How to test an alternator with a multimeter or without: step-by-step

How to test an alternator

Outlines steps for how to test an alternator with a multimeter or without one, plus symptoms and solutions for the most common alternator problems.

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Every car has a generator that is responsible for generating electricity: the alternator. The term comes from a time when an alternator was only responsible for the lights on the vehicle. A modern alternator supplies the now numerous power consumers in the car with the necessary energy. Although alternators are not part of the wearing parts, defects are not uncommon. But the alternator itself is not always the cause. However, as a starting point, we will also outline below how to test the alternator itself too.

Alternator: costs
Functions of the alternator
Power of the alternator
Function of the alternator
How to test an alternator: the Charge indicator light
How to test an alternator: Defective alternator
How to test an alternator with a multimeter
Alternator regulator
V-belt and freewheel

Alternator cost

If the alternator is defective, a new alternator costs between $150 and $400 or more, depending on the vehicle, generator output and alternator manufacturer. In addition, there are the costs for the installation, for which between $50 and $250 or more are usually due. In extreme cases, the prices of powerful and water-cooled alternators can be over $700.

Used replacement alternators are recommended for repairing older cars at fair value. The prices of such refurbished alternators start at around $50. In addition, there is a deposit for old parts, which is refunded when the defective alternator is sent or handed in. When buying, you should make sure that all required parts (possibly also a new alternator regulator) are included or have to be purchased additionally. Authorized dealers also offer replacement generators in original quality for around 60 to 70 percent of the price of a new part. So after you have gone through the how to test alternator steps below, it is wise to shop around if you decide you have to replace it.

Functions of the alternator

Nowadays the alternator is usually a three-phase generator. While driving, it produces electricity for all electrical consumers in the vehicle – ventilation and rear window heating are the major ones. But even the engine control unit needs current pulses in order to function. In addition, the alternator is responsible for ensuring that the starter battery is charged or kept constantly full while driving.

Because the battery in turn is the energy store in the car and, so to speak, forms the backup of the generator, sometimes even while driving. If the combustion engine is switched off, the battery discharges, for example if consumers such as the parking lights remain on or if the parking heater is always used for short journeys. This works fine for a while, but soon the battery is no longer enough to start the engine.

Alternator power

Many are familiar with this from old mopeds or scooters: When the engine is idling, the headlights only light up gently. If, on the other hand, the throttle is turned, the light shines brightly. Those days are over since the early 1980s and with the use of the three-phase generator.

Even at idle speed (between 650 and 900 revolutions of the crankshaft), modern alternators provide sufficient power and thus electricity to be able to supply consumers in the car. Compared to the older systems (direct current and alternating current generator), three-phase generators are not only more powerful, but also more reliable, smaller (about the size of a handball) and lighter.

The output of the alternator is matched to the consumers in the car. It may be a maximum of ten percent less than the power consumption of all consumers in the car if they are activated at the same time. In an emergency, the ten percent difference is bridged by the battery, the aforementioned back-up.

Due to the steadily increasing number of electrical consumers in the car, the charging current of the alternators has continued to increase. While around 50 amps of charging current was sufficient for a compact car in the 1980s, today alternators generate between 100 and 250 amps, depending on the vehicle type. Incidentally, it is not only the comfort functions that have been extended in modern cars that are among the pantographs, but also engine technology such as high-pressure pumps for the fuel supply or the injection nozzle solenoid valves.

Functions of the alternator

To put it simply, the generator converts the mechanical energy of the crankshaft into electrical energy by rotating it inside. The excitation field is brought about by a rotor the size of a fist and usually induces three-phase alternating voltage, which is fed into the vehicle electrical system after rectification. In most cases, the generator is driven by the V-belt or ribbed belt, which is connected to the alternator via a pulley.

In addition to generating electricity, the alternator charges the starter battery while driving.The alternator uses engine power to generate electricity, around five hp are possible. That costs fuel. Therefore, modern alternators are demand-driven in order to make the energy conversion more efficient. When accelerating strongly, the generator output is reduced and thus the resistance is reduced. On the other hand, in phases of overrun, in which the engine does not consume fuel due to the overrun cut-off, more power is drawn from the alternator and the resistance increases.

How to test an alternator on a car

How to test an alternator: the charge indicator light is the first indication of defects

An important “tool” for checking the operation of the alternator itself is installed right in front of the driver’s eyes. This is the charge control lamp in the instrument cluster. The generator or battery symbol lights up whenever the ignition is switched on.

If it still lights up when the engine is active and idling, this could be a sign that the consumers that are switched on are not being supplied with the necessary power and that the battery is currently supporting the system. A quick push on the accelerator is all it takes to clarify this. Most of the time the light goes out suddenly and everything is fine. If the light continues to glow, this could be a sign that the alternator is defective.

