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Allan G, BSA_WM20, Bustednukel, gavin eisler, NickL, quinten
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Original Post (Thread Starter)
#883940 06/26/2022 9:58 PM
by Stasch
Fresh off the "I left the petcocks open over night" caper, now I think I have a bad rectifier on my 67 Lightning. This is what happened. In the middle of a short ride bike seemed to be running fine. Stop sign ahead. Decelerated. But before I came to a full stop, the bike shut down. No motor, no lights or horn. Couldn't restart. Got her home. Battery several months old. Charged it up. No problem. Did some on-line research on rectifier testing. Found one that made sense and this is what I did. Started the bike. Took out my trusty Klein Multi-meter. Checked the battery. At idle it measured a constant 12.75 volts DC. Revved it up to 5000 rpm, it never got above 12.75 volts DC. From what I understand, the high rpm measure should have been around 13.50 to 14.50 volts DC to show there is enough excess current to power all the essentials as well as charge the battery. Since the rectifier regulates current and charges the battery, and since it appears that my battery is not being recharged, and according to the results of my test, it appears I need a new one. BTW, the rectifier (Lucas) on there now is original to the bike.

Several issues. First, did I do a proper test, and if the results are correct, do I have a bad rectifier, or is there something more sinister going on? Stator?
Second, if I do need a new rectifier, what kind should I get? Stay with Lucas at around $125-150.00 a pop or go with a Japanese repo for around $60-75.00? Or other? Third, should I ditch the rectifier route altogether and install a solid state device instead, and if so, what model works best for my bike? And where do you put such a thing?

I am a local rider, no long trips. I am no motorcycle mechanic but I do know some things. I can probably swap out a bad rectifier for a new one no problem. But the installation of a solid state regulator might be above my skill level. If I have to take it to the experts will it hurt the bike to drive it 5-6 miles to the shop, or should I have it towed? Think I can make it on a full battery charge, during the day, no lights. Just don't want to make things worse like frying my entire electrical system. Thanks folks.
Liked Replies
#883956 Jun 27th a 12:24 AM
by quinten
Your bike is not charging
1.the rectifier may be bad
3.The stator may be bad
2. The wiring between the two ... may be bad
( find the problem before spending money .)

the original lucas finned thing is a rectifier ... not a regulator .
the original Lucas zener does all the over voltage regulation
The rectifier Works in concert with the zener .

You can check the output of the stator using the multimeter on AC volts
pull both stator wires ... run the bike off battery only ... test stator output for ac voltage .
This isn't a definitive load test , but it will tell you if the stator is bad ...
You can also hook the stator directly to a spare 12 volt headlight ( resistance loads like headlights work AC and DC )
and do a more " shade-tree load-test ."
They light will grow brighter as rpm increase and depending on headlight wattage ....blow if you push the RPMs too high .

If you have a bad rectifier
You can replace it with an original looking finned rectifier , for more money , but keep the original look .
Upgrade to a more modern "cubed" rectifier ... for as little as five bucks .
These Cubed rectifiers are used on millions of things so you don't need
To pay extra for a motorcycle specific part .
3 members like this
#883973 Jun 27th a 06:50 AM
by sammysnail
Stasch, I've used one of the $5 square full-wave rectifiers for years on my Bonneville with no problems. The original ones seem to be prone to failure due to vibration. The square ones are a direct replacement. I'm looking at one now. They are encapsulated in resin and vibration-proof. I get mine at any electronics store - 35amp full wave (or bridge) rectifier. These are what Quinten refers to above as a "cubed rectifier".

The base is metal and 1" square, with a 5mm mounting hole in the middle. There are four male spade connectors sticking up out of the resin that fills the square. One connector at each corner. There is a printed marking on one side of the square base. Adjacent to the left hand connector is the mark "+", and adjacent to the right hand connector is "AC".
"+" is obviously the positive connection (earth connection to the frame or the battery on your '67 BSA). Assuming that your '67 BSA is positive earth like my '69 Triumph.

Unmarked, and diagonally opposite on the rectifier, is the "-" or negative terminal, which goes to the battery.The other two diagonally opposed terminals, one of which is already marked "AC", are connected to the alternator. It doesn't matter which way around they go.

The original rectifiers are finned for cooling, but I have never found overheating to be a problem with the square one. If it bothers you, you could put a big aluminium washer of any shape behind the rectifier as a heatsink when you mount it to the frame using a bolt through the central 5mm hole.

I hope the above is of use. Replacing the rectifier is a quick, easy, and reversible way to approach your problem. If it doesn't provide a solution you may have to consider looking at your Zener Diode. This is the device that regulates the generated voltage by shorting to earth when the voltage is over a given value (13.7 from memory).
Harder to test and not cheap to replace, the Zener is the electronic device in the middle of the deeply finned cast aluminium plate mounted at the top of your front frame tubes. In the airflow to help dissipate the heat generated when it is subjected to excess voltage. You can never tell, but I would not expect a Zener to fail in the way that you have described. I think it would be more likely to open-circuit and cause excessive voltage in the system.



This is what's inside it:

The triangle thing with a line across the point is a diode, which allows electrons (i.e. current) to travel through it in one direction only.

2 members like this
#884455 Jul 1st a 02:03 PM
by DMadigan
The "cube" with the hole in the middle needs a heat sink. Mount it on a piece of aluminum so the whole back sits flat on it. Thermal compound is a good idea also.
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