Just got my newish-to-me '85 K100 running (had Hall sensor and other problems - purring like a kitten now woohoo!) 

However the fan wasn't coming on automatically. It has a manual fan switch, and I just dug into the electronics box to find the previous owner seems to have cut the fan relay wire and just connected that to the switch override. (The fan does turn on with the override).

Two questions:

1. For now, is it OK to ride it and just turn on the fan once it's warm and keep it on? 

2. How to go about fixing this?

- I know my temperature sensor works because when I ran it for 30 mins to see what's up, the fan didn't come on but the temp warning light did and then the bike cut out

- Should I just reconnect the fan wire from the override switch to the fan relay output (taped up in foto below) and see if it works? If not, I have a spare from another '85 - then I'd try that one and see. Or should I try to do the multimeter test of the relay as per http://www.ibmwr.org/ktech/fan-diagnosis.shtml ?

timz10000 wrote:
- I know my temperature sensor works because when I ran it for 30 mins to see what's up, the fan didn't come on but the temp warning light did and then the bike cut out

- Should I just reconnect the fan wire from the override switch to the fan relay output (taped up in foto below) and see if it works?

Don't see any problem with this...to test if the relay functions or not.
You can keep the override switch as is, just check that it's a fuse in the circuit.