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Electrical System

SOLVING INTERMITTENT ELECTRICAL PROBLEMS

Edited from an article by Larry Carley, ImportCar Magazine, November 2001

New vehicles today have more electrical accessories and gadgets than ever before, so it’s not surprising that intermittent electrical problems are one of the "most dreaded" repairs for even a highly skilled technician. Intermittent faults that come and go have always been a challenge to troubleshoot because of their elusive nature. But with the added complexity of multiplex circuitry, multiple microprocessors and what seems like miles of wires in today’s electronic-laden vehicles, the task may seem overwhelming.

Intermittent problems are the kind of dilemma you would like to send back to the car dealer or to your competitor down the street - but that’s no way to succeed in the repair business today. You have to be able to solve any kind of electrical problem, whether it’s easy to diagnose or not.

One of the keys to nailing down an intermittent electrical problem is to first verify that the problem exists. That means recreating the operating conditions that caused the problem to appear in the first place, either in the service bay or on the road. If you succeed in doing that, you’re more than halfway to figuring out what’s causing the problem. And what if you can’t duplicate the conditions that cause an intermittent problem to act up? Then you’ve got your work cut out for you.

The worst kind of intermittent problems are the ones that appear randomly with no apparent pattern. Diagnosing these kinds of faults can be as frustrating as chasing ghosts. But unlike ghosts, these gremlins do exist and must be exorcised by systematically ruling out each and every possible cause until hopefully you’ve found the culprit.

The key to solving intermittent electrical problems, therefore, is having a strategy for identifying and isolating electrical faults.

ELECTRICAL FAULTS
The one thing you always have to keep in mind when troubleshooting electrical circuits and electronics is the power loop. For any electrical or electronic device to work, power has to flow in a complete loop. Electrons from a power source (the battery and charging system) must flow into a circuit, pass through a component and then return back to the original power source. If there’s an open or a short anywhere in the circuit, the electrons won’t make the round trip and the component won’t work. What’s more, if there are not enough electrons making the round trip due to low voltage or excessive resistance, the component may not function normally.

One of the first things that should always be checked when troubleshooting any kind of electrical problem, therefore, is the battery. Measuring the open circuit (no load) voltage of the battery with a digital voltage ohmmeter (DVOM) will tell you if the battery is producing enough volts to operate the circuits and devices to which it is connected. The next thing that should always be checked is the output of the charging system. A weak alternator that can’t keep up with a vehicle’s electrical demands may be creating an intermittent shortage of electrons.

Low voltage can cause a lot of weird problems in today’s vehicles. Such problems can include:

• Fuel injectors and ignition coils that misfire;
• Solenoids, relays and magnetic clutches on A/C compressors that fail to engage;
• Pumps that run too slowly; and
• Electrical motors that may not have enough torque to complete their assigned tasks - things like cranking the engine, delivering adequate fuel pressure or even raising or lowering a power window. Computers and sensors that require specific operating voltages for accurate operation and communication can likewise be left in the dark if they don’t have their normal supply of volts.

A weak battery or charging system may produce enough volts to operate some components under light load, but not produce enough volts for normal operation when other demands come on line. Underlying causes here include anything that may interfere with the voltage leaving the battery or returning to the battery. Such factors might include sulfated cell plates within the battery; bad diodes in the alternator; loose or corroded battery cables; loose, corroded or missing ground straps; damaged or loose wiring harness connectors; corroded terminals or connectors; and undersized wiring.

Low voltage can often be traced to excessive resistance at some point in the circuit. Usually this means a loose or corroded connector, a faulty switch or relay or poor ground. To find the point of high resistance, you can use a DVOM to do a "voltage drop test" at various points throughout a 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.

ACCESS TSB RESOURCES
Another thing you should also check when confronted with an intermittent electrical problem is an electronic database of vehicle manufacturer technical service bulletins (TSBs). No shop today should be without this kind of information access.

TSBs can be an invaluable resource for identifying and fixing intermittent electrical problems - especially those that seem to defy diagnosis. If the same kind of problem has occurred repeatedly on a significant number of vehicles, chances are the vehicle manufacturer has already studied the problem and published a fix as a TSB. And even if there’s no TSB available for the problem you’re facing, you may find other TSBs that can prevent future problems from occurring.

Most TSBs include an accurate description of the problem, a diagnostic procedure for verifying the problem and, most importantly, a factory-authorized repair procedure for eliminating the problem. Some fixes may be relatively simple like relocating a wiring harness, moving a connector or wrapping some tape or insulation around a wire to prevent it from chaffing. Other fixes, though, may be more involved and require replacing segments of a wiring harness or entire components.

DIAGNOSIS
Any time a circuit or electrical component stops working, whether permanently or intermittently, there can only be four possible causes: no voltage; low voltage; a bad ground connection; or a failed part.

If there’s no voltage to the circuit or component because the battery is dead or there’s an open or short in the power supply circuit, current won’t flow and the device won’t work.

If the available voltage in the circuit is too low because the battery isn’t fully charged, the alternator isn’t producing enough amps, there are loose or corroded connectors, undersized wiring or excessive resistance in the circuit, the device may not have enough power to function or it may function erratically.

If the electrons can’t get back to the battery because of a bad ground connection, that too will prevent a device from working.

