Zener diode

Should be on the inside of the right hand alloy Z plate the footrest is bolted to.

https://andover-norton.co.uk/en/shop-drawing/223/support-plates-riders-footpegs-rear-brake-pedal-and-linkage

The facts above re the zener diode is about right. and the British motor cycle charging is poor BUT converting to the modern reg/rect will not get you more power. The reg/rect will stabilise the voltage better. And of course the Zener system has been with us 50. yrs, so not a bad record. Get the Zener voltage checked and it should be good. If out of spec (14.7V) Then move on.

I have sold rectifiers for over 40 years sold and tested Zeners and sold regulator/rectifiers for nearly as long. My opinion is that the zener has been more reliable than the modern reg/rect. The reg/rect has many components inside all subject to possible failure, And the design criteria sometime needs consideration.  I say again if your Zener checks out as ok keep with it.

Alan Osborn is correct; there is nothing wrong with a known zener diode.  My set-up on my 1978 Mk.III Commando is now around 45 years old and it is still working perfectly.  Likewise, my Boyer system, fitted in 1982, still functions well.  My machine starts third or sometimes second kick from cold and charges correctly.  Some faults with zener diodes were identified as owners incorrectly tightening them; the book says 24 inch pounds for the fixings - that's two foot pounds.

14 commando all with zener. If respected thermally, they will last longer than a human life.

Electronics, with capacitors, are not so blessed.

New podtronics still in box on shelf  after 30 years. One day, when I feel wild and reckless, I'll try one.

I have a 1971 commando and is not charging. I have checked the alternator out put and have 8.3 v one each out put wire.
It is a two wire alternator the rectifier has been replace. For an electronic type. Still no charge. Will the Zener diode stop the bike charging? Is there a test for the Zener diode with a multi meter? Regards Byron

Put a voltmeter accross the battery and check voltage when stopped and then running , should see a rise . Disconnect zener and test again .Its likely little difference. If a big rise then its possible its the zener . But its more likely an issue with alternator or wiring.

You cannot check the alternator with a multi-meter unless you understand electrics very well. There is a test for the alternator using a lamp on my web site, if you do that you will have a decent result. If there is doubt about the modern reg/rect replace it with a basic bridge rectifier (that was on the bike before) NO zener. This will restore an-unregulated charge if alternator is good. The zener is there to stop over charging only, if it fails open circuit-you will over charge. If it fails short circuit you will blow the fuse. It can be checked with a volt meter across the battery if you know what you are doing. I can check zeners by return of post (with postage|)

From Ben C-I have a 1971 commando and is not charging. I have checked the alternator out put and have 8.3 v one each out put wire. It is a two wire alternator the rectifier has been replace. For an electronic type. Still no charge. Will the Zener diode stop the bike charging? Is there a test for the Zener diode with a multi meter? Regards Byron.

The above Voltage check is meaningless please ignore. changing the rectifier for a reg/rect could also be a backward step. Lack of charge on any/all Lucas alternator bikes mid 50s to 1976 (MKIII) first check the alternator as per my instructions on AOSERVICES.co.uk. If the alternator proves in good order, replace the rectifier with a basic bridge rectifier (original equipment type or purchase from AOServices) disconnect any zener. Check battery voltage is 7.5 or 14.7 (ish)

......the zener failed open circuit on my '72 Commando.
It cooked the battery, acid all over the rear wheel and Akront aluminium rim.
Big mess.
Run with the headlight on.