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Diagnosing the Battery, Charging, & Ignition System

What do you really need to know about diagnosing and repairing starting, charging and ignition systems? Two things, basically: that electrical and ignition problems are often difficult to diagnose (especially intermittent faults), and that a LOT of electrical and ignition parts are replaced unnecessarily in an attempt to repair problems that are hard to diagnose.

Throwing parts at a problem until it goes away is a very expensive way of fixing faults. The "no return" policy of many parts stores means that once you've installed an electrical or ignition part on your car, you own it.

What if the part turns out to be "defective?" In many instances, there's nothing wrong with the part. The parts store will usually exchange it under warranty. But if the second part also fails to fix the fault, it's obvious something else is at fault. So don't be too quick to blame "defective" parts when a newly installed part doesn't seem to work any better than the old one, or when you get a comeback. Do your diagnostic homework, then isolate the fault and identify any parts that may need to be replaced.

Scan Tool Diagnostics

Some parts stores now offer customers a free "diagnosis" if their Malfunction Indicator Lamp (MIL) is on. A store employee will plug a code reader or basic scan tool into your vehicle and read out any codes that appear. The code may provide a clue as to what's going on, but it seldom tells you which part needs to be replaced. Further diagnosis is almost always needed with a scan tool such as AutoTap, a digital storage oscilloscope and/or other test equipment. So don't put too much faith in parts store diagnostics. These people are not in the repair business. They are in the parts business and are anxious to sell their customers parts. Whether these parts actually fix the problem or not is not their primary concern.

One thing to keep in mind about scan tool codes is that codes are primarily for emission faults, not electrical faults. Ignition faults such as misfires certainly qualify as emission faults and will trigger the MIL lamp and set a code if the rate of misfire is high enough to cause a problem. A low battery, or an unusually low (or high) charging voltage may set a code, but a weak battery, poor ground or bad starter probably won't.

Even when you have a code, you'll often have to do additional tests to find out what's causing the problem. A misfire code, for example, will tell you the engine is misfiring and which cylinder is the culprit -- unless you have a P0300 code which indicates a random misfire that can't be isolated to any given cylinder. But even when you have a cylinder-specific code, you still don't know if the misfire is due to fuel, ignition or compression. The cause might be a fouled spark plug, bad plug wire or weak ignition coil. Or, it might be a dirty or dead fuel injector. Or, it might be a compression problem due to a burned or bent valve, a leaky head gasket or a rounded cam lobe.

And what do you do when you have a no start/no code condition? The problem might be no ignition, fuel or compression. Or, it might be a bad battery, starter, ignition switch or safety circuit, or anti-theft immobilizer system if the engine won't crank.

Start with the Battery

Many driveability and starting problems that are charging, starting or ignition related may be due to low battery voltage. It's a simple thing to check, yet many technicians overlook the battery as a possible source of trouble. Low battery voltage can also affect fuel delivery by causing the fuel pump to run slower than normal. This, in turn, causes, low fuel pressure and a lean fuel condition. Under some conditions, a low battery may even prevent one or more injectors from opening normally causing lean misfire and/or hard starting.

Automotive lead-acid batteries must be kept at or near full charge for optimum performance and longevity. If a battery is run down or becomes fully discharged, undesirable changes start to occur on the lead plates inside the battery. The plates develop a layer of sulfate that resist recharging and reduce the battery's ability to store power. If the battery is chronically rundown or discharged, it shortens battery life significantly.

Average battery life under the best of conditions is only about four or five years in most vehicles, and only about three years in places like Arizona and New Mexico where summer temperatures typically soar into the triple digits. Many motorists who are driving vehicles with batteries that are four, five or six years old may not realize their batteries are failing until their engine fails to start and they are stranded.

Cold weather also puts a two-fold strain on the battery by reducing its power output (up to 50% at 20 degrees F!) and increasing the amps needed to crank a cold engine (up to double the amps at 20 degrees F).

The best way to check battery charge is with a digital voltmeter. A fully charged battery should read 12.65 volts. A reading of 12.45 volts equals about a 75% charge and is good enough for further testing. Anything less means the battery is low and needs to be recharged.

The battery's state of charge doesn't tell the whole story because a fully charged battery may also be a weak or failing battery that can't provide its normal amp output under load. Battery condition can be determined one of two ways: by load testing with a tester that applies a calibrated load on the battery (this requires the battery to be fully charged for accurate test results), or by testing with an electronic "conductance" tester (which does not require a fully charged battery for accurate test results).

Conductance testers send a frequency signal through the battery to reveal how much active plate area is available to hold and deliver power. As a battery ages, its conductance declines. Shorts, opens and other cell defects also affect conductance, so measuring conductance gives an accurate indication of battery condition.

Many electronic battery testers also analyze the battery's "Cold Cranking Amp" (CCA) capacity, which can be used to estimate the battery's remaining service life. Some testers also measure the amps drawn by the starter while cranking the engine, and analyze the charging system's output under load once the engine is running.

If a vehicle needs a new battery, it should have the same or higher CCA rating as the original equipment battery specified by the vehicle manufacturer. Batteries should be fully charged before they are installed. Other items that should always be inspected and may need to be replaced include battery cables, anti-corrosion washers for the battery terminals, battery tray and/or battery hold-down hardware and clamps.

Charging Checks

The vehicle's charging system should also be checked to make sure it is operating correctly and is capable of keeping the battery at full charge. As a rule, the charging voltage with the engine at fast idle should usually be about 1-1/2 to 2 volts higher than battery voltage.

