Rotating Electric Update – UnderhoodService

Rotating Electric Update

Question: There is a lot of talk in the industry regarding returns on non-defective parts. So, how can repair shops and technicians reduce the number of warranty returns on rotating electrical parts?

Answer: Misdiagnosis of starting and charging problems is the main reason many starters and alternators are returned. The issue isn’t the quality of the replacement parts or installation errors (though mistakes can and do happen). The problem is failing to pinpoint the exact cause of the starting or charging problem. Many times the cause is a bad starter or alternator. But sometimes it may be something else.

Cranking problems can be caused by 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, park-neutral safety switch, brake safety switch, the anti-theft immobilizer system or a mechanical problem inside the engine (such as a coolant leak causing hydrolock).

Charging problems can be caused by a faulty voltage regulator (internal or external); a faulty PCM (if the PCM controls voltage regulation); loose or corroded battery cables; a loose or missing ground strap; loose, corroded or damaged wiring in the charging circuit; a blown fuse in the power distribution center; or a slipping drive belt.

Q: A technician may be asked to have a customer’s electrical part bench tested to verify the part is bad and needs to be replaced. How does bench testing help reduce warranty returns?

A: Bench testing requires some know-how on the part of the jobber or retailer where you purchase units, but most machines are relatively easy to hook up and use. Bench testing a starter not only verifies the starter spins but that it draws the proper number of amps and spins at the specified rpm. Bench testing an alternator not only verifies the unit produced current, but that the unit’s voltage and amperage output is within specifications. Bench testing can also be used to verify a part’s condition if you bring it back, saying that it didn’t work when you installed it. If the starter or alternator tests OK, you may have missed something and you’ll need to address additional diagnostic work.

Jobbers and retailers also may bench test new or reman starters and alternators to verify they are working properly before they leave the store. This helps assure technicians and shop owners that they are buying a quality part and that the part should function properly after it is installed (assuming their diagnosis was correct).

Q. What causes alternators to fail?
A: The main cause is usually overheating caused by overloading the charging system. The higher the load on the alternator, the higher its operating temperature. If the unit gets too hot, the rectifier diodes or soldered connections on the brushes or armature windings may be damaged.

Alternators are designed to maintain battery charge, not recharge dead batteries. For this reason, batteries should always be recharged with a battery charger if they have run down. New batteries should also be charged before they are installed. This will reduce the load on the alternator and reduce the risk of overheating and failure.

An alternator’s maximum output capacity should equal or exceed the maximum load created by the vehicle’s electrical system. This includes the voltage needs of the ignition system, fuel system, lights, A/C, radio and other power accessories. Alternator output is proportional to speed, so to achieve maximum output the engine has to be running at 2,000 to 2,500 rpm or higher to keep up with high electrical loads. Sitting and idling for a long period of time (especially during hot weather) with the A/C, lights and radio on may overtax the alternator and cause it to overheat and fail.

Did You Know:
Most charging systems that are working properly produce a charging voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off.

Q: Why don’t starters fail at the same rate as alternators?
A: Starters are replaced less often than alternators because they are only used to start the engine. Fuel-injected engines usually require little cranking to start, so the starter doesn’t have to work very hard, except during cold weather when the oil in the engine thickens and makes it harder to crank. Prolonged cranking is what kills many starters because heavy cranking causes the starter to overheat.

The starter is mounted on the engine or transmission bellhousing and engages teeth on the flywheel to crank the engine. A one-way, over-running clutch is used to protect most starters against damage should the starter remain engaged after the engine starts.

There are several different types: direct-drive starter motors, gear-reduction starter motors and permanent-magnet starter motors (reduced-size starters with permanent magnets inside instead of wire coils). It’s important to handle permanent-magnet starters with care because banging them on the counter or floor may break the magnets inside.

Because of the high load on the starter, good electrical connections are extremely important. Loose, corroded or undersized battery cables may not deliver enough amperage to crank the engine at normal speed, causing hard starting. Starter drives (which can be replaced separately on many starters) can also fail, preventing the motor from engaging the flywheel. A bad solenoid or relay will prevent the starter motor from cranking at all. Accurate diagnosis of a starter problem is important to prevent unnecessary parts replacements and returns.


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