Extractor fan won't switch off

[Whoever]

Any electricians out there please?
The extractor fan in our windowless downstairs toilet (which is activated by switching on the light via a pull chord )refuses to switch itself off, even when the light switch is pulled again to switch the light off.
It normally runs for a few minutes then switches itself off automatically but has now been running for several hours non-stop.
How can we get it to switch off?

[redman41]

Are you able to access the Timer Mechanism of the Fan or is it a sealed unit ? The Timer is sticking or has become Faulty. Isolate the Circuit by taking out Fuse in Mains Box or Switch off the Circuit Breaker and open up Fan Timer and have a look.

[Whoever]

The fan is a 30 year old Silovent make. It's built into the loo ceiling and there's no sign of any external timer, so presumably the time in somewhere inside?
So competent electricians in this house unfortunately. We've now stopped it by temporarily switching off the circuit breaker but this means we can't use the lavatory light any more as every time we switch it back on it restarts the fan again which still won't switch itself off.

[buildersmate]

Thes type of fans have 3 wires - 4 if it includes an earth.
There is a permanent live, a switched live, a neutral and the earth. The switched live is connected via the light switch and activates the timer mechanism. The timer and the fan need permanent electrical supply such that they can work 24/7.
Clearly the timer has gone kapput (technical term) and doesn't cut the supply to the fan after x minutes of overrun.
They aren't really serviceable and you've done pretty well if you have had 30 years of service.
To cut the fan off, you will have to find and disconnect this permanent live wire. Then you can work the light but the fan will do nothing.

[Whoever]

builders mate - Many thanks for your explanation. As neither of us feel sufficiently competent to go excavating into the depths of the fan, it looks like we'll be going to the loo in the dark or leaving the door open and using the light outside to illuminate our activities.