diagnose starter alternator
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Diagnose Starter and Alternator Problems

Copyright AA1Car Adapted from an article written by Larry Carley for Counterman magazine

Do you need a new starter because your engine won't crank? Many starters are replaced unnecessarily and are later returned back to the parts store because they are thought to be defective. When tested, there's no fault found. The real problem is misdiagnosis. Professional technicians will usually make the correct diagnosis, but many DIYers are simply making a semi-educated guess - and they sometimes guess wrong.

One of the best ways to find out if your starter is bad is to have it "bench tested" at a parts store. If it cranks at normal rpm under load, the problem is something else. Replacing the starter won't fix your problem. If it fails to crank, the starter needs to be replaced.

When you buy a new or rebuilt starter, compare the old and new units to make sure the replacement starter is the correct one for your vehicle. Is the mounting surface and bolt configuration the same? Is the flywheel gear the same (count the teeth if you're not sure). Does the replacement starter have the same electrical connections?



Because most of the starters that are sold in the aftermarket today are remanufactured units, you can usually exchange your old starter for a partial credit against the price of the new starter. If your old starter has been disassembled, has parts missing or has a cracked case, you may not receive full credit (or any credit) for your old unit. Rebuildable cores for many late model vehicles are valuable, so make sure you get full credit for the exchange.


What happens when you turn the key and try to start the engine?

If the answer is, "Nothing," you should check the battery, battery terminals, battery cables and ignition circuit to make sure voltage is reaching the starter.

If the battery is low or has corroded terminals or loose cable connections, the starter may not crank because of low voltage.

If the solenoid that energizes the starter motor is faulty or has loose electrical connections, it will prevent the starter from cranking, too.

A faulty ignition switch, park/neutral safety switch on the transmission linkage, clutch safety switch on the clutch pedal or a wiring problem are other faults that can also prevent a starter from cranking.

starter cutaway


Starter motors come in several varieties. Most older vehicles have a rather large, heavy starter motor that has field coils around an armature. On newer vehicles, the size of the starter motor has been downsized either by using permanent magnets in place of field coils, or by using reduction gears to multiply the torque produced by a smaller motor. As a rule, permanent magnet and gear reduction starters are more expensive units to replace. Permanent magnet starters must also be handled with care because the magnets can be easily cracked and ruined if the starter is dropped on a hard surface.


Starter problems can be caused by worn brushes (carbon pads inside the motor that supply current to the rotating armature), by shorts or opens in the armature or field coils or by worn bushings that increase drag or allow the armature shaft to rub against the pole shoes.

Continuous and prolonged cranking is very hard on a starter motor because it generates excessive heat. If not allowed to cool down every 30 seconds or so for at least a couple of minutes, the starter will be damaged by continuous cranking.

You should have your old starter bench tested to determine if it needs to be replaced. Using a battery and a pair of cables to jump the starter will only tell you if it spins, not how many amps it is drawing or how fast it is cranking. To accurately test a starter, a test stand that can measure amp load, voltage and rpm is required.

A good starter will normally draw 60 to 150 amps with no load on it, and up to 250 amps under load (while cranking the engine). The no load amp draw will vary depending on the type of starter. If the amp draw is too high, the starter needs to be replaced. The same is true if the starter doesn't achieve the specified rpm.

Excessive starter draw can be caused by high resistance within the starter itself, worn brushes, or grounds or opens in the armature or coil windings. It can also result from increased internal friction due to shaft bushings that bind or an armature that is rubbing against the housing (if the starter is noisy, it's probably dragging).

starter motor drive

Sometimes the starter motor works fine but the drive gear won't engage the ring gear on the flywheel. If the drive gear mechanism can be replaced separately, there's no need to replace the entire starter. A bad solenoid can also cause starter problems. The solenoid acts like a relay to route power directly to the starter from the battery. It may be mounted on the starter or located elsewhere in the engine compartment and is usually connected to the positive battery cable. Corrosion, poor ground at the solenoid mount or poor battery cable connections will prevent the solenoid from doing its job.

If the starter tests okay but fails to crank, another possible cause may be a bad ignition switch, neutral safety switch or clutch safety switch. A low battery and/or loose or corroded battery cables can also prevent the starter from cranking the engine.


The charging system consists of an alternator (that generates electricity), a voltage regulator (that controls the alternator's output) and the battery (that stores amps). The charging system's job is to keep the battery fully charged, and to supply voltage to meet the vehicle's electrical needs.

Cranking an engine pulls amps out of the battery. These must be replaced, or over time the battery will eventually run down each time the engine is started and driven. As soon as the engine starts, the charging system automatically senses the need for amps and starts recharging the battery. It also produces as many additional amps as are needed to keep the ignition system, fuel injectors and electrical accessories running. As a rule, the charging voltage is about two volts higher than battery voltage.


There are zillions of different OEM part numbers for alternators, so aftermarket suppliers try to consolidate applications as much as possible.

What's really important is how the alternator is wired (A-circuit, B-circuit or I-circuit), the type of voltage regulation (external regulator, internal regulator or computer-controlled regulation) and the physical hookups (bolt hole locations and indexing, wiring connectors and pulley dimensions).

When an A-circuit regulator loses positive voltage, the alternator will overcharge if the field still has power. If the regulator loses its power. If the regulator loses its ground, the system will go dead. With B-circuit systems, just the opposite is true. If a B-circuit regulator loses its ground, the alternator will run wild and overcharge. If it loses positive voltage, the alternator will go dead.

A replacement alternator doesn't necessarily have to look the same as the original, but it must function the same electrically, have the same pulley dimensions and be a bolt-in replacement. With consolidated applications, it is sometimes necessary to modify or change the wiring connectors as well.

alternator components


Most charging systems that are working properly should produce a charging voltage of about 13.8 to 14.3 volts at idle with the lights and accessories off. Always refer to the vehicle manufacturer's specifications. Many Asian vehicles can have higher charging voltages (up to 15 volts).

