Download Understanding basic vehicle electrical Circuits This two? Part topic

Understanding basic vehicle electrical Circuits
This two? Part topic introduces our new beginners to diagnostics course; it focuses on
simple test procedures, avoiding complex tools, without compromise to the end result.
As automotive technology continues to evolve and become even more complex,
mechanical systems are being enhanced, or even replaced with electronic components
and systems. Electronic suspensions and anti- lock brakes are two of the major
changes that have taken place under today’s vehicles. But coming soon are fully
electronic “brake by wire” systems that have no hydraulics whatsoever!
The 2003 Mercedes- Benz SL was the first production vehicle to feature an electronic
brake system. The brakes are still applied hydraulically, but the amount of force is
controlled by a computer rather than the driver’s foot. When the driver steps on the
brake pedal, a command is sent to the module which then decides how much pressure,
if any! Is needed and where to apply it. The system can react much more quickly in an
emergency situation and can even break just the outside wheels when braking in a
turn.
As sophisticated as this new Mercedes system is, future brake systems will likely do
away with the hydraulics altogether and be fully electronic. Small servo motors at
each wheel will squeeze the pads against the rotors. Brakes may not even have any
friction linings but be fully magnetic. All kinds of new electronic brake systems have
already been developed by Robert Bosch, Continental/ Teves, Kelsey Hayes, Delphi
and others for future vehicle applications.
As vehicle manufacturers move toward 42 volt electrical systems, were also going to
see a growing number of vehicles equipped with fully electronic steering. Hydraulic
pumps and gears will be replaced with electric motors.
The familiar scenery under today’s cars is radically different. So if you’re not
comfortable dealing with the electronics on today’s cars, you’re going to be
challenged even more in the years ahead as suspension control, steering, anti lock
braking, traction control, stability control and power train management become even
more integrated and less distinct as separate systems.
The purpose of this article is to introduce you to some of the basics of electrical
circuits and electronic troubleshooting.
Voltage supply
Regardless of how familiar you are with basic electrical circuits, keep two things in
mind: all circuits need a power source or supply voltage, and all circuits require
continuity otherwise the power wont flow through the circuit, to the final component,
a ground path.
The first thing to check, therefore, when there’s an electrical problem, is reference
voltage on load. The load in a circuit is the component, be it a light bulb, relay, pump,
solenoid or whatever. All you need to check voltage is a digital multimeter, or even
better a power probe hook.
A simple 12 volt test light that glows when there’s voltage will tell you if the circuit is
complete, , but it wont tell you how much voltage, or what the current flow and circuit
resistance is.. Many components, like relays and solenoids, won’t function properly if
the current flow is incorrect.
If the voltage is low, check the battery, battery cables and charging system. A fully
charged battery should read 12.7 volts with no load on it. A reading of 12.4 volts or
less would indicate a low battery and a possible battery or charging system problem.
The battery should be recharged and load tested to evaluate its condition. The
charging system should provide about 13.5 to 14.5 volts at idle, but look up the exact
specs for the vehicle because the charging voltage can vary.
If the charging system output is low, the alternator, voltage regulator or diode pack
(rectifier) inside the alternator that converts alternating current (AC to direct current
(DC) may be faulty , remember, problems with the battery and charging system can
affect every electrical circuit and component on the vehicle. So always make sure
these components are working properly before moving ahead with your diagnosis.
Suppose you check a circuit and find no voltage at the load point, but the battery and
charging system are working fine. Check the fuse or circuit breaker that protects the
circuit, or the power relay that supplies voltage to the circuit to see if that’s the
problem.
If you find a blown fuse, replacing the fuse may restore power temporally. But unless
the underlying cause of the overload is found and corrected, your “fix” won’t last.
Whatever you do, never substitute a fuse of greater capacity to keep a fuse from
blowing repeatedly. If a fuse is blowing, it is because there is a short or overload in
the circuit. The fuses job is to open the circuit and protect the wiring against further
damage. If the wiring gets too hot, it can start a fire. That is something you don’t
want. Be sure to always use a replacement fuse that has the same amperage rating as
the original.
A faulty circuit breaker or an open relay will have the same effect as a blown fuse.
