tech talk - june 05

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TECHTALK

34 JUNE 05 MOTOR INDUSTRY MAGAZINE WWW.MOTOR.ORG.UK

In part 1 (May issue) we discussed aspects

of electrical circuits and related test

equipment which must be understood if

you are to diagnose electrical faults

accurately and quickly. Here, we look at

the further application of such knowledge

to the fault-finding process.

REVISION IN BRIEF

In part 1 there was a detailed look at the 4

essential electrical rules: A digital voltmeter

displays the difference in voltage between

where you place the black probe and where

you place the red probe; the voltage after the

last resistance in a circuit will always be zeroproviding current can flow; volt drop will occur

across a resistance, providing current can flow;

and the volt drop across a resistance in a series

circuit is in direct proportion to the comparative

resistances values.

So far, we have studied the application of

these rules to the diagnostic process with a few

examples of open circuit faults (breaks in the

circuit).

Now we will discuss other fault types and

use tricks of the trade to enable you to

accurately diagnose faults very quickly andconfidently.

Short circuit faults

Open circuit faults are only one of several types

of fault that you could encounter. A short

circuit fault is where current flows in a circuit

(when it should not be) due to an unwanted

route to earth.

The location of the short to earth dictates

the effect that it will have on the circuit. It could

lead to a blown fuse/tripped circuit breaker,

melted looms or worse, a fire but it couldalso lead to a discharged battery through the

permanent on condition of an electrical

consumer. This latter problem is more generally

known as a parasitic drain fault, which we

will return to later.

A short circuit fault will blow a fuse when

there is no resistance in the circuit to limit

current flow. An excess of current flow in a

wire will lead to rapid heat build up resulting

in the blown fuse (hopefully!). A fuse is a weak

link by design and its current rating is such

that it will fail before the circuit can be

damaged. Once the fuse has blown, current

can no longer flow in the circuit, and so the

circuit is protected. A blown fuse can be a very

good indication that a short circuit has

occurred.

The circuit in example 1 has suffered a

short circuit fault. The short has led to the fuse

blowing. If we wish to diagnose the fault using

a voltmeter, we have a problem. The voltmeter

will confirm that the fuse has blown (batteryvoltage into the fuse, 0 volts out) but it is little

use in identifying the location of the short.

By linking the symptom (blown fuse) to the

circuit layout, we know immediately that the

short must lie between the output from the

fuse and the supply terminal of lamp 1. We

can be sure of this because if the fault were

after the lamp no symptom would be

presented, as this is an earth wire anyway; if

the fault was before the fuse, the fuse would

not have blown and we would have had a fire

to deal with. This is the reason that fuses are

placed as physically close to the battery as

possible that way, more of the circuit is

protected.

If the fuse is replaced with a new one, the

fuse will simply blow again. A method to

alleviate this problem would be to place a testlamp across the fuse block terminals (fuse

removed), thereby placing a resistance in the

circuit. Now that there is a resistance to current

flow in the circuit, the circuit will not be at risk

of damage. The short circuit fault will result in

the test lamp illuminating. To diagnose the

exact location of the short we now start to

disconnect connectors in turn on the circuit

ESSENTIAL ELECTRICAL SKILLS PART 2

Example 1

Example 1 test light use


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WWW.MOTOR.ORG.UK MOTOR INDUSTRY MAGAZINE JUNE 05 35

and observe the reaction of the test lamp.

When the test light goes out (upon

disconnection of a connector), the fault must

lie after that connector but before the

connector that had no effect on the test light

(upon disconnection).

The connector that you disconnect first will

be dictated by ease of access and minimal trim

removal. Remember that trim rarely refits as

well as the manufacturer fitted it in the first

place.

In example 1, the first connector to be

disconnected is d. If the test lamp goes out

we know the short must have occurred in

between the lamp and connector d. In this

instance the test lamp stayed illuminated and

so the fault isnt there. The next process is to

reconnected d and disconnect c. Bydisconnecting c the test lamp goes out telling

us that the fault lies between c and d.

Important! It is rare for a fuse on a

modern motor vehicle to protect only one

circuit. Because of this, the test light will

not go out, it will simply dim once the

fault is isolated. This is because your test

light is now supplying the other circuits

that the fuse protects, and current will be

flowing in the series circuits that you have

just created with your lamp. It is common

for relays to chatter as your test light iscreating volts drop on the supply side of

their windings.

Hint always choose a test lamp with

a high wattage bulb. The higher the

wattage, the lower the resistance the bulb

will have. This ensures that the difference

in test light brightness is more noticeable

when the fault has been isolated through

disconnection.

PROGRESS CHECK

Refer to the diagram in example 1 to answerthis question:

The test light went out (or dimmed

significantly) upon disconnection of connector

a, but the test lamp was unaffected by the

disconnection of connector b. Where does the

fault lie?

