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Mar 22 2020

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Auto electrical repair

Troubleshoot Car Electrical Problem


by Larry Carley copyright 2019 AA1Car.com

Troubleshooting electrical problems can be a frustrating task, but it does not have to be if you keep a few simple rules in mind: Every circuit needs a power source; most electrical devices require a minimum voltage to function correctly; and all circuits require continuity. Consequently, most electrical problems are caused by low voltage (or no voltage), excessive resistance or a loss of continuity.

SAFETY CONSIDERATIONS

Safety is always an important consideration when working on automotive electrical systems. Except for the high voltage side of the ignition system, and the high voltage battery and circuits in hybrid vehicles, there is NO danger of being shocked. Twelve volts (12v DC) is not enough to be felt. The danger is accidentally shorting out a hot circuit and damaging the wiring, PCM or other onboard electronics, or starting a fire.

CAUTION: If your vehicle is a hybrid with a high voltage battery, there is a risk of being shocked if you come into direct contact with the high voltage battery, wiring or other hybrid components. for more information on this subject, see Hybrid Safety Hazards

CAUTION: When doing electrical repairs or replacing electrical or electronic component, the battery should ALWAYS be disconnected to eliminate any risk of causing an accidental short. Disconnecting the battery will cause most PCMs to forget their learned settings. This may cause driveability issues or require a special “relearn” procedure with a scan tool, so to avoid this kind of hassle use a 9 volt “memory saver” that plugs into the vehicle’s power receptacle (cigarette lighter) to maintain voltage to the battery, or connect a 9 volt alkaline battery to the PCM power supply.

For more information on safety, see Battery Safety .

ELECTRICAL CIRCUIT CHECKS

All electrical circuits require voltage to operate the components connected to that circuit. So if there is no voltage, there is no function. The first order of business when troubleshooting electrical problems, therefore, is to check for the presence of voltage at the load point in the circuit.

The load point is the element that the circuit is supposed to power, such as a light bulb, wiper motor, blower motor, idle stop solenoid or whatever. And, all you need to quick check it is a voltmeter or a 12-volt test light that glows when there is voltage. A voltmeter is the best tool for this purpose because it will give you an exact reading, but a test light is OK for performing quick voltage checks.


Using a test light is a quick way to check for voltage, but a voltmeter is more accurate.

Suppose you find no voltage at the load point. Ah ha, you have discovered your first clue about the problem. Check the fuse, fuse link or circuit breaker that protects the circuit, or the power relay that supplies voltage to the circuit.

If the problem is a blown fuse, replacing the fuse may restore power temporarily, but unless the underlying cause for the overload is found and corrected, your “fix” probably will not last. Whatever you do, do not substitute a fuse of greater capacity. A larger fuse may be able to handle a greater load but the wiring and the rest of the circuit cannot. A circuit designed for a 20 amp fuse is designed to handle a maximum of 20 amps. Period.

A faulty circuit breaker or an open relay will have the same effect as a blown fuse. Circuit breakers are often used to protect circuits that may experience brief periods of overloading such as an A/C compressor clutch.

The easiest way to check a circuit breaker is to bypass it with a jumper wire. Your jumper wire should have a replaceable inline fuse to protect the circuit against damage. Use a fuse of no greater capacity than what the circuit itself uses. If you do not know, use a 5- or 10-amp fuse to be safe. If the circuit works when you bypass the circuit breaker, you have isolated the problem. 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 voltage, the points close and battery voltage is routed through the main circuit. Relays are often used in circuits to reduce the amount of wiring that is required, and to reduce the current that flows through the primary control switch. Thus, a relatively low amperage (make that cheap) switch, timer or sensor can be used to turn a much higher capacity relay on and off.

VOLTAGE CHECKS FOR CAR ELECTRICAL PROBLEMS

Every electrical device also requires a certain amount of voltage to operate. A light bulb will glow with reduced brilliance as the voltage drops. But for some components, there is a threshold voltage below which it will not operate at all. A starter motor may crank the engine more slowly with reduced voltage but, if the battery voltage is too low, it may not crank at all. Minimum threshold voltage is especially critical for such components as solenoids (which need a certain amount of voltage to overcome spring resistance), relays, timers, buzzers, horns, fuel injectors (which are solenoids, too) and most electronics (the ignition module, computer and radio).

Checking the load point for full battery voltage will tell you whether or not sufficient voltage is getting through, and to do that you need a voltmeter. The battery itself should be at least 70 percent charged and read 12.43 volts or higher (12.66 volts is fully charged). If the battery is low, it should be recharged and tested. The output of the charging system should also be checked, and be about 1.5 to 2.0 volts higher than battery base voltage (around 14 to 14-1/2 volts). If the battery is OK, your voltmeter should read within 1 volt of battery voltage at the circuit load point in any given circuit.

Low circuit voltage is usually caused by excessive resistance at some point in the wiring. Usually this means a loose or corroded connector, a faulty switch or relay or poor ground. To find the point of high resistance, use your voltmeter to do a “voltage drop test” at various points throughout the circuit. If the voltmeter shows a drop of more than a 0.4 volts across any connector, switch or ground contact, it means trouble. Ideally, the voltage drop should be no more than 0.1 volts.

If low voltage is detected in a number of circuits, do a voltage drop test across the battery terminals and engine/body ground straps. Loose or corroded battery cables and ground straps are a common cause of voltage-related problems. Clean and tighten the battery cables and/or ground straps, as needed.

Sometimes undersized wiring can cause low voltage. It is not something you will find in many original equipment wiring circuits, but it is a common mistake that is made in many do-it-yourself wiring installations for aftermarket accessories. The higher the amp load in the circuit, the larger the required gauge size for the wiring.

Wiring Gauge Sizes and Amp Loads

NOTE: These values are for copper wire at a maximum temperature of 140 degrees F (60 degrees C)).

Auto electrical repair



Auto electrical repair

Auto electrical repair

Diagnostic tips for solving automotive electrical problems
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