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How to Diagnose an Alternator or Electrical Problem

I've worked in a variety of hands-on professions and love to write about topics that can save readers from needless expenses.


Diagnosing alternator problems can be a bit tricky because the electrical system in a car comes from two places: the battery and the alternator. I'll give you a little background so you can understand what both systems do.

  • The battery has one primary function, which is to start the car. It provides the high amount of voltage needed to turn the engine over. After the engine is running, the alternator takes over.
  • The alternator is basically a generator that provides an even amount of low-level electricity, which it generates with a circular bundle of copper wires and a rotating magnet.

After the engine starts (thanks to the high voltage provided by the battery), the alternator provides the electricity required to power the car's many electrical functions, including the lights and radio, the spark, charging the battery, and others.

What Happens When Your Car Has an Electrical Problem?

When your car has an electrical problem you will notice any of the following:

  • The car's power slowly dies while you're driving.
  • The car's lights begin to dim.
  • You have trouble starting the car.
  • The car won't start at all.

What to Check to Diagnose Your Car's Electrical Problem

AlternatorBatterySpark Plug or Ignition CoilSolenoid or Starter

If the problem is a bad alternator, your car will slowly lose power. You'll be driving down the road, your lights will dim, and you'll lose power and die.

If the problem is with your battery, then you'll generally only have trouble starting your car since that's the battery's main function. Your lights may also be very dim.

When an ignition coil goes bad, the result is similar to a failed alternator: The car loses power and won't start.

If your car's solenoid or starter is bad, the lights will shine brightly but the engine will not start.

Once the alternator stops working, so do all of the electrical systems required to keep a car going.

Other problems can also cause a car to not start. It could be a bad starter, corroded cables, or a broken connection somewhere between the battery and starter.

When a coil goes bad there may be smoke or odor. Open the hood immediately and check.

A failed solenoid can make a clicking sound when you try to start the engine.

What Causes a Faulty Coil or Spark Plug?

Spark plug or coil problems can also make your car lose power and die. The ignition coil turns the battery's low voltage to the thousands of volts needed to create an electric spark in the spark plugs, which ignite the fuel. Coils can fail due to heat or vibration, or when there is a bad connection to the spark plugs.

As described above, when the coil goes bad, parts of it often melt and there's sometimes smoke or an odor. This will only really help you if you open up the hood immediately after it happens and check. If you check hours or days after the fact, then you won't be able to smell anything anymore.

Dual ignition coils (the blue cylinders at the top of the picture) on a Saab 92.

Dual ignition coils (the blue cylinders at the top of the picture) on a Saab 92.

Test Your Systems Before Going to the Auto Parts Store

Before you go out and order a new battery or alternator, test some of these different systems. If your alternator, coil, battery, or starter is bad, the car probably won't start, and it definitely won't be drivable.

Try to get as much information as possible so that you can get the right parts quickly when you go to the auto store. Depending on which system is bad, you should be able to fix it yourself.

Alternator Prices and Installation

Make sure that your current alternator is beyond repair first before getting a new one. Alternator prices can vary greatly by make and model, but generally speaking, new alternators should run around $250 without labor for installation.

If you're going to order and install the part for yourself, then it shouldn't be too expensive—just a few hundred bucks. If you need to have it installed by the mechanic, then the price might be a bit higher.

Depending on where your alternator is located within the compartment, the installation might only take an hour. But if the alternator is difficult to access, it might require the removal of other parts and it could take many hours.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2010 Benji Mester