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Discussion Starter · #1 ·

I have 1964 6T which had an electrical fault that fried the wiring.
Replaced wiring bike ran fine for a couple of rides of about about 30 miles each.
However today went to start it and ran on one cylinder Battery measured 3 volts (thought I must have left the key on)
Replaced with another battery that only showed 11volts but I thought it better than 3.
Still running on one cylinder, found the white wire off one of the coils (the one that attatches to both coils) put it on and the bike was away tested battery with engine running at 2000rpm showed 13.5 volts.
Went to go for a ride, but as soon as I reved the bike the ammeter went off the scale on the + side and the engine cut out.
Started it again had a bit of trouble to get it to idle,revved it up and same thing ammeter off the scale and engine cuts out
I am a bit lost. Do you think it could be the Zener or the rectifier?

Any advice gratfully received



162 Posts

thats a new one on me.

i'm trying to remember something i heard about getting alternators to put out high voltages.

eh! can't remember what it was.

but whatever regulates your voltage is out of whack is my SWAG (scientific wild assed guess) crossed connection? wrong correction? broken connection? a shorted or blown gizmo?

get out the wiring diagram for that area you tweaked and trace a few circuits?

rectifiers and diodes basically have the same function to turn back and forth current (alternating current) into one way direct current (DC)

6,617 Posts
Hmmmm... 12v on a 64T. I believe this means the charging system has been converted from 6 to 12 v, yes? I'm a newb to Triumphs, but believe, at least for the Bonnie that 66 was the first 12 v year.

Looking at the wiring diagram for a 12 v system, the zener diode, which is your voltage regulator, is the white wire which goes to your coils. It actually comes from the alt and thru the ign switch to the coils. The wire is white only from the switch to the coils and on to the zener. If it was disconnected, you had no voltage regulation and the battery got full power from the alt. The zener should be connected to the neg side of both coils, and the zener body grounded to the frame thru it's heat sink. The zener bleeds off excess charging current by turning on and conducting when voltage gets too high. The 6V system had no auto regulation like this. It used the ign switch to switch in the alt coils to provide low, med and high current output. My point here is if your bike is a 6V to 12 conversion, then you have to figure out if it was done to duplicate a factory 12v system. Then you can begin to sort it out.

The 6V system has a white wire going from the ign switch to the coils also, so that is not a clue you can use. Also, if your bike is a 12v conversion, the zener could have been connected anywhere starting at the center terminal on the alt an anywhere along that center terminal circuit. So find that zener and make sure it is connected to your alt center tap.

My guess is there is another white wire that came off the coils or wherever it was connected to the center tap of the alt and if you reattach it, your ammeter will stop pegging and your voltage regulation will be back in play. If you find that is not the case and you can trace the white wire from the coils to the zener and it is connected properly, I would guess that your zener is kaput.

There is a test procedure for the zener in the shop manual. I've heard this is a somewhat fragile component. If it is loose and cannot reject heat to it's heatsink, it will fry. Make sure it is tight to the heat sink. But don't break it by overtightening.
Please report back what you find.
Good luck,
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