Hello , 

My regulator gives out approx 14.8 v dc for the fan circuit. 

When the fan runs the voltage drops to about 9.5 v.

The engine sometimes cuts out when idling and the fan comes on,  it is also very very difficult to start if the fan is on.

I have tested the resistance of the stator yellow to ground and I have 8 ohms.

I think I'm right saying the stator should not have any connection to ground,  ?? 

Can someone tell me what resistance I should have and on which leads, I think it's yellow and green approx 16 ohms.. does that sound right. 

Thanks in advance Gary

The problem with these "simplified" wiring diagrams is that they don't really show the electronics involved.

From my experience, the heavy-gauge yellow wire is for a stator coil that powers the fan.  One end of that stator coil is connected to the bike's chassis ground.  The coil's resistance will be quite low (maybe 1 ohm) and the wiring itself will contribute to the reading.  Your symptoms / measurements suggest a poor connection to ground at the stator end. 

Edited February 26, 20251 yr by konrad

Stator should have low resistance as you are seeing, the service manual should indicate the correct resistance.  The stator is part of your alternator and should output well in excess of 12 volts AC which is best observed using an oscilloscope.  The fan requires the DC 12 volts and imposes the greatest load on your electric system, but for your voltage to drop so significantly with only the fan running I would be inspecting and testing everything from alternator windings, ground connections, rectifier and the fan motor itself.  

This is why you should;d always buy trials bikes in pairs, it makes it so easy to trouble-shoot components when you have one that works and one that fails.

Thank you for the advice... the fan pulls dc 1.8 amps when running. 

I would have expected a higher current....... 

Would anyone know what the resistance of the fan winding should be,  it's a comex fan...

Thank you

Measuring the resistance of a brushed DC motor is not very informative. The resistance will vary based on rotor position.

But I have a new Comex fan in my  spares.  Powered from exactly 12.0 VDC, it draws 1.8A.  At 13.8V it is more like 2.2A.  This is with the fan sitting on a bench.  Impeding the airflow with something (like a radiator) will change the current draw somewhat.

Edited February 26, 20251 yr by konrad

Thank you for all the reply, that has been great help

I had to replace my regulator/rectifier several times after the fan cut out, spark quit and both. I replaced the whole system with a used Kokusan but now you can buy a new replacement ignition and keep your stock flywheel and stator.

https://www.thehellteam.com/cdi-regulator-rectifier-external-ignition-coil-gas~182025

Yes thank you I have been running this cd16204, for the last 2 years. 

I think I may have a stator issue....