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Motorcycle Maintenance

Read This First Before You Test And Diagnose Your Motorcycle Rectifier

How To Test And Diagnose Your Motorcycle Rectifier
Issues with the electrical components of a bike can be caused by the regulator rectifier. Do you have this problem but you have a new battery and a working alternator? Well, I understand why you want to test the rectifier. Testing the rectifier is crucial for maintaining a healthy battery. There’s an easy way to do it involving a neat tool. Here’s a guide on how to test and diagnose a motorcycle rectifier.
You can test your motorcycle’s rectifier using digital multimeters. This tool measures the electrical value of currents. By measuring the value of both the positive diode and the negative diode, you can find out whether the current is being regulated and whether AC power is converted to DC.
I know that this doesn’t help you a lot. So, here’s a step-by-step guide on how to use multimeter probes.

What Is A Rectifier

I guess most of you have some idea what the rectifier does, otherwise, you wouldn’t be looking it up. But I can’t be for sure, so I want to say a bit about rectifiers. You can’t really fix or test something if you don’t know how it works.

The rectifier regulator is an electrical component that converts the AC power generated by the stator coils into DC power. That’s the rectifier portion. This component also regulates the current. It allows for enough power to go through while it blocks that excess power. Without it, your battery won’t charge while the bike is running. The same happens when the rectifier is faulty. But what also may happen with a faulty rectifier is that it may overcharge the battery.

So, there you go, a component that has two functions, it rectifies and regulates currents. Therefore, it’s called a regulator rectifier. Usually, it’s located near the radiator, but it depends on the bike model. Check your manual to locate it.

Testing A Rectifier With A Digital Multimeter

Testing A Rectifier With A Digital Multimeter

Now, before we get into it, a couple of things to know. You will be working with an electrical system. This is not something that anyone can and should do. Take caution here, you may get shocked if you are not exactly handy. If you feel like you are not up for this, go to a technician to do the same test. Also, you will need a digital automotive multimeter. 

  1. Remove the rectifier block.
    Check your bike’s manual to find out where the rectifier block is located. You shouldn’t have a problem taking it out. Try finding a YouTube tutorial for your exact model. If you can’t even find the rectifier, you shouldn’t do this. It’s better to go to an expert for the first time. After that, you will know how to do it.
  2. Set the multimeter to diode function.
  3. Connect the multimeter positive cable to the positive diode. Then, connect the negative terminals to the stator coils/inputs.
    The multimeter should not show any readings. If it shows any reading up to this point, that’s a bad rectifier.
  4. Connect the negative terminal to the positive diode, and then the positive terminal to the stator coils.
    You may get something here on the multimeter but pay no attention. It doesn’t matter that much, it’s bad if there are readings on step 3.
  5. Connect the positive probe to the negative diode, then the negative probe to the stator coils. 
    You shouldn’t get readings here again. If the multimeter shows something here, you may have charging problems caused by a bad rectifier.

Testing A Regulator With Multimeter

This one is a bit easier, you don’t have to access something that’s hard-to-get-to. You will just be dealing with the battery. Basically, we’ll be testing how well the battery is being charged.

  1. Set the multimeter to DC voltage.
  2. Start your bike.
  3. Connect the probes with the battery terminals.
    Most automotive multimeters come with two black cables that have a ‘+’ or ‘-‘ sign inscribed. But some have a red and a black cable. If you have one of those, the red cable goes to the positive terminal.
  4. Check the reading.
    While your bike is idling the battery should be showing readings of 12-13 volts. Anything lower than that means the battery is not charging. Anything higher means your battery is being overcharged.
  5. Rev your engine.
    Give one or two strong twists to the throttle.
  6. Check the readings again.
    Immediately after revving the engine, the readings should be a bit higher. Around 13.5 volts to 14 volts. This is the normal range. If the readings are the same after revving, you may have a bad alternator. Anything higher than the average means the battery is overcharging.

There you go. That’s how to test the rectifier portion and the regulator. If the meter is showing readings outside the average range, you need to fix something.

Why Do Rectifiers Go Bad

Why Do Rectifiers Go Bad


You may be wondering: What caused this? The rectifier is not something that needs maintenance. So, I understand how some of you may be surprised. Well, it does not have anything to do with you. At least, negligence on your part can’t be the cause.

Rectifiers mostly fail because of overheating. As I said, most rectifiers can be placed near radiators. So, that heat may be too much for the rectifier. This usually happens when the parts that surround the rectifier produce too much heat.

Another reason why rectifiers overheat is the battery. Any bad groundings, bad connections can mess with the current and overheat the rectifier regulator.

Signs Your Motorcycle Rectifier Regulator Is Failing

  • Cold starts. 
    Depends on how bad the situations. The battery may be draining, so you will get some cold starts. Eventually, the battery will die.
  • Dimmed headlights.
    This may be harder to notice, especially during the day. Check the headlights as soon as you start your bike. While you are riding the battery may charge a bit so the headlights will lighten up.
  • Extra bright headlights.
    If the battery is being overcharged, then the headlights may get extra bright.
  • Electrical components failing.
    The excess power will start messing with the electrical system. Bulbs will start dying.

How To Fix A Rectifier

With all of this in mind, you are thinking about fixing the rectifier. Unfortunately, rectifiers can’t be fixed. At least, they are not worth it. So, you will need a new rectifier. Getting a replacement should not be that expensive, somewhere around 20$-100$.

It costs more if the rectifier caused a lot of damage though. We already explained how a failed rectifier regulator can damage your electrical components, especially the battery. If you didn’t catch this in time, chances are, you will need a new battery too.

Related Questions And Other FAQs

Will A Motorcycle Start With A Bad Rectifier

A bad rectifier will not necessarily stop the bike from starting. A fully charged battery will start the bike regardless of the rectifier. A bad rectifier causes bad starts only when it puts a drain on the battery. Also, it takes a while for the battery to go dead.

What Are The Symptoms Of A Bad Stator

The stator affects the performance of the engine. It provides power for the spark plugs. With a bad stator, the engine may not be able to run because of missed sparks. You will realize you have a bad stator once the engine starts running poorly.


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