Tech Tip: Testing A Lucas Zener Diode

Tech Tip: Testing A Lucas Zener Diode

A few weeks back my 1968 Triumph TR6R was experiencing some serious electrical issues.

Backfiring, popping, spitting and everything else in between. I thought to my self how strange? The problem made its self evident literally 1 day after a 100 mile trip.

What could it be?

At this stage I'm trying to figure out what could cause this problem but first let me take a few steps back and figure out my "last steps" that may have caused the problem.

During my trip my engine reached 1,000 miles. My typical routine is to torque the head, adjust valves along with checking / setting the timing.

After setting the timing I took my TR6R for a quick spin and noticed a slight "hick up" around 3500 RPM. I knew something was a miss as this was completely out of the character of my bike. At this point I ignored the "hick up" and continued to ride the motorcycle but was still thinking about the performance.


Setting Ignition Timing On A 1969 Triumph TR6R

 Degree wheel attached to adjust and check the ignition timing


Back on the bench I checked the timing about three times and both cylinders fired at 38BTDC. 

When it comes to timing I like to check, double check and triple check just to make sure everything is set properly. No mistakes!

Another quick spin and the problem was still there. This time it was time to do some investigation work. Back on the lift.


Testing Ohms On A Lucas MA12 Coil

 Using my meter to check the primary resistance on my Lucas MA12 coils


Next was to remove the petrol tank and check the coils for excessive resistance. Upon removing and checking the coils I found that both coils had the adequate amount of resistance (primary and secondary) according to a Triumph workshop manual. In fact, both coils were stamped 1967 making them original to this motorcycle.

Another thought was it could be the condensers but I do not have a proper way to test the condensers. Since condensers are inexpensive I went ahead and replaced them anyways.


Old Lucas 6CA Condensors

 Lucas 6CA condensers - original to the bike since 1977


Both original condensers were marked 1977 which I would assume that is when they were replaced a long time ago.


 Checking Battery AC Voltage Using Meter

 Measuring AC voltage

Next step is to check the battery voltage to make sure that AC from the Lucas alternator was not leaking past the rectifier to the battery. A quick test with my voltage meter indicated that there was no AC voltage in my battery when the motorcycle was running. 0.00 AC volts to be exact according to my meter.


The mystery goes on...

Once running the ammeter does show evidence that the electrical system is charging. One could assume that the rectifier and diode is working or perhaps the battery may mot be fully charged.


Perhaps a bad ground?

Prior to the rebuild I replaced and cleaned just about every electrical component and connector except the coils, ignition and condensers. I felt very confident that a ground issue would be unlikely but still possible.


Triumph Heat Sink & Zener Diode


A quick check with my meter and found no resistance through my wires and all ground locations where clean and solid.


The last check..

With myself checking just about every component on this motorcycle there was only 1 piece of hardware left... the zener diode.


Lucas Zener Diode

 Typical Lucas zener diode fitted on most 12V British motorcycles


The zener diode plays a simple yet important role in the way a charging system will behave. Another word for a zener diode is a voltage regulator as it sheds excess voltage back into the frame when it exceeds a certain voltage threshold.

Zener diodes current only travels in one direction. Current should never travel back into the harness on the diodes we use for our old British machines.

Another tale-tale sign that my diode could be suspect is that my ignition warning light and speedo light where both burnt out. Typically 12V bulbs are rated slightly higher to take the voltage fluctuations but nothing excessive.


The zener diode test

There are many ways to test a zener diode both on the bike or on a bench. Since I felt testing the diode on the bench was over the top I descided to test the diode on the bike to see exactly how it was performing in "real life".


Checking Zener Diode Voltage With A Multi-Meter On Gas Tank

 Meter attached to the parcel grid for testing


I strapped my voltage meter to the gas tank parcel grid to secure it. Using my 2 leads I attached the black cable to the diode spade and the positive cable to the diode ground stud.


Let's go for a spin...

Right away I noticed the voltage going to the diode was rather high.

In first gear riding @4000 RPM my voltage readings where off the chart - anywhere from 25-30 volts. At this RPM my motorcycle began to pop and backfire.


Did I find the problem? - Keep reading


The problem I'm now sure to you is very evident. The diode had failed and was no longer regulating voltage to the frame.

My motorcycle was punching upwards of 25-30 volts straight to the diode. No wonder I was burning bulbs and experiencing poor performance.


The fix

 Front Heat Sink


Since isolating the problem I decided to remove the zener diode heat sink and bracket. I checked the ground and noticed the ground "could have been better".

Perhaps a weak ground may have caused the diode to either burn out perhaps over load.

I used a wire brush to clean all steel surfaces to ensure the diode had a clean ground to shed voltage to the frame.

 Lucas Zener Diode In Heat Sink

 NOS zener diode installed - painted black?


I purchased a NOS Lucas zener diode from a local builder that he laying around. I installed the new diode applying dialectic grease on the mounting surface to allow better heat transfer. This installation method was shown in a Triumph service bulletin.


Time for another test


After installing the NOS zener diode I took my 1968 for another spin around the block. I again strapped my meter to the tank to monitor the voltage going to the diode. At this point I want to make sure my problem is fixed and perhaps something else.

While on the road I monitored the voltage. Ranging from 14V to 15.7 volts at different riding speeds and RPMs.

I opened the throttle just passing 4000 RPM and there was no popping or any type of hesitation. The old trusty TR6 was back on the road and running strong as before.



Here is a clip of me testing my Triumph TR6R the day I installed the zener diode back onto my frame. You can clearly see how the diode works. In my opinion, it's better than any bench testing method.


More info


Triumph Workshop Manual

 Triumph workshop manual indicating how to bench test a Lucas zener diode


In a Triumph workshop manual you will find what the voltage rating is for a Lucas zener diode. You will also find many ways to bench test a diode should yours have any troubles.


Here is a chart of what Triumph states the diode should shunt at


 Part Type Brand Voltage Max Ground
Zener Diode Lucas 12V 15V (approx) (+) Positive


The figures in the chart and photo I would say are close approximates. The closer the better.  I can't imagine a diodes rating at exactly what Triumph recommends. As long as the diode falls close you should be fine.


Thanks for reading


Owning a vintage British motorcycle can be channeling at times. The whole time the diode was at fault but it was nice learning how to test and find the problem during this journey.

The testing methods can be applied to both Norton and BSA motorcycles regardless of the model as long as a diode is fitted.

I hope you have found this blog informative as some of you may also be dealing with this problem.

Last thing! I didn't want to upgrade to a solid state or Podtronics regulator only because I want to keep this bike original using all the period correct hardware. Once set up properly it will last you years to come...

If you have any questions please let us know by commenting in the section below. 

Click here for other tech articles

Thanks for reading


  • Tony

    Zenor diode is charging at 16v lights off.
    13-14v lights on.
    The replacement does exactly the same thing.
    Already cooked one battery.

  • larry

    after sitting all winter I charged battery and installed in my 66 T120R ,started bike,it ran for 2-3 sec. Bike would not restart ,found fuse blown,removed zener ,turned on ignition again and short was gone. Does this sound like a failure of the Zener diode?

  • larry

    after sitting all winter I charged battery and installed in my 66 T120R ,started bike,it ran for 2-3 sec. Bike would not restart ,found fuse blown,removed zener ,turned on ignition again and short was gone. Does this sound like a failure

  • larry

    after sitting all winter I charged battery and installed in my 66 T120R ,started bike,it ran for 2-3 sec. Bike would restart ,found fuse blown,removed zener ,turned on ignition again and short was gone

  • larry

    after sitting all winter I charged battery and installed in my 66 T120R ,sta

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