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Symptoms of a Bad Stator: Troubleshoot your Motorcycle Charging System


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If you’ve got problems with your motorcycle’s charging system or electrics, the alternator system, including the stator, is one of the things you may need to check.

This page will give you an overview of motorcycle “alternators,” which consist of a few components, and how to test if the system is working.

Parts of a Motorcycle Alternator System

The alternator system on a motorcycle consists of a few separate parts, including:

  • A stator.
  • A rectifier.
  • A regulator.

Motorcycle Stator

The stator is one of the main parts of your motorcycle’s charging system.

A stator looks like a round, woven coil of wires, and it sits around the magnets connected directly on the flywheel.

As the engine rotates, the stator creates alternating current.

This power is then sent to the regulator and rectifier units to convert it to DC to charge the battery and power the bike.

Motorcycle Rectifier

The motorcycle’s rectifier is another part of the charging system.

As electricity is produced, the rectifier controls which type of electrical current get to the battery to charge it.

AC is produced by the stator and the rectifier converts this to DC to send to the battery.

Motorcycle Regulator

Then the motorcycle’s voltage regulator controls how much current gets to the battery so that it doesn’t overcharge.

Overcharging the battery can cause it to overheat and damage itself and other electrical components.

The regulator is responsible for maintaining consistent output at the right voltage.

Motorcycle Alternator vs. Car Alternator

The function of a motorcycle alternator and alternators found on other vehicles is the same. It creates, converts, and manages electrical current to charge the battery and keep the vehicle running.

The main difference are:

  • Motorcycle alternators are a few separate units while car alternators are a single contained part.
  • Motorcycle alternators create AC directly from the rotation of the engine while other types of alternators spin indirectly from the movement of the engine, typically with a series of belts.

How Does the Stator Work?

A motorcycle alternator is the system responsible for charging the battery.

A motorcycle’s alternator is made up of a few main parts that are wired into the electrical system to convert mechanic energy into electricity.

This alternator system works together to:

  • Produce alternating current (AC) as the engine rotates.
  • Convert the AC into direct current (DC).
  • Sending the DC to the battery to keep it charged and maintain proper voltage through the electrical system.

When the motorcycle’s alternator system is working properly, the battery will stay charged, a strong spark with reach the spark plugs, and all the lights will work properly.

When the charging system isn’t working, the spark may be weak, the bike may not start, the battery won’t charge, the lights may drain the battery, or the lights may kill the bike altogether.

You can learn more about your motorcycle charging system and testing the battery here.

Stators, regulators, and rectifier units can wear out over time.

If you’re working on an older bike, you may need to replace some of these parts to get your charging system working again.

Symptoms of a Bad Stator

If a motorcycle’s charging system isn’t working properly, it won’t keep the right voltage in the battery and power to other components may be weak.

Tracing the source of your motorcycle problems to the stator can be difficult, and typically comes after a bit of trial and error troubleshooting other systems.

Most often, motorcycle stator problems make it so your motorcycle won’t start.

In this case, you’ll have problem already taken the steps to check whether or not the battery itself is the issue.

Other symptoms of a bad stator and charing system include:

  • Engine misfires.
  • Backfire.
  • Poor running conditions.
  • Weak spark.

If you’ve ruled out other components after troubleshooting the issue, you may indeed be experiencing a bad stator.

In this case, the next best course of action is usually to replace the entire unit.

Will an engine run with a bad stator?

Whether or not your engine will run or stay running with a bad stator will depend.

A lot of times, if the bike does start, it will start running poorly as it continue to run and drain the battery or at varying RPMs.

How to Test a Stator

You can test the components of your motorcycle’s charging system with a multimeter.

There are typically 3 wires of the same color and a ground wire that come from the stator.

To test the stator:

  • Set the multimeter to Ohms.
  • Measure the continuity/resistance between the wires of the same color coming from the stator unit.
  • The multimeter should read between around .5 to 3 Ohms.
  • There should be no continuity between the three wires and the ground wire.

Refer to your shop manual for the exact specs, and replace if necessary.

How to Test a Rectifier

To test the rectifier, connect your Ohmmeter to one of the wires and the ground/mounting stud of the rectifier and record the reading.

Next, reverse the leads of the Ohmmeter and check the readings again.

The two reading should be very different. Repeat for all wires.

If any one of them has a reading that is too close or if there is not continuity, the rectifier may be bad.

How to Test a Regulator

To check that the voltage regulator is working properly, set a multimeter to the 20 volts DC range and connect the positive and negative leads to the positive and negative leads on your batter.

As you rev the engine, the volt reading should get up to around 14 volts. If it’s more than that, the battery will overcharge. If it’s too much less, the battery won’t charge at all.

Some old regulators can be opened up, cleaned, and adjusted if they are not working correctly.

Others may need to be replaced if they are not working.

Modern units typically combine the rectifier and regulator into one component.

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