JAGUAR XK User Manual

Circuit T esting

Contents

Introduction 2
Testing Equipment 3
Electrical Units and Quantities 4
Rules Governing Electrical Circuits 5
Basic Electrical Circuit Faults 6
Voltmeter Tests 8
Ohmmeter Tests 10
General Approach to Circuit Testing 11
Diagnostic Strategy 12
Professional Electrical Practices 13
Circuit Failure Testing 14

1

Circuit T esting

Introduction

Most Jaguar Service Training courses concentrate on the technical details of a specific vehicle
system; how the components function and how to determine if the system is functioning cor­rectly. Advanced Diagnostics courses have a different focus; they concentrate on the process
used to diagnose faults. They are for professionals who already have practical knowledge and
experience in the systems covered by the courses.

We have all developed diagnostic techniques that we use on a regular basis and this course
provides the opportunity to examine and further develop these techniques and share them with
other technicians.

To get the most from Advanced Diagnostic training, you should practice the structured diagnostic
process that will be presented and apply some critical thought to the process that you use. Time
spent developing your diagnostic technique here will be paid back as you solve problems more
efficiently at the dealership. Combining the structured diagnostic process with your existing
experience will result in:

• More effective troubleshooting

• More “fixed right the first time” repair

• More confidence in the repair
Most importantly, the enhancement of your diagnostic skills will increase customer loyalty and

promote your professional image.

2

Circuit T esting

Testing Equipment

Because sensitive electronic circuits can be damaged by using analog (dial type) meters, test
lights and many types of circuit testers, only digital multimeters (DVOM) should be used.
Analog meters require too much power to be used in circuits with sensitive digital components.
DVOMs require very little power. In addition, DVOMs are more accurate, enabling precise value
measurement. By using a DVOM with a combination digital / analog display or a MIN / MAX
mode, it can be determined if the measured value is increasing or decreasing during the test.

DIGITAL MULTIMETER (DVOM) WITH LEADS AND CURRENT PROBE

OFF

V

V

300mV

Ω

A

A

TAD/1.01

Jaguar diagnostic equipment – both PDU and JDS – have digital multimeter capabilities and can
be used for circuit analysis.

When performing electrical tests, refer to the applicable Electrical Guide to determine the
circuit construction, associated circuits, wire colors and connector, splice, component and
ground locations.

NOTES

3

Circuit T esting

Electrical Units and Quantities

The international engineering and scientific communities have adopted standards for quantities
and units in order to do away with the confusion caused by converting between the various mea­surement systems used by individual countries. The ISO (International Organization for
Standardization) published the standards in their documents ISO 31 and ISO 1000. The units
used in this standardized measurement system are known as SI (Systéme International) units.

Selected Units
Base unit Symbol SI unit Symbol

Electric current I Ampere A
Electric potential E Volt V
Electric conductance G Siemens S
Electric resistance R Ohm Ω
Quantity of electricity Q Ampere hour A h
Electric capacitance C Farad F
Time t second s
Power P Watt W

Multiples or decimal fractions of SI units are shown by prefixes or prefix symbols before the name
of the unit. Refer to the chart below.

Selected Quantities

Quantity Prefix Symbol

1,000,000,000 (billion [milliard]) giga G

1,000,000 (million) mega M

1,000 (thousand) kilo k

100 (hundred) hecto h

10 (ten) deka da

0.1 (tenth) deci d

0.01 (hundredth) centi c

0.001 (thousandth) milli m

0.000001 (millionth) micro µ

Examples: 2,000,000 Ohms (two million Ohms) is written as 2 MΩ (two mega-Ohms)

6/1,000 Volt (six-thousandths of a Volt) is written as 6 mV (six milli-Volts)

NOTES

4

Circuit T esting

Rules Governing Electrical Circuits

Ohm’s Law describes the relationship between voltage and resistance in solid and liquid conduc­tors: Electrical potential

(R)

. The formula is written as

Ohm’s Law can be useful during diagnoses to help determine the effect of voltage, current flow
or resistance in a circuit. If two values are known, the third value can easily be calculated.

