Hi guys,
I have installed a TrailTech onto my 1985 K100RT. It seems that I do not have a proper charge from the alternator when the bike is running now. I'm getting 12.7V with ignition off and 12.3V with the engine at a high idle (around 2000rpm).

I have only had the bike for a few months so I'm not 100% that it wasn't like this before I swapped the instruments over.

I have swapped the voltage regulator and brushes with a different unit with no improvement and I have put a different battery in with no improvement.

So, do I need the charge light connected for the alternator to work properly?

I have the alternator out and apart at the moment and have cleaned the slip rings, measured the winding resistances which all seem pretty close to what my Haynes manual recommends. Does anyone know how to check the diodes?

Thanks,
delToro

Simple answer is yes, you need a charge light connected. The alternator will not start to charge without it.

That's why people let their bike idle thinking they're charging the battery and they don't notice the charge light is still lit. Give it a rev so it goes out, then it'll start charging.

As Hover said, light on? It's not charging.

I wonder how may more times could I've said "charging"?

The charge light provides a path for current to "excite the field coil" before the alternator can provide current on it's own.
Ignore the dull glow at night and when loads increase

Thanks so much everyone. It's working now! A good alternator clean out probably wasn't a bad thing anyway

And you must also use a bulb with the correct wattage = 3W.

Ok, thanks Inge.

Is it possible to use a correctly sized resistor in place of the bulb? Given the 3W bulb I would pick something close to 48 ohms if my calculations are correct. (P=V^2/R)

A resistor would probably also function, I don't see any reason why it shouldn't.

But I would prefeer to keep the bulb....to monitor the charging system.....
an alternative----, in the middle of nowhere with a empty battery.