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By Larry Carley
Technical Editor

One reason why alternators have a high failure rate is because they’re always working under a load. Generating electricity to recharge the battery, run the fuel pump, injectors and ignition system, and power all of the vehicle’s lights and other electrical accessories places a substantial load on the alternator that generates a lot of internal heat. This can burn out the diodes in the back of the alternator that convert alternating current (AC) to direct current (DC) — especially in vehicles that spend a lot of time idling with high electrical loads. Heat can also damage the rotor and stator windings, brush connections and wiring leads.

Most alternators fail electronically long before they wear out mechanically. Even so, the shaft bushings and brushes are also wear items that don’t last forever. These parts can also be ruined if the alternator is mounted low on the engine and is subjected to road splash that contains salt water or debris.

On 1994 to 2004 Mitsubishi Montero SUVs with a 3.0L V6 engine, the alternator is mounted directly under the power steering pump. On these vehicles, the alternators have a higher-than-normal failure rate due to contamination by power steering fluid. If you’re replacing a bad alternator on one of these vehicles, be sure to inspect the power steering pump and hoses for leaks — and fix any leaks that are found before installing the new alternator.

If the charging system fails for any reason or does not produce enough power to meet all of the vehicle’s electrical demands, the battery quickly runs down. Once battery voltage drops below a certain threshold, the onboard electronics, ignition and fuel systems may stop working normally or cause the engine to stall. A low battery also may not have enough reserve power to crank the engine, so the vehicle will be stranded until the charging problem can be diagnosed and repaired. Charging problems can be caused by electrical faults in the alternator or voltage regulator, poor wiring connections at the battery or alternator, or a slipping or broken drive belt. Since most late-model import alternators are internally regulated, a failure of the regulator means the alternator must also be replaced. In cases where the powertrain control module (PCM) controls voltage regulation, a problem in the voltage regulation circuit means the PCM will have to be replaced.

The actual output voltage produced by the alternator will vary depending on temperature and load, and will usually be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems produce 13.8 to 15.3 volts with no lights or accessories on. You can check the charging voltage by touching the test leads of a DVOM to the positive and negative battery terminals. If the DVOM is not auto ranging, set the scale to 20 volts, and select DC. Then take your reading with the engine idling.

Here’s another check few technicians do, but it’s one that can easily detect bad diodes in an alternator. Switch your DVOM to AC and check the voltage again. If all of the diodes are doing their job, there should be no AC voltage reading at the battery. If you get a voltage reading, it means one or more diodes are leaking and the alternator needs to be replaced. Leaking or shorted diodes can often cause a visible fluctuation in the output voltage of the charging system. This may cause the headlights to brighten and dim, or the instrument lights to flicker.

Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to rundown overnight. The normal key-off current drain on a battery may be as high as 300 to 400 milliamps right after the engine is shut off. But as a rule, the key-off current drain should usually be less than 50 milliamps one hour after the engine has been shut off and left undisturbed. Once all the modules go to sleep, the current drain drops significantly. Always refer to the vehicle manufacturer’s key-off electrical drain specifications if they’re available.

Diodes usually fail as a result of overheating or overloading. But they may have been damaged by a short in the charging light indicator circuit, or a poor connection between the alternator output terminal (B+) and the battery positive terminal. If a vehicle has a history of repeat alternator failures, one of these may be the cause.

The alternator’s current output (the number of amps it produces) should also be tested. Current typically increases in proportion to the electrical load on the charging system and engine speed. But the PCM may alter the curve depending on what kind of charging strategy has been programmed into it. The PCM may boost the charging current at low rpm when loads are high, or reduce charging output to smooth idle quality, or temporarily reduce the charging voltage to maximize engine power when accelerating at full throttle.

Maximum output is typically achieved at speeds above 2,500 rpm, and may range from 80 up to 150 amps or more depending on the rating of the alternator. A “good” alternator should be capable of producing up to 90% or more of its rated output at 2,500 rpm when it’s placed under load.

If the charging voltage is low, or the alternator isn’t putting out enough current to keep up with the electrical loads that are placed upon it, don’t automatically assume the alternator is bad and needs to be replaced (unless you’re bench testing the unit out of the vehicle). Many times an alternator is not working properly because of poor electrical connections in the charging circuit.

Loose or corroded connections on the back of the unit can increase resistance and restrict the current flowing through the circuit. So can broken or frayed wires inside a connector or the alternator wiring harness. The connectors and wires may appear to be okay visually, but unless you actually test them, you have no way of knowing if they’re making good electrical contact, and are clean, tight and undamaged.
If the wires and connectors are not checked, you may replace the alternator only to discover the new unit you just installed is “no good.” Now you get to replace it again and, on some vehicles where the alternator is buried under a lot of other stuff, that can be a lot of lost time and labor.

