Every editorial product is independently selected, though we may be compensated or receive an affiliate commission if you buy something through our links. Ratings and prices are accurate and items are in stock as of time of publication. Don't test an alternator by disconnecting the negative battery cable. Use a
voltmeter instead to conduct a simple, safe test. If you have a newer battery but your car won’t start, you may have a bad alternator. If you’re tempted to test an alternator by disconnecting the negative battery cable, don’t do it. A good alternator may indeed keep the engine running without the negative cable, but this was never a good test. In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every
electrical device in your vehicle. The second you disconnect the battery, the voltage regulator pegs the alternator to put out maximum power. With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “simple test” could end up costing you several thousand dollars for new electronics. Step 1Learn how to how to test an alternator without disconnecting your vehicle's battery.
Introduction
Tools Required
Here’s a video on how to properly and safely test your alternator.
Project step-by-step (2)
Conduct a Voltmeter Test
Step 2
If Your Alternator Passes
- If you connect the meter leads to the battery terminals and the meter shows that it's in the 13.8 to 15.3 volt ranges (engine running, lights and accessories off), that means the alternator is working as it should be.
- If the alternator is functioning correctly, it's likely that your dead battery was caused by a computer module that isn't shutting down when you turn off the car. If your alternator passes the voltmeter test, get your vehicle into a shop and pay a professional mechanic to find and correct the misbehaving module.
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Do It Right, Do It Yourself!
Originally Published: January 04, 2019
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When the alternator’s output voltage is unstable or intermittent, it is necessary to test the voltage regulator. The regulator regulates the alternator’s output voltage between 13.8 and 14.5 V and controls the power supply to the rotor windings. As long as the output voltage is below the maximum value, the regulator feeds the rotor windings that create the magnetic field through the brushes. The rotor’s rotation under the magnetic field’s influence in the stator windings generates voltage, which is sent to the vehicle’s electrical network via the diode plate. When the output voltage exceeds the maximum allowed voltage, the regulator cuts off the power supply to the rotor. Without excitation voltage, there is no magnetic field in the rotor, and the generation of voltage in the secondary windings is interrupted. The output voltage drops, and when it falls below the minimum value, the regulator turns on the power supply to the rotor windings again.
The voltage regulator regulates the alternator output voltage by turning on and off the excitation voltage.
Various alternator voltage regulators
Irregularities in the operation of the voltage regulator can be power interruption, too high output voltage, and very rarely too low output voltage. Apart from the voltage regulator, other factors can influence the appearance of these irregularities.
To check the correctness of the regulator, we can use special testers or check its operation ourselves with the help of a few elements. We need a voltage regulator (0-20 V), a low-power bulb, and a multimeter for measuring DC voltage for a manual correctness test.
We connect the minus wire from the voltage regulator (0-20 V) to the ground of the alternator voltage regulator, and the plus wire from the voltage regulator (0-20 V) to the 12 V (D+) output port on the alternator voltage regulator. We connect the light bulb to the DF and D- alternator voltage regulator connections or the brushes. We connect the black measuring pipette of the multimeter to the ground and the red one to D+ of the alternator voltage regulator. Let’s set the multimeter to measure DC voltage.
Voltage regulator testing
When all the necessary test elements are connected to the voltage regulator, we turn on the power source and increase the output voltage. The light bulb is getting brighter. We monitor the voltage on the multimeter and the operation of the light bulb. With a correct regulator, when the voltage exceeds the value of 14.6 V, the bulb should turn off. With a further increase in voltage, the bulb does not work. Then we lower the voltage, and when we reach a value of about 13.8 V, the bulb lights up again. We increase and decrease the voltage on the regulator several times. The bulb should always turn off when 14.6 V is exceeded and turn on again when the voltage drops below 13.8 V. If it reacts differently than described, the regulator is faulty and needs to be replaced.
To perform an alternator voltage regulator test, follow these steps:
Connect the minus wire from the voltage regulator (0-20 V) to the ground of the alternator regulator, and the plus wire to D+. Connect the bulb to the DF and D- alternator voltage regulators or the brushes. Connect the black measuring pipette of the multimeter to the ground, and the red pipette to D+ of the regulator. Set the multimeter to measure DC voltage.
From the information system, read the required values of the regulation voltage for the given regulator. Turn on the power supply regulator and increase the supply voltage from zero upwards. Monitor the operation of the bulb that is lit. As the voltage increases, monitor the value on the multimeter. When the bulb goes out, read the voltage value on the multimeter. Increase the voltage a little more and if the bulb is out, start reducing the supply voltage. When the bulb lights up, read the voltage of the bulb ignition. Repeat the procedure a few more times. Compare the voltage of turning on and off the bulb with the required voltage values of the regulator regulation. If they are outside the rear frame, replace the alternator voltage regulator.