Alternator testing

Hi all, i’ve got a charge fault on my Lemans2 which i think is the stator. The three phase connecters are measuring 1.5ohm when they should be 3.8ohm + - 10% and the output at 4000rpm is 2.2 volts so i’m convinced it’s the stator, or have i missed something?. Any bright sparks out there who can confirm my suspicions before i buy a new stator would be very welcome.

Hi there,
Here’s the go to guide. There’s a great section on fault finding in Guzziology too. Bit more expensive than ‘free to access’ 'though :open_mouth:
Good luck

There is a trick of putting 12v across the stator which solves a lot of probs with charging, others will know more of this.

Sturdy wire from the positive terminal of the battery touched on the slip ring(s) f the rotor.
Usually when the bike has been standing or with a replacement alternator.
CAUTION: There is potential for sparks
All the best

My workshop manual says:

Rotor field winding, 3.4 Ohms. (The spinning bit with the slip rings.)

3-phase stator winding resistance, 0.36 Ohms. Yes 0.36. As measured across any pair of the yellow wires.

However, and I’m sure I must have mentioned it before, it is notoriously difficult tp measure very small resistances with a multimeter, just the oxides or ‘dirt’ on the probe tips and whatever they’re touching could easily add 1 or 2 Ohms. Also the meter has to be calibrated properly at true zero Ohms.

I did have my suspicians about very small resistances and my ability to measure them with the cheap multimeter. I did try to measure the output from the heavy weight three phase cables and achieved just over 1.5 volts at 2000 rpm rising to over 2.5 volts at 4000rpm. I’m fairly certain that the voltage output is very low, unless i’ve completely messed up.
The rotor measures 3.8 Ohms across the slip rings, the brushes look good with good springs, continuity checks out everywhere and i’ve replaced the rectifier and tested the regulator all checks out ok.

Problem solved, i removed the stator and discovered the rear brush had intermittent resistance which turned out to be a break in the wire to the brush.Ten minutes with the soldering iron and good as new.
Thanks to all the replies.

There ya go it’s often something dead simple :smiley:

I had something similar happen on my R80, brush stuck in its holder so not touching fully.

Spot on.
Glad it’s sorted