T250 Overcharging

[ConnerVT]

When you are seeing that +20V, check the DC voltage from the negative terminal and frame ground. Also, actually mount that Tympanum unit to something metal (with a solid ground to the frame).

Voltage regulators typically work by shunting (bypassing) the excessive input power to ground. I don't know if the Tympanum sends it to the black wire or the unit's case. In this situation, I suspect a bad ground as the culprit.

[jabcb]

When you are seeing that +20V, check the DC voltage from the negative terminal and frame ground. Also, actually mount that Tympanum unit to something metal (with a solid ground to the frame).

Voltage regulators typically work by shunting (bypassing) the excessive input power to ground. I don't know if the Tympanum sends it to the black wire or the unit's case. In this situation, I suspect a bad ground as the culprit.

+1 on mounting the Tympanium on the frame. It doesn't have cooling fins & benefits from using the frame as a heat sink.

But its popular on vintage British bikes that are positive ground. So it can't rely on its case being negative grounded.

[ConnerVT]

But its popular on vintage British bikes that are positive ground. So it can't rely on its case being negative grounded.

You are correct. I had forgotten that. So it would make sense that it shunts through the black (negative) wire.

Guess now is that the negative terminal of the battery is not solidly grounded (as it should be).

[oldandslow]

Ok, this is odd...Looking at the wiring from the generator you will find a black wire and a white wire that run from the points and go to ground when the points are closed and allow the ignition coils to fire when the points open. Also a blue wire that comes from the neutral switch. A black and white wire is also present and it runs to ground.
That leaves a yellow and green wire, a red and green wire, a green and white wire and a solid green wire which all seem to originate from the stator coils and therefore are responsible for the charging current.
Now the weird part, when I disconnect every wire mentioned above except for the two wire wires from the points and the blue wire to the neutral light ground and then start the engine the charging voltage will still run up over 20vdc at 4000rpm.
During the restoration I removed the flywheel and stator and reinstalled them as they were. The connectors, wiring and sheathing from the stator/timing case were not tampered with as they were in quite good shape and only cleaned and reinstalled, they remain as they came from Hamamatsu.
How can the generator be supplying charging voltage with the wires from the stator disconnected?
Once again, any help will be greatly appreciated.

[jabcb]

With the battery disconnected & the all of the wires going to the stator/points/neutral switch disconnected:
1) measure resistance between stator Y/G & R/G wires
2) measure resistance between stator Y/G & G wires
3) measure resistance between stator Y/G & G/W wires
4) measure resistance between each of these 4 stator wires & ground (some place on motor that isn't coated like the stator housing)

If your multimeter is not auto ranging, then use the lowest reading scale that it has.

What brand/model multimeter do you have?

[oldandslow]

UEi DM 383 Digital Multimeter with new battery set at 200

Y/G & R/G - 1.5
Y/G & GREEN - 2.6
Y/G & G/W - 1.1

Y/G & Grd - No display
R/G & Grd - No display
G/W & Grd - No display
Green & Grd - No display

[jabcb]

Your stator readings look ok.

With the 4 stator wires (Y/G, R/G, G & G/W) disconnected, you got 20 VDC.
What do you get at the same location with the bike switched on but not running?

What voltage do you get across the battery + & - terminals
with the bike on but not running & @ 4k rpm? (take these readings at the battery)

[oldandslow]

Ok, I feel a bit of the fool. I finally realized that what I think I have been seeing can't be happening so I borrowed my son-in-law's Fluke auto ranging multimeter.
With the Tympanium in place and wired as the "recommended" diagram that jabcb provided these are the results.
Key off - 12.3 vdc
Idle - 13.1 vdc (no headlight)
4500 rpm - 14.2 vdc (no headlight)
Idle - 12.3 (headlight on)
4500rpm - 13.2 (headlight on)

What do you think?

I had tunnel vision, I believed in the accuracy of my 20 year old meter because I knew it was a high quality instrument that we provided to our field technicians back in the day.
I guess you are never too old to learn.
Anyway, thank you jabcb and to all for the help.

[jabcb]

Those numbers look good.

12.3V is a bit low for an unloaded battery. Would be good to charge it up.

Your flat-bottom headlight has been NLA for quite some time & can be pricey.
From time to time its available on eBay for a reasonable price -- its worth keeping an eye out & getting a spare when it happens.