Basic vehicle electrical circuits

New technology doesn't mean you should ignore basic fault finding

Published:  01 March, 2013

As automotive technology continues to evolve and become more complex, mechanisms are being enhanced, or even replaced, with drive-by-wire 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 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. When the driver steps on the brake pedal, a command is sent to the module which then decides how much pressure is needed and where to apply it. The system can react more quickly in an emergency situation and can break just the outside wheels when braking in a turn.

Future brake systems will likely do away with hydraulics altogether and be fully electronic. Small servo motors at each wheel will squeeze the pads against the rotors and brakes may not have any friction linings but be fully magnetic. All kinds of new electronic brake systems have been developed by Bosch, Continental, Kelsey Hayes, Delphi and others for future vehicle applications.

As vehicle manufacturers move toward 42 volt electrical systems, we're also going to see a growing number of vehicles equipped with fully electronic steering. Hydraulic pumps and gears will be replaced with electric motors.

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 won't 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 a dedicated automotive electrical probe.

A simple 12 volt test light that glows when there's voltage will tell you if the circuit is complete but it won't tell you how much voltage, or what the current flow and circuit resistance is. Many components, such as 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.

If you find a blown fuse, replacing it may restore power temporarily 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 fuse's job is to open the circuit and protect the wiring against further damage - you don't want the wiring to get too hot as it can start a fire. 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 automotive probe (the 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 bypass the circuit breaker, you've isolated the problem and can replace the circuit breaker.

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.

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