Answer available at the end of this article.

Example 2 shows a parasitic drain fault. This

is a fault on a circuit where there is an earth

located between a consumer and its switch

(providing the switch is on the earth side of the

circuit). This way, even if the switch is open or

closed, current is able to flow through the

consumer and it will remain on.

As the switchs position has no affect on

whether the consumer is on or not, electrical

control of the consumer is no longer possible

and it will run unnecessarily.

The diagnosis of such a fault can be simple

or complex depending upon the nature of the

problem and the circuit that has been affected.

In example 2, it is likely that the fault symptom

will be visible through a simple visual inspectionof the vehicle i.e. you will notice that the light

is on (a courtesy light for example).

In this instance, circuit disconnection is

again the method to use in order to identify the

location of the fault. We know the fault must

lie in between the lamp and the switch, and so

the fault can only lie in two places; between

the lamp and connector e, or connector e

and the switch. If we disconnect connector e

and the lamp stays on, the fault must lie in

between the lamp and connector e; if we

disconnect connector e and the lamp goesout, the fault must lie in between connector

e and the switch. In the example shown, the

lamp would go out when connector e is

disconnected.

If the consumer affected by a parasitic drain

fault displays no visible or audible symptoms

(such as a boot light), your approach needs to

be different. The customer will complain of a

flat battery (due to drainage through the

parasitic fault), but a look at the vehicles service

history will show that its been fitted with a

new battery already. Firstly, confirm that there

is not a charging fault on the vehicle. Once you

have confirmed that the vehicle is charging

efficiently, you should suspect parasitic drain.

Confirm this by measuring current draw from

the battery with all systems switched off. To do

this, place an ammeter in series with the

negative lead of the battery and measure the

current flow. Always fit the ammeter to the

negative side in order to observe safe practice

regarding correct disconnection of a battery.

Important! You should ensure that you

fit the ammeter without breaking theconnection that the vehicle has with the

negative post of the battery. This can be

done by ensuring that your ammeter leads

are in place before lifting the lead clamp

free of the battery post. This is necessary,

as some faults will disappear temporarily

if the circuit powers down.

Do not forget to remove the keys from

the ignition key cylinder and lock all doors.

Leave the vehicle for at least 10 minutes

before taking your reading as multiplex

systems that may be present take sometime to go to sleep.

As a guide, the battery draw should be no

more than about 0.1 amps (it will never be 0

amps due to systems that never power down

completely such as the clock and ECU memory

functions). If your reading is significantly more,

then you have confirmed a parasitic problem.

If you are unsure of what is an acceptable

current drain rate, the amp hour rating of the

OEM battery is often a good indication. For

example, if the battery is rated at 100

amp/hours, a current draw of 1 amp would

Example 2


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render the battery flat in 4 days (100 hours).

Not too good if you have left your vehicle at

an airport for 2 weeks!

Your next step should be to confirm that

the vehicle has not been fitted with any

accessories recently that could be causing

excessive drain (car phone kits etc.). If all is well,

then you are looking for a genuine parasitic

short circuit fault.

PARASITIC SHORT CIRCUIT

FAULT DIAGNOSIS

Start removing fuses one at a time until there

is a significant reduction in battery current draw

(leave your ammeter in place). The fuse that

affected your ammeter significantly is

protecting the faulty circuit. Through effectiveuse of the wiring diagram you can now

approach your fault through disconnection as

previously discussed. Be careful, though. Some

fuses will be protecting circuits that do not

power down, such as the clock and ECUs. You

are looking for asignificantreduction in battery

draw.

HIGH RESISTANCE FAULTS

Example 1

In the circuit pictured in example 1 there is an

unwanted resistance, i.e. a corroded terminal

in connector a. As has been previouslystated; a resistance is a resistance to current

flow, and so it will reduce the amount of

current able to pass through the lamp, causing

it to glow dimly. We can locate this fault with

the use of our voltmeter.

The first measurement to make would be

at terminal d of the lamp. In this example

we do not get 12 volts at terminal d, so the

resistance must be on the positive side. We

then measure the voltage at terminal a on

the circuit to check supply voltage is correct. In

this example the supply voltage is 12 volts. We

then check the voltage at terminal b of

connector a. The measurement is 12 volts

so we now know that the wire between a

and b is good. We then check the voltage

at terminal c of connector a. In this

example we find that the voltage is

considerably less than 12v so we have now

confirmed that the fault (high resistance) lies

in connector a between terminals b and

terminal c.

Example 2

In example 2 the corroded terminal is in

connector a that is now positioned on the

earth side of the lamp (the resistance). The

symptom will be identical to the problematic

circuit in example 1 i.e. a dim bulb.

