Can a Car Run with a Bad Alternator?

Your car’s alternator works with the battery to supply the electricity that powers all the accessories in the car, like lights, radio, and heated seats. It also provides the power to your engine ignition. 

Alternators can last for a long time, but problems can occur that decrease its ability to produce power for your car’s electrical systems. Fortunately, even without a working alternator, the car should still function until the battery has been drained. However, it’s safest to address the issue immediately

How Long Can I Drive with a Bad Alternator?

The amount of time you have before the car completely dies will depend on how much charge the battery had before the alternator failed, how many accessories you have running in the car, and the driving conditions. It’s also important to note that many newer vehicles use electricity to power engine cooling fans, power steering, and a host of other systems, so the power drain can happen very quickly. Your best bet is to pick up and install a new alternator immediately or get to a service center as quickly as possible to have it replaced.

how long to drive on a bad alternator
How long can you drive on a bad alternator?

Potential Issues Driving with a Faulty Alternator

There are some potential safety hazards that come when driving with a faulty alternator. The main issue is that most new cars use electrically assisted power-steering and when the power fails, the steering effort goes up quite a bit. This could be very hazardous at highway speeds, especially with inexperienced drivers, or if driving in areas with many curves. You’ll also be doing your battery no favors, since completely draining a battery reduces its service life. If your alternator has quit and the battery charge is depleted, your car could stall, leaving you stranded in traffic.

Signs of a Faulty Alternator

The faster you recognize the signs that your alternator might be going bad, the better off you’ll be, so it pays to know these signs of a faulty alternator:

  1. Your car won’t start. If your car refuses to start and then dies quickly after being boosted, you may have an alternator issue. If your battery is simply dead (but the alternator is still good), your car should run after being boosted.
  2. Your headlights are dimmer than usual or are dimming and getting brighter at random intervals. This is a sign that your alternator is providing inconsistent voltage and may fail soon. 
  3. Your battery warning light appears on the dash. The “battery” light actually represents the charging system, not just the battery. If it appears on your dash, you’ll want to check that your alternator is still producing consistent voltage. Most alternators should be producing between 13 and 14.5 volts, if they are operating properly. 
  4. You are hearing strange noises from under the hood of the car. If you hear new rubbing, grinding, or squealing noises from under the hood, it’s a sign that something is wrong. Start by opening up the hood and looking, listening, and smelling for the location of the problem. If you can pinpoint the location, you can start to diagnose many car care issues.
Signs on when to replace your alternator

How Replace an Alternator

There is great satisfaction in diagnosing an issue with your car, buying the parts and tools for the job, and successfully completing a repair. Anyone can work on cars. You just need to be patient, remember where you put all your parts, and do a little research before you start. Read on for a general guide on how to replace the alternator in your car.

  1. Order your alternator according to your car’s year, make, model, and engine size. 
  2. Detach the negative cable from your car battery. You’re working on the electrical system of the vehicle, so you don’t want to injure yourself or cause problems with the car due to a short circuit.
  3. Locate the alternator. They’re usually about the size of half a loaf of bread and it will have a pulley on the front with a belt attached. The belt will probably wind around a bunch of other pulleys on the front of the engine.
  4. Remove the belt from the alternator pulley. One of the other pulleys on the front of the engine will be a tensioner pulley which is attached to a spring-loaded arm that moves back and forth to keep the belt tight on the rest of the pulleys. Most tensioners have a nut on the front so that you can use a box end wrench or a socket and ratchet to release the tension on the belt. Be careful because if your wrench slips off, the tensioner will spring back quickly and could injure you. If there isn’t a nut on the front of the tensioner, it probably has a female hex key insert that you can use for the same purpose. Some tensioners, often on older cars, are mechanical rather than spring loaded and simply need a bolt loosened.
  5. Disconnect all the cables that are attached to the alternator. The electrical circuit to the battery will be a wire that is bolted to the alternator. The control circuit will usually use quick connectors that have a tab that you pry back with a little screwdriver while you pull the connector. Be careful not to break the tabs! While you’re taking off the cables, inspect them to make sure they’re in good condition (not cracked or worn).
  6. Remove the bolts that connect the alternator to the engine. Most alternators will have two to four bolts. Carefully remove the alternator.
  7. Installation is the reverse of removal.
  8. Once you have the new alternator installed, test it by starting the car and removing the negative battery cable. If the car continues to run without the battery connected it means that your alternator is producing power. You can test how much voltage your alternator is producing by using a multimeter. Set your multimeter to Volts (at least 20) and then touch the disconnected negative cable to one of the multimeter leads while touching the other battery cable with the other multimeter lead. It should read around 13-14.5 Volts.

What Does an Alternator Replacement Cost?

If you do the work yourself, you can save a bunch of money. Even if you purchase all the parts and tools that are required for this repair, it will still cost less than going to a mechanic.  The other factor is the one to three hours of labor costs, at $100-$200 per hour, for a total cost to replace an alternator will probably be between $600 and $1,000 or more, depending on the cost of replacement parts.

Working on your own vehicle is a fun challenge; it develops useful skills, as well as a sense of pride in knowing that you can solve problems on your own. Invest some of the money you save in tools and pretty soon you’ll have everything you need to do maintenance and repairs on your fleet. Whether it’s parts you need or tools to get the job done right, you’ll find it at AutoZone.

This you alternator is going bad? Don’t forget that you can head to your local AutoZone and get your alternator tested for free.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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