) and connect one lead
of an ohmmeter to a known good earth point.
Connect the other lead to the wire or earth
12•2 Body electrical system
1380 Rover 800 Series Remake
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