How to check a car alternator

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  1. How to test your alternator
  2. The Complete Guide to Testing Your Car Alternator
  3. Testing battery and charging system
  4. Automotive Editor
  5. How to check your car's alternator

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David Taylor 28th Feb Your lights are dim Are your car headlights dim? You can smell burning rubber Have you smelled a rather unpleasant whiff of burning rubber recently? Your warning light is on Many modern cars make it easy for you spot an alternator issue because they have a dashboard warning light. Your car battery is flat While a car battery can fail of its own accord, it can also be caused by the alternator not functioning correctly.

Step One: Set your Multimeter The first step is to open up your bonnet and set your multimeter to DCV which stands for direct current volts. Set the multimeter so it points to 20 DCV.

How to test your alternator

Before connecting your multimeter, locate which one is which. However, if your car battery terminals are not clearly marked, remember that the positive terminal is always the bigger one.


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  • Step Two: Locate Positive and Negative Battery Terminals?
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The red wire of the multimeter should be connected to the positive battery terminal and the black wire of the multimeter should be connected to the negative battery terminal. At this stage, the multimeter should ideally read around This means that the battery is properly charged.

The Complete Guide to Testing Your Car Alternator

Step Four: Start the Engine To check the functionality of the alternator, you need to switch on your car engine. Only when the car engine is running can you check the efficiency of your alternator. Step Five: Check your Voltage Reading When the multimeter is connected to battery terminals and the engine running, a good alternator will show a reading between A voltage reading within this range means that the alternator is running efficiently enough to charge the car battery.

If the voltage reading is over If the voltage reading is below The engine will eventually stop running because of the insufficient voltage it is getting to operate the ignition system. The result? You may find yourself stranded on the roadside! Whenever your battery warning light is on , or you recently replaced your battery or alternator, you need to test your voltage output. Testing is also essential if you hear a buzzing radio sound through your speakers. This may indicate alternator diodes gone bad, and leaking AC voltage.

If your car battery is overcharged, it will trigger a chemical reaction that will emit a noxious sulfur-like smell. This will indicate a voltage regulator failure that paved the way for the overcharging of the system when this happens.

Easy way to test Alternator at home

Your battery indicator light will not only turn on. If you need to check the charging system, it is never a good idea to disconnect the car battery while your engine is still running. This may trigger a major electrical system malfunction like a main computer short circuit. Before , it was common practice to remove the battery cable for testing the charging system.

The battery is utilized as an electric shock absorber that helps stabilize the system. If you disassemble the battery cable, it will trigger a voltage spike that can potentially cause damage to major electrical components such as the computer.

Testing battery and charging system

Thus, this is no longer done for motor vehicles today. As mentioned, the alternator helps in powering all your car's electrical systems. These include the starter motor, lights, and speaker system. A malfunctioning or lousy alternator can cause a battery discharge that may result in electrical system problems in your car. Worse, it may leave you stuck in the middle of nowhere just when you need to be somewhere else. It is, therefore, a good idea to be aware of some simple tests, and knowing the early indications of alternator malfunction to avoid a car breakdown.

Before proceeding, you need first to gather the materials you'll need for testing your alternator:. If the results of your tests show regular voltage readings, but problems such as a discharging battery, battery warning lights turned on, or car electrical systems issues persist, you may need to perform further testing. It is a good idea to seek the assistance of an expert mechanic who can perform a better job of testing or replacing your alternator. He switched on the sidelights and headlights and they worked.

He then revved the engine and blew every bulb. As pointed out by tlhIngan, use a multimeter to test the alternator. EDIT: Because there has been so much conjecture and comments thrown in below this answer, I'm adding some stuff from what appears to be an authoritative web source. Here are some excerpts on why you should not remove the connection to your battery with the engine running I think it pretty much regurgitates what JasonC says in the comments, but I'll throw it out here anyway : Your battery does more than just provide electricity.

Nobody should EVER run your engine without a battery. Could you elaborate on the mechanism by which this damages the alternator? Certainly this test has false negatives for a bad alternator, as you mentioned, but it shouldn't have false positives. Aug 2 '16 at It is not just a mechanical device but an electronic circuit including the regulator.

Related Questions and Answers

The car battery is an important part of the complete electrical circuit. Of course one could design an alternator so that the electronics would be guaranteed survive this test, but the extra cost and complexity would be pointless for normal use. Maybe damage by disconnecting the battery is impossible in the base design, so making modifications is unnecessary. I've heard this before and also would like to know how exactly damage could occur in such a situation.

The alternator generates more heat as a consequence, which has many poor possible outcomes.

Automotive Editor

The components in the alternator are designed to handle typical loads. Exceed design specs on electrical components and all bets are off. Electrical engineers will be all too familiar with the problems associated with exceeding design limits. Heat is the mechanism of destruction here. IhavenoideawhatI'mdoing - IIRC, the alternator pulls the voltage from the battery to set up the field current. Without it, it can go crazy. I'm not an electrical engineer, though, so could have that wrong. Here's an easier and safer way to test an alternator.

Get a multimeter With the car on but all accessories off, read the voltage between the battery terminals a healthy alternator will read about 14V With the car on and all electrical accessories on, read the voltage again a healthy alternator will read about 13V. While this is true, it doesn't answer the question at large.

How to check your car's alternator

Please reread the question. I'm not saying you're wrong in what you are saying, I'm just saying it doesn't answer it. SickoPsycho SickoPsycho 51 1 1 bronze badge. Gargravarr Gargravarr 1 1 gold badge 4 4 silver badges 8 8 bronze badges. Featured on Meta. Feedback post: Moderator review and reinstatement processes. Post for clarifications on the updated pronouns FAQ. Linked 0.

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