Alternator charging output increases in proportion to the electrical load on the charging system and engine speed. Charging output is controlled by a voltage regulator, which may be mounted inside or on the back of the alternator ("internally regulated"), or somewhere else under the hood ("externally regulated"). On newer vehicles, the powertrain control module (PCM) regulates charging output.

If an alternator is overloaded, it may overheat and fail (check for aftermarket sound equipment that may be overloading the stock alternator!). An alternator failure will cause the battery to run down and go dead. Symptoms of a charging problem include a low battery, dim headlights, hard starting or a charging system warning light.

Many parts stores have an alternator test stand and can test alternators to determine if they have failed or are capable of putting out their rated amp capacity. When done correctly, bench testing an alternator is a good way to verify your diagnosis and/or to confirm the output of a new or reman alternator before it is installed.

A replacement alternator (new or reman) should have the same or higher amp rating as the original. If the replacement comes with a pulley (some do not), make sure it matches the original (same diameter, width and belt type). If your vehicle has a high amp, aftermarket sound system, you should consider upgrading to a high output replacement alternator.

Related items that may also need to be replaced include the regulator (externally regulated applications only), drive belt, battery cables and/or battery. If you're installing a high output alternator, larger gauge battery cables, ground straps and charging wire may also be required.

Starter Checks

An engine that won't crank may have a bad starter, or it may have another fault such as a bad starter solenoid starter drive, a problem in the ignition switch circuit, the park/neutral or brake safety switch circuit, or the anti-theft immobilizer system. If the battery is fully charged and the starter is receiving normal battery voltage but fails to crank (or cranks slowly), the starter is probably defective and needs to be replaced. Many parts stores can also bench test a starter to check motor rpm and amp draw.

The most common cause of starter failure is prolonged cranking. This causes the motor to overheat. Starters also wear out after years of service if the brushes, bushings, or starter drive become worn. Starters also have a solenoid that routes current to the motor when the ignition switch is turned to the crank position. A defective solenoid will prevent the starter from working. The starter drive mechanism that engages the flywheel may also stick or fail prevent the starter from working.

A replacement starter (new or reman) must have the same bolt pattern and electrical connections as the original, and the same number of teeth on the drive gear. Handle permanent magnet starters with care because the magnets are brittle and can be easily cracked if you drop the starter. Related items that may also have to be replaced include battery cables and engine ground straps.

Ignition Diagnosis

Primary and secondary ignition patterns can reveal a great deal about the health and performance of the ignition system. Learning how to use and read ignition patterns on a scope takes some time, but once you know what to look for diagnosis becomes much easier. Essentially you look for significant differences in firing voltages between cylinders, or abnormal spark lines that indicate coil triggering or dwell problems.

Spark plugs eventually wear out (even long life 100,000-mile plugs) and can become fouled by carbon deposits. A fouled plug may misfire causing a loss of power and fuel economy, and a big increase in exhaust hydrocarbon (HC) emissions -- which may eventually damage the catalytic converter. Fouled or worn spark plugs can make an engine hard to start and run poorly. Replacing the spark plugs can restore normal performance -- but it won't fix a misfire problem if the cause is bad plug wires or a weak coil.

Under normal driving conditions, a set of standard spark plugs will usually last about 45,000 miles. Refer to your vehicle owners manual for the recommended replacement interval. Long life plugs, which have a center electrode made of a wear-resistant metal such as platinum or iridium, typically go up to 100,000 miles before replacement is needed. If your engine has standard plugs, consider upgrading to long life plugs.

Spark Plug Wires

The spark plug wires (ignition cables) carry high voltage from the coil or distributor to the spark plug. Some wires have carbon-impregnated fiberglass strands inside to carry the voltage while others have spiral-wound "mag" wire.

Plug wires can deteriorate with age causing misfires, hard starting and poor performance. Wires should be replaced if the boots or terminals are loose, damaged or corroded, if the wires are cracked or sparking, or if their internal resistance exceeds specifications.

On late model engines with coil-on-plug (COP) ignition systems, there are no plug wires because the coils are mounted directly on the spark plugs. This also complicates ignition diagnosis with a scope because special adapters are required to pick up an induced ignition signal from the coils.

Ignition Coils

Coils seldom fail, but when they do they can't provide voltage to the spark plug(s). On a single coil ignition system, a coil failure will cause the engine to quit and prevent it from starting. On a multi-coil DIS or COP ignition system, it will cause a misfire in only the cylinders supplied by the bad coil.

Coils can be tested by measuring their internal resistance with an ohm meter. If the coil's "primary" or "secondary" resistance is out of specifications, replace the coil.

Caution: Never disconnect a COP coil from its spark plug while the engine is running. Doing so may damage the coil or other electronics!

Ignition Module

Ignition modules may receive a trigger signal directly from a distributor pickup (magnetic, Hall effect or Optical), a crankshaft position (CKP) sensor or the PCM. A fault in any of these other components or the wiring can prevent the ignition system from firing.

An intermittent no start and/or stalling due to loss of spark may be a clue that the ignition module is failing, or that it is losing its trigger signal from the crank sensor. Check the resistance of the crank sensor (both hot and cold, not just at room temperature), and check to see that the module is receiving voltage and a trigger signal from the crank sensor or PCM. If the module has good inputs, but is not switching the coil(s) on and off, the module has reached the end of the road.