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.