Alternator and Starter Diagnosis

Rotating electrical parts such as starters and alternators often have to be replaced on older, high-mileage vehicles. Fuel injection has helped prolong the service life of starters by allowing engines to start more quickly when they are cranked. Such is not the case with alternators. Higher electrical demands on charging systems have increased alternator failures.

Starter Troubles

Alternator and Starter

A starter that’s failing may crank the engine too slowly for a quick start, or it may not crank the engine at all. Often, the problem is not the starter but a low battery or a loose or corroded battery cable connection. So, check the battery charge and condition first.

A good battery should be capable of accepting and holding a charge from a battery charger, and should be at least 75 percent charged (12.5 volts or higher). If the battery’s voltage is low and it doesn’t take a charge, your customer needs a new battery. Load-testing the battery or using a conductance tester to check its ability to take and store a charge can also confirm the need for replacing the battery. The average service life of a car battery is only about four to five years and can be shorter in hot climates. So also consider the age of the battery when evaluating its condition.

If a DIY customer is not sure what might be causing his cranking problem, offer to bench-test his old starter. If the starter passes the tests, the slow cranking or no cranking problem is obviously something other than the starter. He should check the battery, battery cables, starter relay and the starter circuit and wiring connections for possible faults. If the starter fails the tests due to low cranking RPMs, excessive current draw or it failure to spin at all, you can sell your customer a new or remanufactured starter with a high degree of confidence.

High resistance within the starter itself, worn brushes, or grounds or opens in the armature or coil windings can be cause of excessive current draw. It can also result from increased internal friction due to shaft bushings that bind or an armature or magnets that are rubbing inside the starter.

A loose starter may crank an engine slowly, noisily or not at all. Loose bolts will make for a weak ground connection. The starter may also flop around, slip, chatter or fail to engage depending on how loose it is. Sometimes the engine won’t crank even though the starter will spin. This is usually an engagement problem due to a weak solenoid or a defective starter drive. A starter drive that is on the verge of failure may engage briefly but then slip. The starter drive has a one-way overrunning clutch mechanism that you can check once the starter is out of the car (and replace if necessary). The drive should turn freely in one direction but not in the other if good. A bad drive will turn freely in both directions or not at all. If a drive locks up, it can over-rev and destroy the starter.

Charging Problems

The first sign of trouble when an alternator is failing is a low or dead battery. On a late-model vehicle, that can not only cause a no-start but also can cause the loss of “learned” data in the powertrain control module and other modules throughout the vehicle. In some cases, certain modules may not regain their normal function after the battery has been recharged because the module requires a special relearn procedure.

The output of the charging system on a vehicle can be checked with a digital voltmeter while the engine is idling. A charging system that is working properly should produce a charging voltage of somewhere around 13.5 to 14 volts at idle with the lights and accessories off (always refer to the vehicle manufacturers specifications). When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, and 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 vice versa.
If the charging output is low, the alternator can be bench-tested to see if it is good or bad. Make sure you use the correct wiring adapters for the unit and that all of the connections are made properly. The bench tester will check the alternator’s voltage and current output, and also look for “ripple voltage” or alternating current leakage that would indicate bad diodes inside the alternator.

If the alternator fails the tests, your customer needs a new or remanufactured replacement alternator. You might use the opportunity to recommend a higher-output alternator if the customer’s vehicle is equipped with a megawatt aftermarket sound system or other high-load electrical accessories.

If the alternator passes the tests, the problem is not the alternator but something in the wiring or the charging system control circuitry. High circuit resistance and ground connection resistance can cause low charging output. Circuit resistance can be checked by connecting a volt meter to the positive battery terminal and the positive terminal on the alternator. With the engine running and the headlights on, there should be less than half a volt voltage drop, and ideally less than one-tenth of a volt drop. A higher reading would indicate too much resistance. The same test can be repeated using the battery negative terminal and the alternator housing to check for excessive resistance on the ground side of the circuit.

If the wiring checks out okay, the problem could be in the voltage regulating circuitry inside the powertrain control module. On many late-model vehicles, charging output can be varied depending on operating conditions as well as electrical load. Problems with other sensor inputs or a defect in the control module itself may prevent the alternator from charging properly.

A slipping drive belt is another common cause of undercharging, especially with V-belts on older vehicles. Serpentine belts usually provide a better grip, but if the automatic tensioner is weak or stuck it can allow the belt to slip under load. Glazed streaks on the belt or belt noise when high-load electrical accessories are turned on with the engine idling can be signs that the belt is slipping.

Alternator slippage and undercharging can also be caused by a bad alternator pulley. Overrunning Alternator Pulleys (OAP) are used on a number of late-model import and domestic vehicles. OAP pulleys have a one-way clutch inside the center of the pulley that slips and allows the pulley to free-wheel when engine speed suddenly drops. This reduces noise, vibration, and harshness in the belt drive system, but may cause a charging problem if the clutch slips when it should be gripping.

Another type of pulley is the Overrunning Alternator Decoupler (OAD) pulley. This type of pulley combines a one-way clutch with a torsion spring to decouple and absorb torsional vibrations in the belt drive system. This provides much quieter and smoother operation than either a solid pulley or an OAP, especially at lower engine speeds (from idle to about 1,500 RPM). But it also can cause slipping and charging problems if the clutch or decoupler spring is defective.

Alternator Replacement

Some replacement alternators come with a pulley already installed and some do not. If the replacement alternator comes with a pulley, make sure the pulley matches the original (same diameter, width and belt type). No pulley means your customer will have to swap the pulley from the old alternator to the replacement. Removing a conventional solid pulley usually requires a gear puller to pull the pulley off the alternator shaft. But on some applications, a threaded OAP or OAD pulley may be used and a special tool may be required to get it off.

OAP and OAD pulleys are more complicated and expensive than ordinary solid pulleys, but are installed on the alternator for a reason. Replacing an OAP or OAD pulley with a less expensive solid pulley is possible, and some replacement alternators come with a less expensive solid pulley installed rather than an overrunning clutch or decoupler pulley. But replacing an OAP or OAD pulley with a solid pulley defeats the purpose of the original pulley and may result in increased NVH, reduced belt and tensioner life and customer complaints.

When a new or remanufactured alternator is installed on a high-mileage vehicle, recommend a new drive belt too. Original equipment belts made of EPDM synthetic rubber are high-mileage belts capable of lasting upward of 100,000 miles. Unlike older belts made of less durable rubber, they don’t crack with age. But they do wear. It’s difficult to see how much the grooves on the underside of the belt may be worn, so belt manufacturers have created special belt wear tools that can check the depth of the grooves to reveal how much they are worn.

The operation of the automatic belt tensioner also should be checked to make sure it is working correctly and is capable of maintaining proper belt tension. Rust and corrosion can cause old tensioners to bind, and a weak or broken spring may prevent it from keeping the belt tight.