Every alternator supplier we’ve ever talked to about this issue says the same thing: Most of the alternators that are returned under warranty have nothing whatsoever wrong with them. The alternator isn’t charging the battery because of other problems on the vehicle like bad wiring connections, bad battery cables, a bad battery or a bad PCM. So save yourself the embarrassment and hassle of a comeback and test the alternator wiring connectors and wiring harness.

You can do this by using your voltmeter to perform “voltage drop” checks across the connections when the engine is running. A voltage drop test is done by setting the voltmeter on the 2-volt scale, then touching the positive and negative test leads on opposite sides of a connection. If there is resistance in the connection, some of the voltage will try to bypass the resistance by flowing through the voltmeter. If you see a reading of more than 0.2 volts, it means trouble. Ideally, the voltage drop across any connection should be zero, or less than 0.1 volts.

Check for voltage drops at the positive and negative battery cable connections, the alternator BAT+ power connection and the engine ground strap(s). Poor ground connections are an often-overlooked cause of low charging output and alternator failure. Voltage drops on the positive side of the charging circuit can cause undercharging. Voltage drops on the negative side can cause overcharging (fools the voltage regulator into thinking the battery is low).

Another approach to reducing comebacks and unnecessary warranty returns is to ask your parts supplier to bench test your customer’s old alternator (to confirm it’s bad), and to bench test the new alternator before you install it (to confirm it’s good). If the old alternator tests good, the problem is obviously not the alternator. You’ve missed something, so get out your voltmeter and start doing those voltage drop checks in the battery and charging circuit.

When a new alternator is installed, check the battery voltage and use a battery charger to bring the battery up to full charge before you return the vehicle to your customer. Also, start the engine and use your DVOM to check the charging output of the alternator. Don’t assume everything is working okay just because you bolted in a new alternator.

Starter failures are not very common these days thanks to fuel injection. Engines start right up in cold weather and don’t require a lot of cranking anymore, so we don’t see the seasonal rise in starter failures that we used to see years ago.

Primary causes of starter failure are overloading and overheating. If an engine won’t start for whatever reason (no fuel, bad gas, no spark or no compression) and is cranked continuously in a futile attempt to bring it to life, the starter can overheat and fail. If an engine won’t start after 15 to 20 seconds of cranking, chances are it isn’t going to start at all.

Cranking problems can be caused by a number of factors. They include a low battery, loose or corroded battery cables, a broken or missing ground strap, a weak starter solenoid, a bad starter drive, a damaged flywheel (broken teeth), starter misalignment, loose, corroded or damaged wiring in the starter circuit, a faulty ignition switch, a mechanical problem inside the engine, or even the wrong oil viscosity (oil too thick for cold temperatures). Yet some technicians jump to conclusions and blame the starter if the engine fails to crank.

As with alternators, the cranking circuit needs to be thoroughly inspected to rule out all the other possibilities before replacing the starter. If full battery voltage is not reaching the starter, the problem could be poor battery or ground connections, a bad starter relay, or a fault in the ignition circuit (ignition switch, Park/Neutral switch or wiring) or even the anti-theft system. If a battery has been disconnected or allowed to run down, the anti-theft system may have lost its memory and will have to be reset with a factory scan tool before it will allow the engine to start.

If the starter is getting its normal dose of power, but is not cranking properly, the fault may be the starter motor or the starter drive. Starters should also be bench tested to see if they’re good or bad before a new unit is installed. A bench tester will check the cranking speed and amp draw, and give a “good” or “bad” indication based on the test results. A good starter will usually crank the engine at 250 to 500 rpm and draw 60 to 150 amps. If the amp draw is too high, or the cranking speed is too slow, the starter is bad and needs to be replaced.

An unusually high current draw and low free turning speed typically indicate a shorted armature, grounded armature or field coils, or excessive friction within the starter itself (dirty, worn or binding bearings or bushings, a bent armature shaft or contact between the armature and field coils). The magnets in permanent magnet starters can sometimes break or separate from the housing and drag against the armature.

If you’re installing a remanufactured starter that requires an exchange core, and it’s the permanent magnet type, handle it with care. Dropping it can damage the magnets inside making the core worthless. Also, don’t disassemble an exchange starter, as your supplier may not give you credit if parts are missing.

After replacing a starter, check the battery to make sure it’s fully charged, and if it’s not, use a battery charger to bring it up to full charge before you return the vehicle to your customer.

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