Circuit breakers are sometimes used to protect circuits that may experience brief
periods of overloading. When the breaker opens, it cuts off the voltage and allows the
circuit to cool down. Then it resets and allows the flow of voltage to resume.
The easiest way to check a breaker or circuit, is to conduct a load test with the hook,
its applied load is adjustable. Set the probe tip load value at no greater capacity than
what the circuit itself uses. If you don’t know, use a 2\5\10amp setting to be safe. If
the circuit works when you by pass the circuit breaker, you’ve isolated the problem.
Replace the circuit breaker. Another very useful hook feature is its ability to measure
current inrush,
This same basic test can also be used to check a questionable relay. A relay is nothing
more than a remote switch that uses an electromagnet to close a set of contact points.
When the relay magnet is supplied with current, the points close and load current is
routed through the main circuit. There are also “solid state” relays that use transistors
to switch the power on and off instead of mechanical contact points. Relays are often
used in circuits to reduce the amount of wiring that’s required, and to reduce the
current that flows through the primary control switch. Thus a relatively low amperage
switch, timer or sensor can be used to turn a much higher capacity relay on and off.
Circuit voltage
As I said earlier, every electrical device requires a certain amount of voltage to
operate. A light bulb will glow with reduced brilliance as the voltage drops. For some
components, however, there is a threshold voltage below which it won’t operate at all.
This includes ABS pump motors, ABS solenoids, variable rate steering solenoid
valves, electronic shock absorber and strut solenoids and motors, even electronic
modules. Low circuit voltage is usually caused by excessive resistance at some point
in the circuit. Usually, this means a loose, poor fitting or corroded connector, or a
faulty switch, relay or ground connection.
To find the point of high resistance, use your voltmeter or hook to do a “voltage drop
test” at various points throughout the circuit. If the voltmeter shows a drop of more
than 0.1 volts across a connector, switch or ground contact, it means trouble. The
connector, switch or ground contact will have to be cleaned or replaced. Another very
useful tool for locating a open or short circuit is the ECT 2000.
Sometimes undersized wiring can cause low voltage. It’s not something you’ll find
with original equipment wiring circuits. but it is a common mistake that’s made in
many repairs or where additional equipment has been installed.





18 gauge wire- 6 amps
16 gauge wire- 8amps
12 gauge wire- 15 amps
10 gauge wire- 30 amps
8 gauge wire- 40 amps

6 gauge wire- 50 amps.
It takes Continuity
Every electrical circuit requires a complete circuit to operate. Correct Voltage supply
on load won’t do any good unless there’s a return back to the battery. The ground path
in the case of metal bodied vehicles may be the vehicle body itself. A poor ground
connection has the same effect as an additional component or resistor. Therefore it
will share its proportion of the load, if the circuit isn’t complete, no current will flow.
Make sure there’s no voltage in the circuit when disconnecting it from a power
source, by pulling the fuse or by testing downstream from the circuit switch or relay.
Ohmmeters can’t handle normal battery voltage, and should you accidentally
complete a circuit through the meter it may cause permanent damage,
Ohmmeters are great for measuring circuit resistance, but you have to use care when
checking electronic components. An ohmmeter works by applying a small voltage
through its test leads, and this voltage can be enough to damage some electronic
components (such as the oxygen sensor). A high impedance (10,000 mega ohm) meter
should be used for electronic testing.
Tracing wires isn’t as easy as it looks because the circuit wire will sometimes change
colour after passing through a connector, switch or relay. Always refer to a wiring
diagram when possible. A much more accurate and professional way is to use the
ECT 2000 where a signal is transmitted through the circuit , a receiver is then used to
trace the signal.
Finding the fault
Now that we’ve covered the basics, what’s the best way of finding a fault fast? It
depends on the nature of the problem.
For a “dead” circuit, the first thing to look for is event activity, followed by voltage at
the load point. No voltage would tell you the problem is in the supply side of the
circuit. Trace back through the fuse box (or relay or circuit breaker) until you do find
voltage. Now look for the open or short that’s preventing the current from reaching its
correct destination. If there is voltage at the load point, the problem is either a bad
ground (the return path) or the component itself has failed. Check the ground by
applying a suitable load.
