The fans themselves, or the relays specifically, are triggered by the
ECU. There isn't any other external fan switch or fan controller module.
The temp sensor in the back of the head is the only temp sensor. It provdes the temp to the
ECU and the
ECU shares the information with the cluster via a data link and the
ECU uses that data to control the fans directly.
If the temp gauge is working fine then the temp sensor and any data it provides is working properly. So it would be something related to the whether the
ECU is triggering the relays or a problem with the relays or their power itself.
As AMB said, the fans don't come on unless the aircon is on. So if the aircon is switch off or not working then they wont come on until the engine gets hot. In a BA/BF they don't come on until the engine gets to about 95 deg. Can't remember what it is for previous models.
It could be a problem with the
ECU, the internal drivers are faulty. It could be a problem with the wiring between the
ECU and the relays, which would be strange because there are 2 wires, would be strange both of them are faulty. Unless you have an active hungry rodent. Or it could be a problem with the +ve side of the relay, that there isn't any power going to it. Check all the fuses for a start.