The other day my fans failed to come on my v-8 converted 1973 914.
Evidently when I put in a new radiator over the winter I failed to suffiently bleed all the air out of the system. I didn't know this, but evidently the thermo sensor for the fans won't operate unless the system is well bled.
I put a bleeder valve on the radiator and spent about an hour thourougly bleeding the system and it solved the problem.
I hadn't really thought the system would need to be that well bled but...learn something new every day, right?
Just thought I would share in the event other converted teener owners may have had the problem and couldn't figure it out.
David

Is your sensor at a high point? Air is a great insulator, so if there's a bubble sitting around your sensor it'll stay nice and cool, even if the water is hot.

Is your sensor on the radiator or on the engine? Bubbles can also impede proper coolant flow. So if the sensor is on the radiator and there isn't much flow to the radiator, it'll never get hot and trip on.

Glad you were able to figure it out.

Indeed the sensor is at a high point on the radiator.
Also solved why my second fan was having an intermitent failure to come on previously (quite by accident). Bad ground that was hidden from view.
All good in teener land tonight

Air collects at any high point, it's good to have a bleeder at each one. I plumbed a heat exchanger at the underdash airbox, so I have 3.