We test the ECU-controlled alternator


The function of the alternator is to produce electricity in an internal combustion powered car. It is the main source of power – the battery has only an auxiliary function, enabling starting and powering the installation when the engine is not running. As soon as the engine starts running, the alternator powers all the vehicle’s electrical systems and charges the battery. Proper condition of the alternator is essential to ensure the proper operation of the entire electrical system and the battery, which will be the first to feel the effects of alternator problems.

How does the alternator work?

The use of alternators in vehicles has been linked to the increasing demand for power. The DC generators used previously proved to be less efficient and more failure-prone. The alternator consists of a rotor (rotor) and a stator (stator). The rotor is a single electromagnet that, powered by two wires (through brushes), produces a magnetic field. This is the so-called. excitation circuit.

When the rotor is driven, the rotating magnetic field induces current in the stator windings, where electricity is generated. Typically, three windings are used, which produce three-phase alternating current (in a similar way to the power grid, with the frequency not being constant and depending on the speed of the motor).

Rectifier and regulator

How then to use the alternator in the DC circuit that the car is powered by? A rectifier is needed to convert AC into DC. A classic arrangement of 6 semiconductor diodes is used to perform this task.

One problem remains – the rotor power circuit (providing excitation). It turns out that not only the frequency changes with rotation, but also the voltages. The rectifier solves the frequency problem, but the voltage must be within certain limits. The alternator requires a voltage regulator to regulate the current flowing in the rotor so that the output voltage is appropriate. It constitutes an electronic circuit, usually powered by 3 additional rectifier diodes.

A classic alternator has a fixed voltage level that ranges from 13.8 to 14.5V. Many of the regulators used raise the voltages slightly as the temperature drops.

Alternator connection systems

A different number of wires are used depending on the design of the alternator and the type of voltage regulator used. The most common designations used to mark the output (some letter designations have different meanings depending on the manufacturer):

  • B+ is the output from the alternator, attached to the positive terminal of the battery.
  • D+ in the simplest circuit of connections is the output to the charge light, it is attached by the ignition switch which allows the alternator to be pre-excited.
  • I, IGN, R (Mazda), 15, IG – regulator on/off switch, powered after the ignition switch (terminal 15).
  • L, IL – control output.
  • DFM, M (Toyota), LI, GLI, FB, FR, DF, FM – load monitoring output (PWM signal).
  • S, M – monitoring input. Connected by a second wire to the battery terminal, it allows to offset the effect of voltage drop on the alternator’s plus wire.
  • W – output (frequency) to the tachometer.
  • E – the weight of the battery.
  • COM, LIN, BSS – digital line for control and communication with the alternator.
  • C – voltage reduction order (Honda).
  • RC, SIG, RLO – regulator voltage control input.
  • R (relay) – output to a relay (external voltage regulator).

Controlled voltage regulator

The output from the alternator is directly connected to the battery so in such a circuit the charging current is not regulated and is limited only by the battery. Lead-acid cells can be safely operated in this way – the current will be large only at the beginning and will limit itself, provided the correct voltage. To provide better control over this process, a circuit was proposed to adjust the voltage so that the current has the desired value. In the simplest variant, the alternator charging voltage is reduced on demand by the car’s computer (C-terminal versions used Honda vehicles). Nowadays, it is more and more common to use control that allows smooth voltage regulation by the ECU, realized by a PWM signal or by a digital data bus (e.g. LIN, BSS interfaces).

The ability to adjust allows you to control the charge, ensuring that the battery is adequately charged, taking into account wear and tear (special clamps are often used at the same time to monitor the current). Changing the set voltage can also have other functions – by reducing the charge we get a decrease in the load through the alternator, so we temporarily have more power.

Some vehicles use the alternator to partially recover braking energy. By suddenly increasing the voltage, an additional load is created, and the energy goes to the battery (the alternator brakes the car to some extent). The disadvantage of this solution is keeping the battery charged to approx. 80% which shortens its lifespan to some extent. An incomplete charge is necessary so that there is the possibility of accepting braking energy.

How to check the ECU-controlled voltage regulator?

Testing an alternator controlled by the engine controller requires providing it with the appropriate control signals. To do this, we need a tester that will provide the right signal according to a given standard. Such a device is the Altalyzer. It gives the ability to check alternators installed in the vehicle, and can also be used on alternator test stands.

Connecting the alternator in the vehicle. The connected tester overrides the signals from the ECU.

When checking the vehicle’s alternator, we must disconnect it from the engine controller (ECU). Connect the tester to the connector on the alternator according to the connection instructions for the specific variant. We then indicate the make of the vehicle in the device menu and can start the test.

When is the alternator functional?

When testing an alternator, we examine how the charging voltage in the circuit under test responds to the voltage set by the tester. If the alternator reaches the required values without problems, we can consider it to be in working order. When testing, keep load in mind – many electronically controlled alternators provide a signal about its load factor (DFM). If you overload the alternator at a given voltage, the voltage will drop. It is also worth noting the correct reading of the load value. Alternators that fail to maintain adequate voltage may have a fault within the regulators themselves or other components in the alternator such as wearable brushes.

The tester's screen displays the following information: voltages (set and measured) and load stage.

Testing alternators outside the vehicle

When testing alternators outside the vehicle, we must remember to connect them properly. This does not apply only to alternators electronically controlled by the ECU, but also to classic designs. Do not run alternators on the test bench without the B+ terminal connected . If the alternator has a separate voltage measurement terminal (terminal S) then it is essential to connect it. The consequence of improper connection can be, in an unfavorable case, excessive voltage increase, which can damage to the component. Sometimes the output to the control (L terminal) also acts as a regulator switch or takes part in voltage regulation.

Additional capabilities

Thanks to the Altalyzer tester’s built-in Bluetooth connectivity, we have the ability to connect to a dedicated Android app. The application allows us to easily record the results of the measurements carried out along with data about the alternator under test, these entries can even be supplemented with a photograph of the component under test.

In addition, we can use the app to remotely control the test run – without extending the cables, we can conveniently control the receivers from the cabin, and at the same time inflict voltage and read the voltage value and load level.

Author: Product Engineer Piotr Libuszowski


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