Finally, if something inside an electrical device has failed (for example, a burned-out bulb; shorted or open windings inside an electrical motor; worn motor brushes; a failed solenoid or relay; or a failed transistor, IC chip or other electronic component inside a module), the device won’t work, either.

By a process of elimination, you should be able to rule out each possibility and hopefully arrive at the correct diagnosis so you can solve the problem.

A classic example would be a light bulb that fails to come on. Let’s say it’s a taillight. You turn on the lights and the left taillight appears to be out. By turning on the lights, you’ve already ruled out the power source as well as a lot of related possibilities. If all the other lights are working, you know that the battery is providing enough voltage to power the lights, that the light switch is passing the current to the light circuit, that the fuse for the light circuit is intact and that current is flowing through the entire circuit.

Okay, so you’ve isolated the fault to the left taillight circuit. Now what’s the next step? Is it the bulb, the socket or the left taillight wiring?

Depending on which is easiest to access, you might remove and inspect the bulb to see if the filament is broken, or check the connector on the inside of the taillight housing to see if the connector is loose. If the filament inside the bulb appears to be intact, the next step might be to test the bulb with a battery and jumper wires, or to install a new bulb to see if that solves the problem.

If the light still doesn’t come on, the problem is either a bad socket, bad ground or a wiring fault. The next steps would be to check for voltage at the bulb socket, and if there is voltage, then check the ground connection. The socket itself should also be visually inspected for corrosion and to make sure the bulb fits properly and makes good electrical connection. The last step would be to check wiring continuity for a possible open or short - and for that you may need a wiring diagram to trace the wires and locate any hidden connectors or ground connections that might be out of sight.

CAUSES OF INTERMITTENT FAULTS
The only difference between an intermittent fault and one that isn’t is that some people think the problem has gone away - but we all know that cars don’t heal themselves. If they did, we’d all be out of work. An intermittent fault that has happened once will almost certainly happen again. So you have to try to duplicate the conditions that caused it to happen in the first place, so you can fix it and prevent it from recurring.

Heat, vibration, moisture and other electrical loads in the electrical system are all factors that can make an intermittent fault come and go.

If something stops working after a vehicle has been driven a certain distance, it usually indicates a heat-related electrical problem. When the wiring gets hot, either from its own heat or from an external source, thermal expansion causes something in the circuit to open up. This could be a loose fitting connector, a crack in a soldered wiring connection or a crack in a printed circuit board. If the circuit starts working again after the vehicle has been shut off for a while or allowed to sit, that would be another clue that the problem is heat-related. Likely trouble spots for heat-related intermittent problems include connectors and relays located near sources of engine heat such as the exhaust manifolds, exhaust pipes, radiator or heater core.

To find this kind of problem, you can use a heat gun (low setting only) or blow comb to direct heat at suspicious connections or components. If the device stops working when you apply heat, you’ve found the problem.

Intermittent problems can also be caused by vibration and movement. If a particular electrical device or accessory quits working under any of the following circumstances - when driving on a rough road, after hitting a bump, when making a sudden turn in one direction or the other, after slamming a door, and so on - it probably means there’s a loose wiring connector, a loose ground connection, or the wiring is moving and shorting out or pulling open. Visually inspecting the wiring in the suspected circuit may reveal signs of chaffing or rubbing. Look for missing looms, wiring supports or other grommets that normally keep the wiring from moving.

To find this kind of problem, a DVOM can be used to monitor the voltage in a circuit while a helper wiggles and flexes various connectors and wires. If the voltage suddenly fluctuates or goes away, you’ve identified the problem.

A third common cause of intermittent problems is moisture and condensation. Water from rain-soaked streets or moisture condensation from a cold morning dew can infiltrate wiring connectors and provide a short circuit to ground. Dirty water is a good conductor of electricity, so pay close attention to connectors that are located in areas that may be vulnerable to road splash and condensation. If you see droplets of water clinging to wires or a connector, it is likely that water may also be inside. Unplugging and inspecting connectors may reveal an obvious moisture problem. Packing connectors with dielectric grease will help keep out moisture and corrosion.

Another way to find a moisture-related intermittent electrical problem is to use a spray bottle filled with water. Spray a fine mist of water at the suspected connectors and wires to see if it causes the problem you’re troubleshooting.

Another possible cause of an intermittent electrical problem may be other electrical loads in system. For example, if you turn on the rear defroster and the headlights go dim or the radio stops working, the high amp load created by the defroster may be robbing the other circuits of their much-needed amps. This shouldn’t be a problem with factory-installed accessories, but it might be with aftermarket accessories that tap an existing circuit for power. A short in an existing circuit can also pull a higher-than-normal current, causing shortages elsewhere in the system.

A WORD ABOUT FUSES
If a fuse has blown, it means the circuit it protects has experienced a momentary overload. In many instances, the overload has been caused by a short. So if the fuse blows again as soon as it is replaced, you’ll have to track down the short. And even if it doesn’t blow, the fix may be only temporary if there’s an intermittent short in the wiring or component that caused the fuse to blow in the first place.

Never substitute a fuse of greater capacity to cure a fuse that keeps blowing. A larger fuse may be able to handle a greater load but the wiring and the rest of the circuit can’t. 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 don’t know, use a 5- or 10-amp fuse to be safe. If the circuit works when you bypass the circuit breaker, you’ve 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’s required, and to reduce the current that flows through the primary control switch.

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