When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage.

The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.8 to 14.3 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might be 14.9 to 15.3 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts.

Charging output of the alternator can also be checked with an adjustable carbon pile, voltmeter and ammeter. The carbon pile is attached to the battery and adjusted to obtain maximum output while the engine is running at 2000 rpm.

Charging amperage is another number that can reveal the condition of the alternator. With the engine idling and no load on the charging system (lights and all accessories off, battery fully charged), the amperage output should be relatively low (typically less than 10 amps). With the headlights and heater blower fan on and the engine running at 2000 rpm, the output should jump to a higher reading, typically 25 to 30 amps or more.

Warning: Never disconnect a battery cable while the engine is running to "test" the alternator. Doing so can cause high voltage spikes that can damage the alternator as well as other electronics.

alternator troubleshooting chart

NOTE: If a vehicle has a history of repeat alternator failures, it might mean the battery is not building up normal resistance as it accepts a charge. This, in turn, makes the alternator keep charging the battery at a higher than normal rate. The result is that the alternator runs hot, overheats and eventually fails from being over worked. The battery charging current should gradually decrease after the engine starts, and taper off to less than 10 amps at idle (with no lights or accessories on) after five minutes of running. If a fully-charged battery is still pulling 20 or more amps after five minutes of idling, the battery is defective and needs to be replaced.

Another way to check alternator output is with an oscilloscope. Observing the "ripple voltage" pattern will tell you at a glance whether or not all the alternator windings are functioning. A "good" pattern should look like the top of a picket fence. If any of the humps are missing, it means one or more of the windings is grounded or open, or there's a bad diode. Most battery/charging system testers also have a test function that can detect bad diodes.


One way to check the integrity of the alternator and diodes on Bosch alternators is to check the voltage readings at the D+ (blue wire) terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the alternator.

Bosch does not recommend full fielding as a procedure for testing alternator output because full fielding may damage onboard electronics.



If the alternator or regulator fails, the battery will run down and the vehicle may not crank or start. Low voltage or amperage output from the charging system will usually cause the alternator warning light to glow, the dash voltmeter to read low, the dash ammeter to show discharge or the Check Engine light to come on. The headlights will also be dim when the engine is idling.

Low alternator output can be caused by a slipping drive belt (it takes up to five horsepower to turn some alternators), one or more defective diodes in the alternator's rectifier assembly or a defective voltage regulator. Loss of alternator output can be caused by a broken drive belt, loose, broken or corroded wiring connections, electrical failures within the alternator or regulator, or a bad external regulator ground or voltage connection.

In some instances, the alternator can produce too much voltage and overcharge the battery. Symptoms here would include low battery electrolyte, damaged battery plates from overheating and/or burned out bulbs. The cause is usually a defective voltage regulator or poor regulator ground connection.

Many alternators are replaced unnecessarily or are returned because of misdiagnosed charging problems. If possible, have your old alternator bench tested to see if it works. If it does, the problem is not the alternator. It could be a bad regulator or wiring connection. Also, have the replacement alternator bench tested before you leave the parts store to confirm it is delivering the proper voltage and current.

Make sure the replacement alternator's amp capacity matches the vehicle's requirements. Don't buy a 45-amp alternator for an application that requires a 90-amp alternator. Overloading an alternator will cause it to fail prematurely. What you may need is a High Output Alternator

On some Japanese applications, it's not unusual to find several different alternators used on the same vehicle. So it may be necessary to refer to the vehicle VIN code and/or the part number on the OEM alternator to identify the unit in order to get the correct replacement.

Also, test and recharge the battery before the alternator is installed. The alternator is designed to maintain battery charge, not to recharge a dead battery. Forcing it to revive a dead battery may overload it and cause it to fail.

Additional items that may also need to be replaced to ensure proper operation of the charging system include the battery cables and drive belt. V-belts should be replaced every four or five years for preventive maintenance.


Neither the starter, nor alternator will function properly if the battery is low or worn out. Average battery life under the best circumstances is four to five years, and is as low as three years in really hot climates.

A fully charged battery should read 12.66 volts with the key off and no electrical load on the battery. A reading of less than 12.45 volts at the battery means the battery is low (less than 75% charged) and needs to be recharged.

If the charging system is functioning normally, but the battery fails to hold a charge, it may mean your battery has reached the end of its service life and needs to be replaced. Load testing a battery or using an electronic tester to test the battery's amp capacity should reveal the health of the battery. If the battery tests okay, the battery may be running down because of a parasitic electrical load. A trunk light that remains on, a relay that remains energized, etc, can all create a steady drain on the battery that will run it down.

If you need a new battery, the replacement battery must have an amp capacity that equals or exceeds the OEM cold cranking amp (CCA) requirements. The group size (height, width and length) must also fit the battery tray in the vehicle, and the posts must have the same configuration.

The battery cables should be cleaned and inspected or replaced if found to be badly corroded, loose or damaged. Installing chemically treated anti-corrosion felt washers under the battery cables will help keep the connections corrosion-free.

diagnose starter alternator More Starter & Battery Articles:

Push Button Start System

Diagnosing a Battery that Runs Down

Starter Diagnosis

Car Won't Start (Possible Causes & Quick Checks)

Engine Won't Crank or Start

Battery Safety & Jump Starting (Read First!!!)

Battery Testing

Charging System Checks (alternator testing)

How To Replace an Alternator

Troubleshooting electrical problems

Voltage Drop Testing

Power Centers: Relays & Fuses

What You Should Know About Rebuilt Alternators, Starters & Other Parts

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