How to test an alternator: Defective alternator

Three-phase generators are in principle maintenance-free and wear-resistant and often last the life of the car. However, if the alternator is broken, the battery is gradually sucked empty. In the worst case, the engine will stop while driving because the engine control unit and fuel pump must continue to be supplied with power. It went better if the driver became skeptical because the car had a hard time starting in the morning, even though a long distance had been covered the day before. Because in this case the starter battery should actually be fully charged.

How to test an alternator: What can break in an alternator?

  1. The carbon brushes of the alternator regulator are worn out
    ➤ Symptoms: The charge control lamp lights up , the battery is not being charged
    ➤ Solution: Replace the brushes or regulator
  2. The alternator regulator is defective
    ➤ Symptoms: The battery is getting hot (overcharging), the battery is weak (no charge), the charge indicator light becomes brighter as the
    engine speed increases, the lamps burn out
    ➤ Solution: Replace the alternator regulator
  3. The belt drive bearings or the freewheel are defective
    ➤ Symptoms: Noise development from the belt drive (whistling, grinding), battery is not charged, charge indicator light is on
    ➤ Solution: Check belt drive and freewheel and replace if necessary
  4. Corrosion or defects in cable connections
    ➤ Symptoms: Charging voltage is not reached, differential voltage between LiMa and battery
    ➤ Solution: Check / replace lines and connections
  5. V-belt has too little tension
    ➤ Symptoms: Charge control lamp flickers, V-belt slips and squeaks
    during the process
    ➤ Solution: Tension or replace V-belt
  6. Short circuit or interruption of the diodes
    ➤ Symptoms: Discharge of the battery while stationary (with a defective positive diode)
    ➤ Solution: Disconnect the battery, replace or repair the alternator
  7. The alternator winding is defective
    ➤ Symptoms: The charge control lamp lights up , the battery is not charged.
    ➤ Solution: Replace or repair the alternator
  8. The rectifier of the alternator is defective
    ➤ Symptoms: Battery does not charge, control lamp is on
    ➤ Solution: Replace or repair the alternator

How to test an alternator with a multimeter

Should a problem arise, it is relatively easy to test an alternator. The charging voltage of the alternator is checked with a multimeter (available from around $20):

  1. Set the voltage range on the multimeter (up to 15 or 20 V)
  2. Connect the black clamp to the ground pole and the red clamp to the positive pole
  3. When the engine is switched off, the voltage of the car battery should be between 12.2 and 13.6 volts (if the voltage is significantly lower, i.e. below 12 volts, we recommend charging with a charger)
  4. Switch on the engine, the charging voltage should be around 13.4 to 14.8 volts at idle speed
  5. Then repeat the measurement at a speed of 4000 rpm, the voltage should remain constant
    That disregarded power plants
    The alternator voltage is between 13.4 and 14.8 volts. Measurements are made at idling speed and at 4000 rpm. Nothing should change in the tension.

If the voltage does not rise to the specified values ​​with the engine switched on (after a few seconds), this indicates a defect in the alternator. To rule out damage to the wiring, the measurement on the contacts of the alternator can be checked. If there is a clear difference, the wiring must be checked!

If the voltage rises noticeably above the value measured when idling at 4000 rpm, this indicates a defect in the alternator regulator. In this case, the battery can overheat and damage the on-board electronics. In the workshop, the mechanic can also check the alternator with an oscilloscope and interpret the measurement values ​​accordingly.

If the voltage regulator is defective

The alternator regulator, also known as the voltage regulator, is integrated in the generator. Its task is, among other things, to keep the generator voltage roughly constant under all loads and speeds. This is the only way to ensure that there are no voltage fluctuations that could damage electronic components in the vehicle. The regulator is designed in such a way that it allows the generator voltage to level off to around 14 volts (alternator charging voltage) so that the voltage is below the gassing voltage of the starter battery.

Often the two small carbon brushes of the alternator regulator are worn out, recognizable by the fact that the brushes are significantly shortened. In older vehicles, only the carbon brushes can be replaced; the cost is a few dollars. If the entire alternator regulator has to be replaced , this costs between $20 and $80. The installation time is between ten minutes and an hour, depending on the accessibility. However, it can also be time-consuming: In engine compartments that are difficult to access, additional units may have to be dismantled to replace the alternator regulator, which makes the matter a case for the workshop.

Check V-belt and belt drive

Also quick to check: the tension of the V-belt and the freewheeling of the pulley. It decouples the belt drive of the alternator from the vibrating crankshaft that drives the belt. Normally the generator runs at about twice the speed of the crankshaft, which can mean between 1400 and 12,000 revolutions per minute – i.e. maximum output. It is therefore hardly surprising that the bearings integrated in the pulley cannot always withstand such loads over the long term.Bearing damage is indicated by a grinding-squeaking noise when the motor is switched on, which comes directly from the freewheel on the alternator. If this is defective, the belt pulley can sometimes even be dismantled with the generator installed and the bearing inside replaced (fastened with Torx or splined). Replacement costs between $20 and $60, depending on the model and quality.