Alternator troubleshooting

DENSO is a true pioneer of starter and alternator technology and is now the world’s largest original equipment (OE) manufacturer of rotating electric parts, with a global market share of more than 25 percent. As a result, its replacement starters and alternators, supplied to wholesalers across Europe by DENSO Aftermarket, are the smallest and lightest rotating machines for their output, delivering unrivalled efficiency, wear resistance and powerful performance.
DSN9363 D

Alternator overview

The alternator is driven by the engine, via the auxiliary belt, and converts mechanical energy into electrical energy and supplies the required power to the various electrical loads. When power supplied by the alternator does not match the required electrical load (when all the electrical equipment is in use, or when the engine speed is low during idling etc.), the battery temporarily supplies extra power to the electrical equipment. However, during normal driving the alternator re-charges the battery to its original strength.

As engine speeds change constantly according to driving conditions, the speed at which the alternator rotates also changes, with the generated voltage changing in tandem. It is then the role of the regulator to control and supply the appropriate voltage to the various electrical loads, as well as ensuring that the battery is charged.

What can go wrong and how to fix it

First, the battery must be fully charged (12.6 volts or above) and the battery cables, terminals and case in good and clean condition. So, start with a visual inspection and performance test before inspecting the charging system.

Checking the voltage drops on the positive and negative sides provides helpful tips to find hidden problems that can cause charging issues. Voltage always flows through the path with the lowest resistance. Therefore, if there is a high resistance somewhere in the positive or negative side, it should be able to be detected via a multimeter.

If there is a voltage reading greater than 0.2 volts at the voltage drop test positive side (output circuit), it indicates there is excessive resistance somewhere in the positive side. So, check all the wiring and connector pins/terminals are intact, clean and corrosion free.

If there is a voltage reading greater than 0.2 volts at the voltage drop test negative side (ground circuit), make sure all the ground connections and contact area are clean and corrosion free. Also, make sure that there are no broken, loose or missing ground points/straps between the engine and chassis.

However, if there is a voltage reading of less than 0.2 volts during these voltage drop tests, continue with further electrical tests.

What is the alternator’s output current?

Regulated voltage inspection – Run the engine at approximately 2000 rpm and check the regulated voltage at the alternator output stud (B+) terminal when current output reaches approximately 10A (Refer to the test standards and values designated by the relevant vehicle manufacturer).

Output current measurement – Turn the headlights to high beam and the heater blower switch to high etc. and again measure the output current with the engine at approximately 2000 rpm. The current should be at or above the standard values designated by the relevant vehicle manufacturer.

Caution: The standard values differ according to the vehicle manufacturer, so refer to the appropriate manufacturer’s service manual for specific information corresponding to alternator specifications.

Are any fuses or fusible links open?

Check the fuses in all the fuse box(es). An open fuse indicates circuit problem(s), which may influence the charging circuit. There may also be several fusible links controlling battery voltage to the vehicle’s electrical circuits. If a fusible link is open, supply voltage will be completely lost to all electrical systems or to the electric circuit(s) that the open link controls. Check the owner’s manual or the vehicle manufacturer’s service manual for the location of each fusible link.

Is the alternator’s drive belt tension within specification?

Check the tension and condition of the alternator drive belt.

If the drive belt is too loose, it will slip around the pulley and cause the alternator to charge irregularly or not at all, and if it’s too tight, internal bearing damage will cause premature alternator failure.

The belt’s condition can also affect the transfer of power from the engine crank pulley to the alternator pulley and prevent the alternator from charging the battery properly. The service life of a drive belt differs depending on the conditions of its use, but it is recommended to replace the drive belt when replacing the alternator.

Is there an alternative way to check whether the alternator works or not?

Another way to check the alternator is using a portable/handheld oscilloscope. Observing the “ripple pattern” can identify open or shorted diodes as well as problems in the stator coil. Any irregularities in the ripple pattern, mean that there is open or shorted diode(s) and/or stator winding(s). Most of the modern alternator test benches have the option to check the ripple pattern and to detect defective diode(s).

Can an alternator fail because of a poor wiring/ground contact or loose connection?

A poor wiring or ground contact increases resistance and causes a voltage drop in the electrical circuits, reducing the flow of the current through the charging circuit. This means the battery is not able to be fully and properly recharged, causing the alternator to keep charging at a higher rate than normal and this may cause overheating and premature alternator failure.

Another common problem that causes failure is a loose battery cable connection to the alternator, which results in intermittent or no charge, alongside a discoloured and/or melted alternator output (B+) stud terminal.

What could be the root cause of repeated alternator failures?

It may be caused by burnt out diode(s) due to an open circuit or a high resistance between the alternator output (B+) and the positive (+) battery connections. In such instances, charging current will flow through the diodes from an alternative route towards the battery causing excessive current flow through the diode, resulting in overheating and failure. Therefore, perform a thorough inspection and voltage drop tests to isolate any problems.

A similar effect may occur when the alternator is forced to charge a discharged battery or when the battery is able to be charged, but not able to set up normal resistance. In such cases, the alternator will be instructed to charge the battery to the maximum rate and will be overloaded for prolonged periods. This will cause the alternator to overheat and the diodes, as well as the stator windings and connections inside the unit, can be damaged and fail. Therefore, check the battery carefully and replace if necessary.

What could be the other root causes of alternator failure due to bad diode(s)?

Another common cause of alternator failure due to one or more bad diode(s) within the rectifier, is reverse polarity. Therefore, NEVER apply reverse polarity to the alternator because it can cause the diode(s) to explode, melt or be perforated by high current flow and fail.

In addition, the diode(s) can also be damaged severely if the battery is disconnected while the engine is running or during a jump start.

What are the key features of selecting an aftermarket alternator?

Vehicle manufacturers specify numerous OE part numbers for their alternators, which is why aftermarket suppliers consolidate their range as much as possible. A replacement alternator does not have to look like the original unit, but it must provide an equivalent output, as well as have the same pulley specifications and fitting interface dimensions.

The most important key features are:

  • Long lifetime and maintenance-free
  • Regulator type is the key feature due to voltage regulation characteristics
  • Pulley type, diameter and number of ribs
  • Fitting interface dimensions such as fixing lug locations, fixing hole diameters, wiring connector, etc.
  • Current output capacity should meet the vehicle’s requirements

Caution: Never use an alternator with lower current output capacity for a vehicle that requires an alternator with higher capacity. For example, do not use a 80A rated alternator for a vehicle that requires a 120A as overloading it will cause premature failure.

DENSO Alternators

DENSO has pioneered new alternator designs capable of producing more electrical power more efficiently, yet in a smaller, lighter unit. In 2000, for example, DENSO introduced the world’s first SC (Segment Conductor) alternator using a rectangular conductor for its stator coil.

Key Facts

  • Completely new, boxed items (no reman) and no core surcharge
  • Range includes Conventional, Type III (alternator with small internal fan) and SC Types
  • Maximum efficiency by offering small size and light weight units while delivering the highest outputs

For further details of the DENSO Aftermarket programme are available online at: