Hi, I am having similar problems with my '89 tenere. I think it is the rectifier / regulator (doesn't produce the correct voltage under load) and maybe also the battery (doesn't hold much charge). I would appreciate if someone could confirm my diagnosis though or suggest more tests I could do.

Bike electrics: standard low beam, rear, and brake lights. Dual hi beam for 120w of forward lighting. Heated handgrips at around 25w. I know this puts me at around 170w at night, which is the rated capacity (12Ax14V=168W) of the charging system.

Symptoms: bike will usually not start. While riding, forward lights visibly dim when I apply the brake.

Ruled out: starter motor - the bike will always jump start.

Suspect: charging system - lights dimming; if I ride for a while without headlights, the bike will usually start next time.

So, I did some standard tests:
- checked continuity between regulator and battery, low (<1ohm) resistance) on both positive and negative.

- Checked resistance of stator coils. Low (~1ohm resistance) on all three, no connection to ground.

- Checked for voltage drops between rr and battery with bike idling and no load. Nothing significant.

- measured voltage across the battery terminals for various load conditions.
Idle, no load: 14.5V
Idle, lo beam: 14.0V
Idle, hi beam + heat + brakes: 10.7V (and dropping as the battery discharged)
3000rpm, hi beam + heat + brakes: 9.8V (dropping)
3000rpm, hi beam + heat: 11.0V

- measured open circuit voltage of stator windings. All measure around 22v at idle, 55v at 3000rpm.

Then I decided to see under what conditions my battery is charging and under which it is discharging. Unfortunately my multimeter is unable to measure current (came with a 10A fuse and a 5A solder track... brilliant). But I can figure out whether the battery is charging or discharging by measuring the voltage across my fuse (in millivolts). I found:
- idling or revving with the lights off, there is a small current charging the battery. (3 - 5mV across fuse)
- with just the low beams, the battery charges slowly. (0 - 3mV across fuse)
- with low beams and heated handgrips (daytime riding) the battery neither charges nor discharges.
- with hi beams, heated handgrips, and brake light on, the battery is discharging quite rapidly - I estimate around 4 - 8A if the bike is idling, or 2A if revving.

I know these tests could be better - use a multimeter that can accurately measure current; use a good, charged battery, but I am currently travelling (in turkey) and everything is more difficult and expensive/wasteful than it would be at home.

So, in the next few days I plan to get the regulator / rectifier replaced or repaired then see if I need a new battery. I guess one other thing that interests me is if I need to be concerned about my current draw and whether it is possible to increase it.

Cheers,

Barry