Most riders absolutely dread the thought of having to fix electrical faults on their motorcycles, and a failing rectifier is no exception to that.
However, if you spend a few minutes to familiarize yourself with each component of the charging system, you’ll be able to easily pinpoint which component is going bad before it stops working completely, and leaves you stranded.
In this article, we’ll take a closer look at the rectifier – what it’s used for, signs of failure, and how to test it. So, if your motorcycle is struggling to charge the battery and you’re wondering whether the rectifier is at fault, then this article is for you.
What is a Motorcycle Rectifier?
The motorcycle rectifier is the silver (or black) aluminum box with large cooling fins, that you often see on the side of a motorcycle. The rectifier is used to convert alternating current (AC) from the stator (alternator) to direct current (DC).
The process of converting AC to DC is called rectification – that’s why it’s called the rectifier. Furthermore, the rectifier almost always contains a voltage regulator inside, therefore some people also call it the Rectifier-Regulator.
Why Does a Motorcycle Need a Rectifier?
Nearly all motorcycles on the road today use a 3-phase AC generator, also known as the alternator, to power the ignition system, fuel injection, and accessories.
Now, the alternator produces alternating current because it’s the most efficient design, but the electrical system of most motorcycles requires direct current to function. This is where the rectifier comes into play.
The rectifier takes the AC from the alternator and converts it into DC, using a full-bridge rectifier. The direct current from the rectifier then goes through the voltage regulator, which keeps the voltage at a steady 14.5 Volts or so, depending on the make and model of the bike.
What Are the Symptoms of a Bad Rectifier?
Once the rectifier starts going bad, it can cause some weird problems with the electrical system. Here are the most common symptoms of a bad motorcycle rectifier:
- Engine idles rough
- Misfiring or hesitation when accelerating
- Dim headlights
- Sluggish acceleration
- Frequent dead battery
- Overcharging (>15.5 V)
- Undercharging (<13.5 V)
Keep in mind that the rectifier usually contains the voltage regulator inside. If the voltage regulator part of the rectifier goes bad, it will overcharge the battery (>15.5 V).
If that’s the case, you will need to replace the rectifier.
How to Test a Motorcycle Rectifier?
Testing a motorcycle rectifier can be a challenging task, due to the electronics inside it. Different make and model motorcycles have different rectifiers, therefore there are no universal methods of testing a rectifier.
However, there is a way to easily determine whether the rectifier has gone bad – the elimination method.
That’s right, by quickly checking the stator and the battery, we can quite easily and reliably determine whether the rectifier is bad. If the stator and the battery are in good working order, then the only component left is the rectifier.
The Elimination Method
Here’s a brief breakdown of the elimination method. First, we check whether the AC generator (alternator) is charging properly.
- Unplug the three-wire AC generator connector.
- Start the engine and maintain cruising RPMs.
- Measure the AC voltage at each of the three wires. It should be no less than 70V for most motorcycles, otherwise, you have a bad stator. Consult your service manual for the exact voltage.
- Plug the AC generator connector back in.
Next, it’s time to check whether the battery is in good condition and whether it can hold a charge.
- Fully charge the battery.
- Check the battery voltage. It should be between 12.4V and 12.7V. If it’s outside the range, replace the battery.
- Measure the battery voltage while cranking the engine
- If the voltage falls to 10.4V while cranking, replace the battery.
Now, if your motorcycle passed all the checks above, then the only component left is the rectifier, therefore it’s very likely that it should be replaced.
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Testing the Rectifier With a Multimeter
In order to test a motorcycle rectifier with a multimeter, you must find the service manual for your bike.
The rectifier is made up of various electrical components and they are unique to each make and model bike. Because of that, the only way to test them properly is by using a service manual that contains all the expected values and ranges.
If that is not possible, you might be able to test a rectifier using the forward and reverse bias test. For more information, please check out this video by Partzilla on YouTube.
What if I Can’t Find a Rectifier for My Specific Motorcycle?
If you own a rare or old motorcycle, chances are that you won’t be able to find a rectifier made specifically for it.
In that case, you can likely use a rectifier from the same motorcycle manufacturer, as long as it has the same number of wires. Now, we highly recommend that you test the output of the rectifier in order to know whether it could be retrofitted to your motorcycle.
For example, I own an old Suzuki Bandit 400, and I wasn’t able to find a rectifier made specifically for this model. To go around that, I bought a rectifier for a Suzuki DR-Z 400, which has the same number of wires and pretty similar connectors.
The three yellow wires on connector 1 were used for the alternator output, while the two black and red wires on connector 2 were used to output DC power.
I connected the three alternator wires to my Bandit 400 and measured the two red and black wires, which I assumed were the DC output. The voltage was a constant 14.5V, which meant that the rectifier is compatible with my motorcycle.
At this point, all I had to do was snip the connectors that didn’t fit together, and replace them with Super Seal connectors.
Final Thoughts
The rectifier is one of the most challenging components of a motorcycle’s charging system to diagnose. It’s a silver or black box with rather large cooling fins, and it’s tasked with converting AC from the alternator to DC, which is required for the bike to run.
However, even though diagnosing the rectifier can be difficult, if you were to look for and use the elimination method, or a service manual – you can test it pretty easily.
Do you have any questions or advice regarding motorcycle rectifiers? Feel free to comment down below!
Nick Kowalski is a diesel mechanic by trade, but his passion lies in motorcycles.
He’s been riding them since he was just a wee lad, and he loves restoring old Japanese street bikes and ATVs in his spare time.
Nick is an adrenaline junkie at heart, and he loves nothing more than hitting the open road on his motorcycle.