Engine Fans Not Running Whilst Engine Running?

Hi guys, and girls:smooch:

New here but I have a problem;

My engine cooling fans are not running whilst the engine is running but start to run when the ignition has been switched off:frown:

I noticed this when the trip computer advised me that the engine was overheating. Got out and noticed that they were not running, switched off ignition and, hey presto, they ran. Got home with the needle creeping up again and let the engine idle for it to overheat again, I left the engine running for a few more minutes and the fans still didn't kick in yet as soon as the ignition was switched of the fans started to run.

Anyone any idea what would be causing this.

Any help would be greatly appreciated as I've got a 200 mile round trip to do on Monday:eek:

Thanks