Testing your Motorcycle Rectifier

Grant Tiller

A Rectifier is the part of the charging system that convert the AC voltage from the alternator into DC voltage which can be used to charge the battery.

Here is a picture of a Lucas alternator rotor (the magnetic bit in the middle) and stator (the bit round the outside with the wires coming out of it)

Grant Tiller

The rotor is mounted directly to the engine’s crankshaft.

As the magnets in the rotor spin round they excite the coils of wire which are embedded in the green resin on the stator.

This produces an AC current.

Grant Tiller

AC is fine for making a lightbulb glow, however in order for it to be used to charge a battery, it needs to be converted into DC instead.

This is where the Rectifier comes in to play.

The “Full Wave Bridge Rectifier” to give it the correct name uses four diodes which basically takes everything below the green line, flips it, and puts it above the line!

Grant Tiller

The is now DC, and can be used to charge the battery!

The ORIGINAL RECTIFIER looks something like this:

Grant Tiller

The diodes on the original units were prone to rattling off, due to the vibrations on cars and bikes.

So the more modern equivalent AFTERMARKET RECTIFIER is potted in an epoxy resin encapsulant. It looks like this:

Grant Tiller

A popular upgrade is to combine the rectifier and regulator (the zener diode) with one, single unit.

The AFTERMARKET REGULATOR/RECTIFIER looks something like this:

Grant Tiller

There are some new types of regulator/rectifiers that can not be tested simply, because they contain microchips and specialist circuitry inside them.

You cannot test the new MOSFET regulator/rectifiers in this way, as they don’t use standard diodes.

Also bear in mind that some of the combined regulator/rectifiers have capacitors in them, which will also affect your results.

If you are running a combined regulator/rectifier, remember that this test procedure only confirms that the rectifier package is functioning as it should. The regulator side is not part of these tests – you should monitor the output across positive and negative cables when the reg/rec is connected to a fully charged battery.

However, most of the regulator/rectifiers in the market (examples include Podtronics, Boyer Bransden Power Box and Tympanium) are simple and easy to test.

If you have a factory original rectifier, make sure you have a new battery in your multimeter – the diodes are chunkier than on the modern equivalents.

Multimeter Settings

Grant Tiller

Most Digital Multimeters have a diode test setting.

It is a line with a triangle on it.

Grant Tiller

The display will read “OL” which means “Open Line” when there is nothing connected to your test leads.

Grant Tiller

With your leads connected to each other, your multimeter will read 0

Grant Tiller

Only newer analogue mulitmeters will have a diode setting.

For example, my Avo doesn’t. But that’s no problem.

Set the meter to read Ohms – it is the greek alphabet Omega symbol

Grant Tiller

With nothing connected, the needle will be at rest, over to the left.

Grant Tiller

With your test leads connected to each other, the needle will be at the opposite end of the scale resting on the stop – reading 1.


Here is the testing procedure as printed in the 1970 onwards Norton Commando Workshop Manual:

Grant Tiller

I don’t think their description of the test procedure is particularly clear. So i have done my best below to make it easier to understand.

Follow the instructions below, alternatively click here to download a set of instructions that you can print out.

Grant Tiller

You should produce a set of 8 numbers – follow the process below, and write the numbers down as you go, so you don’t lose track.

In the tests below – the green arrow on tests 1, 3, 5 and 7 indicate that you should see a value on your multimeter.

This value will vary per unit but importantly the value from all four readings should be the same or very nearly the same.

For the remaining tests – the STOP sign on tests 2, 4, 6 and 8 shows no voltage is flowing.

Grant Tiller
Grant Tiller
Grant Tiller
Grant Tiller


If you are not seeing the desired values on just one of these tests, it means that you have a malfunctioning component (diode) within the rectifier.

Unfortunately, these are not user serviceable or repairable, so you need to replace it.

Make sure you throw the defective part away, rather than put it in a drawer in your workshop. This will make sure it doesn’t find it’s way back on to a bike in the future!!!

Click here to download a set of instructions that you can print out.

Grant Tiller

Hopefully this will help someone somewhere!

8 replies

  1. Thank you Grant for all you do to support the Classic British motorcycle community! You do more than you may realize and many classics are on the road due to your support. Excellent and intuitive instructions on rectifier testing.

  2. Thanks for the info.
    I ran the test on a Podtronics single phase regulator rectifier and my readings were backwards from what they should be. on the tests that state there should be current flow there is nothing and the tests were there should be no current flow I’m getting 7.5-8 ohms. Do you think the wires could have gotten crossed in the manufacturing?

    • Hi Brian,

      Yes indeed – I have come across manufacturing defects a few times.
      Unfortunately (and it is very much depending on the manufacturer) not all manufacturers test every unit during the assembly stage.
      Instead, some companies test by removing a couple of reg/recs from each batch and testing on a percentage basis.

      It’s only when you get into the big companies like Shindengen where manufacture is automated, and quality control standards demand testing at multiple stages.
      These guys supply to the likes of Honda and Yamaha, so the standards are extremely high which means consequently the failure rate is very very low.

      The majority of the units we come across on a day-to-day basis in the classic bike world are made by hand in smaller numbers, and in order to keep the price down, a lot of the steps are outsourced, which is where the quality slips.

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