Yes I've got a multimeter just not sure how to test the rectifier with it, help appreciated.

The bike doesn't have a battery and the head light currently doesn't have a bulb.

Test the power output from the wires coming out of the engine.  The alternator produces alternating current, set the meter on AC and the voltage to handle about 20 volts if it has a setting.  Touch your negative probe to the engine for a good ground and use the red probe to test output from a white or yellow wire or wires exiting the engine, need to do that when it's running.  Should be able to stick the probe into the connector plug if it still has one.

Can you take photos of what you are working with?  might help.

Thanks I'll give it a try.

The fastest way around this was to by pass the thermostat or just connect 12 volts to the fan. This time if year the thermostat does not matter.

16 hours ago, lineaway said:

The fastest way around this was to by pass the thermostat or just connect 12 volts to the fan. This time if year the thermostat does not matter.

They already tried that, now looking for 12 volts DC output for the fan and that ain't happening without a working alternator, regulator and rectifier circuit.

On 8/13/2023 at 6:39 AM, Ian589 said:

I've got a old 1999 Bultaco Sherco 2.5 not been used in a few years but the fan doesn't seem to cut in.

I've tested the fan separately and know that that works, I've also bypassed the temperature sensor but still the fan doesn't work. ...

 I knew there was a reason I left this thread alone that first day!

27 minutes ago, jackyma09 said:

Ian may want to consider checking the wiring connections and potentially replacing the fan relay if he suspects a possible issue with either. It's important to ensure proper functi­onality by invest­igating potential problems related to the wiring or the fan relay itself.

Fan doesn't have a 'relay' (magnetic activated switch for controlling high power to something like a starter motor) only a temperature controlled switch on the radiator.  He's still dead in the water if there is no regulated ~12 volts DC power to supply the fan.  

No AC power coming out of the engine is an alternator problem inside the engine.

Today cleaned all electrical connectors and earth points, replaced the rectifier and I now have a working fan thet now cuts in when the bike warms up. Even shortened the HT lead and fitted a new iridium plug so the bike starts easier, finally happy days.

Thanks everyone for your help.