Again, the voltmeter is the tool of choice.

In this example, the voltage at terminal b ofthe lamp will be supply voltage. This confirms

that the resistance is not on the supply side

and therefore must be further along in the

circuit on the earth side.

The voltage rules previously discussed tell

us that The voltage after the last resistance in

a circuit will always be 0 volts providing current

can flow. With this rule in mind, we would

expect the voltage at terminal c of the lamp

to be 0 volts. However, the lamp is not the last

resistance in this example, the fault is (the

corroded terminals). When we measure thevoltage at terminal c, and find that it is

perhaps 3 or 4 volts (dependent upon the

comparative resistance values of the lamp and

the corroded terminals see rule 4 at the start

of this article), we know that we are looking

for a high resistance fault on the earth side.

When we check the voltage at terminal d

of connector a we will have the same

voltage reading as found at terminal c of

the lamp. This confirms that the circuit

between terminal c of the lamp and

terminal d of the connector is good. A

voltage check at terminal e of connector a

will show 0 volts. We now know that we are

after the last resistance, so that resistance must

be in connector a. Fault found!

COMMON MISTAKES!

Example 1

It can be seen that the circuit in example 1

has a break in the wire on the earth side. The

technician in this example has placed both

probes of the voltmeter onto the circuit,

rather than putting the black probe onto

chassis earth. As can be seen, the meter is

displaying 0 volts. This is because of rule 1 -

A digital voltmeter displays the difference in

voltage between where you place the black

probe and where you place the red probe.

The voltage where the red probe has beenplaced is 12v and the voltage where the black

probe has been placed is also 12v. There is

no difference between 12 and 12, so the

meter displays no difference 0 volts. It is

easy to misinterpret this reading as meaning

no supply!

There are circumstances under which

using the meter this way is advantageous

checking earth wires is typical. If in example

1 you had first confirmed that the supply to

the lamp was good using the voltmeter with

the black probe on the chassis (12 volt readingdisplayed) you could then confirm that the

earth wire is good or bad by using the meter

as pictured. If the meter now read 0 volts

when placing the black probe on the earth

wire, the earth must be faulty.

In example 2, it can be seen that there is

a high resistance fault in connector A. It can

also be seen that the voltmeter is displaying

12volts, and therefore is not indicating any

problem on the supply side of the lamp. The

reason is that the technician has disconnected

the circuit at the lamp in order to probe the


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terminal. This action has prevented current

from flowing and therefore no volts drop has

occurred across the resistance (the fault). Rule

3 - Volt drop will only occur across a resistance,

providing current can flow.

Always keep the circuit complete and back

probe terminals!

Although these articles are detailed, you

have only 4 things to remember, and these are

our voltage rules quoted throughout.

Remember them, and electrical systems will

hold no fear for you!

Answer to progress check: The harness

must be shorted to ground in between

connectors a and b.

ProAuto is an automotive technical

training company based in Shrewsbury,

Shropshire. Its core business is the design

and delivery of technical training to the

automotive industry, which includes

vehicle manufacturers, component man-

ufacturers, diagnostic equipment man-

ufacturers and independent garages. The

company runs courses from numerous

venues nationally. Further details on

www.proautotraining.com, email info@

proautotraining.com, or telephone 01743

709679.Example 2

In a series of local training sessions for

motor factors, garage proprietors and

seminars for representative bodies such

as the IMI, Gates is advocating a re-

evaluation of drive systems maintenance.

In less than a generation, the role of the front-

end drive systems has been completely

transformed. Drive systems no longer simply

support the engine plant. They drive a rangeof key engine components that todays high

performance engines depend upon and

deliver the power to the auxiliary systems such

as air conditioning. So, the drive systems in

todays vehicles not only make the vehicles

more efficient and reliable they also make

them more comfortable to drive.

However, belt and tensioner man-

ufacturers Gates says the approach to drive

systems maintenance has not necessarily

evolved at the same pace as technological

change. The principles that may have beenappropriate when changing a belt in the

1970s do not apply to a belt installed as part

of a scheduled change on todays vehicle,

commented the companys technical

manager Iain Mitchell.

The engine layouts are more complex

and tensioning systems can differ slightly on

subsequent models in the same range. The

installed tension of the belts must be set

precisely, using a tool designed to do the job.

Theres no room for error. Failure of one drive

can easily lead to trouble with the other so it

makes sense to check over the components

of both the timing belt drive and the accessory

drive at the same time. Unfortunately, not

everyone always appreciates that, added

Mitchell.

IMI members from Cheshire and more

recently from the Shropshire region are

among a growing number of technical

specialist audiences to have heard the Gates

case for a complete overhaul of the front-enddrive systems as part of every scheduled belt

change. The seminars they attended covered

belt technology, installation procedures, fault

diagnostics and tension setting techniques.

