How To Diagnose A Failing Alternator

A Toyota alternator can last as long as 100,000 miles or more. But it’s possible for an alternator to fail early due to unforeseen circumstances. If you have a feeling that your alternator is on the fritz, this diagnostic guide will help you confirm it.

Telltale Signs Of A Bad Alternator

Old alternator

When your alternator starts to fail, you’ll know. The signs are so obvious. Some signs include:

  • Battery dies, even though it's not that old
  • Vehicle can be jump-started but stops running shortly after the jumper cables are disconnected
  • Battery indicator light showing on the dash. Some vehicles use "Gen" or "Alt" lights instead.
  • The smell of burning rubber
  • Whining or screeching sound coming from the engine bay
  • Alternator voltage output has dropped from its typical output

What Causes An Alternator To Go Bad?

Alternators don't last forever. It's because the components within or connected to them wear out over time. Here are some of the most common causes of a bad alternator:

  • Bad diodes
  • Failed internal wiring
  • Worn alternator bearings
  • Failing voltage regulator

When your alternator goes bad, the best thing to do is to replace it. It's a fairly simple job that we talk about here. Instead of paying a huge markup at a Toyota dealer, you can order a genuine OEM alternator from us. We offer rock bottom prices, same-day shipping, and no hassle returns. Here's a catalog of all OEM Toyota alternators in our inventory. Look up your model to see if we have an alternator for your car.

Inspecting Related Parts First

In some cases, the alternator is fine, but a related part is interfering with its performance. It doesn't take a lot of time to inspect the parts connected to the alternator. Before checking the alternator, inspect the following parts:

  • Drive belt
  • Wiring harness
  • Alternator fuse (if there's one)
  • The lead between the alternator and the battery

Checking The Alternator With A Voltage Meter

alternator voltage meter

Image Credit: Junky DIY Guy

If all the related parts look good, then it's likely that the alternator is going bad. Grab a friend and a voltage meter and then take these steps:

  1. It's best to let the car sit overnight, so you can measure the resting voltage of the battery.
  2. Make sure any accessory that draws battery power is off. Keep the doors closed, so the dome lights aren't on.
  3. With the voltage meter, check the voltage of the battery.
    1. It should read between 12.6V and 12.8V.
      1. If it's between 12.6V and 12.8V, then the battery should still be good. It's still possible that the alternator is bad, though.
      2. If it's under 12.6V, the battery is not fully charged. If the voltage at 11.9V or below, the battery is fully discharged and may be bad. It's possible that the alternator is still good and just the battery is bad.
  4. Touch the voltage probes to the battery terminals.
  5. Have a friend turn the engine on and leave it running.
  6. With the voltage meter, check the voltage of the battery.
    1. When the engine first starts, the voltage should read between 14.1V and 14.7V. After a few minutes, the voltage will drop.
    2. If the voltage drops to under 13.7 V, then the alternator isn't keeping the battery charged. The battery is being drained without the alternator replenishing its energy.
    3. If the voltage remains between 13.7 V and 14.7 V, then the alternator is still charging the battery.
    4. If the voltage right after the engine starts never makes it above 14V but instead sits around 13.7V-13.9V, the alternator is getting worn out, and will probably fail soon.

Do the alternator and the battery seem good, but you still have a hard time starting your car? It's possible that there's a parasitic draw somewhere. The problem should go away once you find the source of the draw.

Please get in touch with us if you have any questions about checking the alternator on your Toyota.