(E)

is equal to the electrical current

E = I x R [E (volts) = I (amperes) x R (ohms)].

(I)

multiplied by the electrical resistance

The diagram at right is designed to simplify the
use of Ohm's Law. The horizontal line indi­cates that two values should be divided; the
vertical line indicates that two values should
be multiplied. To use the formula, substitute
the known or measured values for their sym­bols, cover the unknown value with your
thumb and multiply or divide, as indicated, to
find the missing value.

For example, if the electrical potential
the current

(R)

is not, divide the electrical potential
the electrical current
resistance

R (ohms) = E (volts) ÷ I (amperes).

Electrical resistance depends on the dimensions, material and temperature of the conductor.
Resistance in metal conductors generally increases with the length and temperature of the con­ductor. Conductors with larger cross sectional areas have less resistance than conductors with
smaller cross sectional areas.

(I)

are known, but the resistance

(I)

to find the electrical

(R)

:

(E)

(E)

and

by

DIAGRAM FOR USING OHM'S LAW

E

IR

E = I x R I = E ÷ R R = E ÷ I

Electrical Power

Electrical power is expressed in watts:

W (watts) = E (volts) x I (amperes).

TAD/1.02

Energy Conversions

1 Watt = 0.0013 HP
1 Kw = 1.341 HP
1 HP = 745.7 Watts

NOTES

5

Circuit T esting

Basic Electrical Circuit Faults

Electrical circuit faults can be categorized as follows:

OPEN CIRCUIT

HIGH RESISTANCE

OPEN CIRCUIT

TAD/1.03

Open circuit

An open circuit is a break in the path of current
flow. If the circuit is powered, a voltage
potential will be present in the portion of the
circuit that is still connected to the power
source.

With parallel circuits, an open circuit in one
branch will stop operation in that branch, but
the other branches will continue to operate.

An ohmmeter test can determine if a circuit is
open (infinite resistance [∞ Ω]).

A voltmeter can also be used to determine an
open circuit. By measuring the available volt­age at various points or the voltage drop
between two points, it is possible to deter­mine the location of the open circuit.

High resistance

A high resistance circuit is a circuit with more
resistance than specified. High resistance
reduces the amount of power (current x volt­age) available for components connected to
the circuit.

HIGH RESISTANCE

(CORROSION)

TAD/1.04

High resistance can be caused by loose, dirty
or corroded connections. Broken strands of
conductor within a wire’s insulation or at a
connector will also increase circuit resistance.

When diagnosing a circuit for high resistance,
disturb the connections as little as possible
until the area of high resistance has been
found. Disturbing connections may clean any
corrosion or dirt, temporarily correcting the
problem and making diagnosis difficult.

An ohmmeter test on an unpowered circuit
can determine high resistance.

An available voltage or voltage drop test on a
powered circuit can also determine areas of
high resistance.

6

Circuit T esting

Short circuit to ground

A short circuit to ground occurs when the circuit
is grounded or partially grounded where not
designed. If the short circuit is located after the
load, circuit control may be lost causing operation
when it is not wanted.

To diagnose a short circuit to ground in a fused
circuit, substitute a voltmeter for the fuse.
Systematically disconnecting circuit compo­nents until the voltmeter reads 0 V will identify
the area of the short circuit.

Short circuit to voltage

A short circuit to voltage occurs when insulation
failure causes a conductor to contact the volt­age of another circuit. The circuit (or circuits)
will operate improperly.

Carefully observe the symptoms and related
symptoms and refer to the Electrical Guide to
understand the circuits involved. Remove
fuses until the circuit is isolated, then measure
resistance and voltage as appropriate to find the
problem area.

SHORT CIRCUIT TO GROUND

SHORT CIRCUIT TO VOLTAGE

SHORT CIRCUIT

TO GROUND

SHORT CIRCUIT

TO VOLTAGE

TAD/1.05

NOTES

TAD/1.06

7