The seminars also explode a few myths. For

example, its made clear that technicians

should never attempt to cure noise by easing

the tension of a belt.

Andrew Vaux, the Gates presenter at

recent events, emphasises the need for

technicians to make thorough checks of all of

the components in the timing belt drivesystem and all of the components in the

accessory drive system as well.

Changing the belt only should never be

an option, says Vaux. Since the lifetime of

all components is more or less the same, the

best solution to guarantee perfect and safe

performance is to change both belts and

metal components using a kit. In many cases,

replacing only the timing belt, tensioner and

pulleys /idlers may not even be sufficient.

While belt kits have been available for

some time, Gates has gone a step further in

order to support its case with the introduction

of the PowerGrip Kit Plus. Depending on

the application, this includes timing belt(s),

idler(s), belt tensioner units, other parts

needed for a complete overhaul (bolts,

springs, etc), oil seal(s) and/or O-rings and a

mileage sticker. In a further move, for certain

applications, Gates is planning to introduce a

Power Grip Kit Plus in early summer that also

includes the appropriate water pump.If theres a worn tensioner, an oil or water

leak or even a damaged main drive system

cover, the lifespan of the timing belt can

be compromised, with serious consequences

for the engine, says Iain Mitchell. The

technician must be sure that at the time of

the scheduled belt replacement, the water

DRIVE SYSTEMS MAINTENANCE


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SMART SERVICESmartrepairer.com is a web site from Power-Tec providinginformation about panel repair on both steel and aluminium, plus

other smart-type repair techniques.

It lists repairers offering this type of work both alphabetically

and by region. There are also sections devoted to tips, seminars

and training.

Further information on 01926 484187.

HELP FROM HELLAAdvice for workshops on air conditioning, electronics, electrics orlighting problems is now available from Hella through a technical

helpline.

Staffed by specialists with a workshop background, the helpline

is available to all aftermarket garages and repairers, supplying answers

to diagnostic, parts operation and fitting queries across a full range

of car, light commercial and CV applications.

The helpline is not intended to service general catalogue and

application enquiries; these will continue to be handled by Hellas

customer services department.

Helpline calls are charged at 1.20 per minute, inclusive of VAT,

and Hella says most enquiries should be resolved within five minutes.To guard against improper use, calls are limited to a maximum

duration of 25 minutes (charged at 30). In the event of a call

exceeding this length, the customer is asked to hang up and dial a

second time.

The helpline number is 09056 700 999. Further information

on 01295 272233.

Forte has extended its sponsorship of engine diagnostic management seminars to cover a

programme of diesel training for independent garages.

The half-day, Saturday morning HDI (High Pressure Direct Injection) and PDI (Piezo Direct

Injection) courses are being run by Eric Mitchell of AT Training. They are specifically geared

towards independent garage technicians who are working on the latest common rail

technology, but do not have ready access to the type of in depth courses offered within franchise

dealer groups.

Cost is 35 (plus VAT) per person.

Contact Rachel Roberts at Forte on 024 76 474069.

More than 140 new car and light commercial model references arelisted in the latest editions of the Mahle and Knecht filter catalogues.

Models using OE filtration supplied by Mahle now include the

Citroen C4, MK5 Golf, Skoda Octavia and BMW M5.

As well as oil, fuel and panel air filters, the catalogues feature

Mahles Micro-Star cabin filters.

FILTER FACTS

pump has the ability and integrity to complete

another full duty cycle because if the water

pump fails, coolant will leak and can

contaminate the belt. Long-term coolant

contamination will eventually lead to timing

belt failure.

On the issue of training, Andrew Vaux

said: In a garage with more than one

technician, or a business or dealership with

more than one location, there are likely to be

alternative approaches to timing belt

replacement procedures - probably related to

differences in the type of training received or

the relative experience of the technicians on

site. Unless everybodys approach is always the

same, oversights occur and problems in the

future may arise such as premature failure

of a belt.

He says that such a procedure should

specify the tools that technicians should

always use, indicate all of the components

that should be inspected, as well highlighting

the parts that should always be replaced.

Ultimately, the overhaul approach

means an additional cost to the car owner,

but the performance and long term reliability

of the engine will be significantly increased.

Formalising the procedure adds a quality

control measure that makes the garage more

efficient, adds a level of protection for the

garage and improves the quality of service that

the customer receives.

Gates provides free support and advice for

drive system diagnostic skills. It also provides

guidance on the introduction of a formal

inspection and installation procedure.

For further information contact:

Iain Mitchell on 01387 242075, email

[email protected].

DIESEL TRAINING FROM FORTE

tech talk - june 05

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