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Will a Car Run With a Bad Alternator & For How Long?

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If your car’s alternator has failed and is no longer charging the battery, you’re forced to either fix it yourself or get the car to your mechanic. But will the car run with a bad alternator and will you be able to drive to your mechanic without spending more money on getting the car towed?

Signs of a Bad Alternator

An alternator supplies current to the vehicle’s electrical system and charges the battery. The charging system of a car consists of the following components: alternator, battery, and the battery light on the instrument cluster. Here are the signs of a bad alternator:

  • Battery light in the instrument cluster
  • Interior and exterior lights going dim
  • Slow power windows
  • Difficulty starting the engine

The failure of an alternator is categorized into insufficient charging and overcharging.

An insufficient charge is usually due to:

  • A snapped or loose serpentine belt
  • Open-loop in the charging circuit (positive “BAT” terminal of the alternator; positive terminal of the battery)
  • Open-loop in the ,,excitation circuit” of the alternator
  • Internal fault in the alternator
  • Faulty voltage regulator

Insufficient charging of the battery is the most common way the alternator fails. Running a car with an alternator that provides poor charge will damage the electrical ignition components of the car (even on older, pre 20000 vehicles) over time. Letting the car’s battery discharge completely will damage the battery’s ability to hold a charge – avoid doing so if possible.

Overcharging is less common, but this condition does considerably more damage to the car’s battery and the electrical system if you ignore it.

Overcharge usually occurs due to:

  • Poor battery condition;
  • Failed voltage regulator.

Irreversible damage to the battery is done when the alternator is overcharging. The battery may warm up and damage its structure and interior. Drying out of the electrolyte is common and produces a smell of rotten eggs.

How To Test An Alternator?

1. Test The Battery

First, you need to check the condition of the battery. This is important because a bad battery could make the alternator overcharge, thus making it difficult to determine if the alternator is bad.

Make sure your car’s battery terminals are free of oxidation and are making good electrical contact with the terminal connectors. Get your multimeter out, set it to Volts DC and check the voltage of the battery (ignition off). A healthy, fully charged battery should have a voltage of 12.3-12.6V. Anything below 12.3V could indicate a bad battery or a bad alternator.

Time to test the battery under load. For this, you can use a special battery load tester or your multimeter.

If you’re using a battery load tester, a voltage of 10.5V and below on a loaded battery indicates a bad battery.

If you’re using a multimeter, you will have to load test your battery using the car’s starter motor as the starter motor is the single biggest consumer of electrical power in your car. Measure the voltage of the battery while cranking the engine, if the voltage of the battery drops below 10.5V – you probably have a bad battery.

To test an alternator accurately, you need to have a good, fully charged battery.

2. Visual Inspection

Give the alternator a good visual inspection. Is the serpentine belt loose? Are any connectors loose? Are the terminals free of corrosion? If the alternator passes the visual inspection, ready your multimeter and continue to the next test.

3. No-Load Test

First, test the charging system without loads (turn off your headlights, blower motor, radio and anything else that could be consuming excess power).

Get an assistant to start the engine and maintain the engine speed at 2000 RPM. Check the voltage of the battery. Expected readings should be between 13.0 – 14.8 Volts.

Anything below 13.0V indicates a faulty alternator or a wiring problem.

On the other hand, anything above 14.8V indicates a bad battery or a failed voltage regulator in the alternator.

4. Loaded Test

This time we will really stress the alternator. With the engine running, turn on your car’s accessories: headlights, radio, wipers, AC and so on.

Keep the engine speed at 2000RPM and measure the battery voltage.

A voltage higher than 15.0V indicates a faulty voltage regulator and an overcharge condition.

A voltage lower than 13.0V indicates a faulty alternator and an undercharge condition.

Will a Car Run With a Bad Alternator?

In short – yes, but It’s not recommended to DRIVE a car with a bad alternator, here’s why:

Your car’s alternator is the main power feed for the car’s electrical system when the engine is running, the battery is only designed to be used as a power source to start the engine. Because of this, your car’s battery is not designed to power the car’s electrical system when the engine is running. This means that you won’t get too far with only using the battery as the power source, and when the battery’s charge is depleted your car’s engine will stall and you will be stranded.

Driving with a bad alternator is dangerous, more so with newer cars that rely so much on electronics. And, if your car has a fully electric power steering, don’t even think about it! It becomes a serious safety hazard for you and the people around you. You will be better off towing the car to the mechanic.

How Long Will a Car Run With a Bad Alternator?

It really depends on the car and the battery. Generally, diesel cars will run considerably longer than gasoline cars. Since diesel engines ignite the air/fuel mixture using the heat that is created when compressing the mixture, they don’t use as much electrical energy as gasoline engines. Gasoline engines require tens of thousands of volts to ignite the air/fuel mixture – this extra consumption of electrical energy reduces the engine’s running time dramatically.

If most consumers of electrical energy (headlights, AC, blower motor and so on) are off, a car could run for 5-60 minutes if it has a gasoline engine.

If the car has a diesel engine, the runtime is extended to from 30 minutes to a couple of hours.

2 thoughts on “Will a Car Run With a Bad Alternator & For How Long?”

    1. Hi, Arlene!

      Alternator replacement on the 2002 ES300 takes 1 to 2 hours, which should cost around $100 to $200 for labor. A new alternator goes for around $150 to $250 (depending on quality), so you’re looking at a $250 – $450 total for parts and labor. You can save on labor by going to an independent shop instead of the dealer.

      Hope that helps. Good luck!

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