The worst kind of electrical problems to troubleshoot are intermittent ones.
Everything works fine in the workshop, but as soon as the customer gets his vehicle
back, it starts to act up again. An intermittent open or short is usually the result of
something heating up and breaking (or making) contact, or something that’s loose
making periodic contact. Loose or corroded connections and switches are often
responsible for this kind of problem, so try jiggling the wires and circuit switch to see
if it changes circuit voltage or resistance. A wire that is rubbing and has chaffed away
some of its insulation can make intermittent contact causing a short, so again,
wiggling suspicious wires will often reveal the problem.
Temperature- sensitive intermittent shorts or opens can be hard to identify because
you frequently have to simulate the exact circumstances that cause them to happen.
Sometimes you can assume what’s happening by the nature of the problem. But its
always more satisfying (and assuring) to duplicate the problem so you know what’s
wrong. Ask the customer when the problem occurs. Does it only happen when the
engine is hot or after the circuit has been on for a period of time? Always consider
applying load as well as movement, it is often necessary to drive the vehicle,
reproducing the exact conditions, if known!
Environmental factors can often play havoc with electrical systems. Road splash and
water can sometimes short out a connector or circuit. Look for obvious signs of
corrosion or damage on connectors and wires.
A final note on repairing electrical faults: when splicing wires, don’t just twist them
together and wrap electrical tape around the connection. Use a solder less crimp on
connector, or twist the wires together, solder them and use shrink wrap electrical
insulation tubing to seal the repair.
Electrical terms you need to know
Voltage
Voltage is the force that pushes current through a circuit. It’s also called the
“electromagnetic force” (EMF) because it pushes electrons along their journey from
one point to another. Voltage is like the pressure that forces compressed air through a
hose, but instead of being measured in pound per square inch, voltage is measured in
units called “volts”.
Amps
Current is the amount or volume of electrons that flow through a circuit. It is a
measure of volume, and is specified in units called “amperes” or “amps” for short.
The analogy with an air hose would be the number of cubic feet per minute of air
passing through the hose. One Amp is equal to 6.3 million trillion electrons (6.3 with
18 zeros after it) flowing past a point in one second! That’s a lot of electrons, but a
relatively small current in many automotive circuits. A starter, for example, can draw
600-800 amps when initially cranking the engine.
Ohms
Resistance is the opposition to the flow of current, or the restriction that impedes the
flow of electrons. Resistance is measured in units called “Ohms”. The flow of air
through a hose can be reduced by pinching it, by reducing the diameter of the hose or
by holding your finger over the outlet. Likewise, current flow through a wire can be
slowed or controlled by components that create resistance.
Ohms law
One volt equals the amount of force needed to push a one amp current through a
circuit with a resistance of one ohm. They call this “Ohms Law”. It can be expressed
in various ways:
Amps= Volts/ Ohms (volts dived by ohms)
Ohms= Volts/ Amps (volts divided by amps)
Volts= Amps x Ohms (amps times ohms)
Understanding Ohms law and the relationship between volts, ohms and amps is the
key to understanding electrical currents and circuits. Ohms law explains why high
resistance in a circuit chokes off the current and causes a voltage drop. It also explains
why an electrical short can cause a wire to rapidly overheat and burn away because of
a runaway current.
Voltage drop
A voltage drop occurs when current flows through a component in a circuit. Voltage
drop is measured with a voltmeter. The voltmeters leads are connected on either side
of the circuit component or connection that’s being tested. If a connection is loose or
corroded, it will create resistance in the circuit and restrict the flow of current causing
an excessive voltage drop. Voltage drops should be less than one tenth volt (0.1v) in a
discreet sensor circuit, and no more than (1v) in a substantial power circuit. When
testing for voltage drop always, always measure the source voltage at the same time.
I feel like I’m repeating myself here but the hook is perfect for this.
For full details of this exciting new course or any of our hands on skills training, zap
the QR or telephone Annette @ ads – global.co.uk 01772 201597
There is a special hook and tuition deal, don’t miss it!