I've had an issue with my K since I bought it 2 years ago, fan wasn't working as it was melted so I replaced it and everything worked as it should. 

After a long ride turning into my driveway shortly after replacing the fan, the fan became intermittent, turning on, off, on etc. I switched the bike off and then no matter what temp the bike got up to afterwards, the fan wouldn't turn back on. Started troubleshooting, put power to the fan and it was working fine, cleaned the grounds under the tank, removed temp sensor cleaned grounds around the stub pipe, noticed a little bit of corrosion around one prong, cleaned it off and applied dielectric grease but none of this got the fan to turn on by itself. I wired in an over-ride switch and now just use that. 

I've since replaced the temp relay switch under the tank, no change and then started swapping temp sensors but this is where things get weird -the bike will only idle with its original temp sensor. Any new replacement (I've tried 3 different ones despite all measuring correct ohms) causes the bike to flood the engine. It's like my ecu will only listen to this old temp sensor which is likely feeding an incorrect signal to the fan? I'm also seeing the overtemp light come on after 20mins of warm weather riding while the fan is running so there's that too... replaced thermostat but no change. 

Any ideas or something I'm missing? Appreciate any help

Did you use ptfe tape on the temperature sensor?

If the bike only idles on the old sensor and not on the replacement that's a clue either the replacement sensor is incorrect or is possibly not making a good electrical connection. It earths through the body so don't use ptfe tape.

If its not earthing it can think engine is cold = overfuelling or can think its over heated = no start.

The temperature sensor has 2 internal variable resistors - one is the sensor for the fuel injection unit and the other is for the fan switch/relay. As you have both giving unexpected results, it does point to an earthing issue as Olaf indicates.

I have an '86 K100RS and had an issue with fan and overtemp light coming on too soon - replaced thermostat and fan relay and then the sensor before it was resolved. Bike had been running fine and the sensor had correct resistance between each terminal to ground when cold.

Haven't used ptfe tape on the temp sensor and played with a number of different crush washers as the schematics call for one to make sure it was properly grounded (I stupidly used an aluminium washer in one attempt...).  I'm beginning to wonder if perhaps the whole stub isn't properly grounded to the block because the sensor itself is now definitely well grounded to the stub? 

So far I've replaced the relay, thermostat and 3x sensors - I just find it weird that the original sensors second variable resistor to the relay is suspect but when I replace it and ensure a good ground, the resistor to the ecu appears to fail...

My thought is do a test after each individual step one at a time to see if the issue remains or is cured. As far as I can guess the only way the temperature sensor can effect idle is by affecting the fuelling.

Have you aftermarket sensors? Are they identical?

If you are getting cheap sensors off Aliexpress they are incorrect for use on the 8v Bricks. The sensor is also situated  in a bad position and is subject to a lot of road crap. As well as cleaning the male connectors on the actual sensor the female connectors on the wiring need cleaning and can loosen off.
Regards Martin.

I started with 2 'substitute' alixepress quality sensors which haven't worked but then I opted for what should be a proper one from Euro Motorelectrics  - made by Endura last. All are experiencing the same problem of flooding the engine on start up except for the original one which doesn't appear to want to operate the fan.

There is two things that the sensor controls one is the fan the other is the fuel injection. I haven't seen one half fail but it's possible. I think I have a spare one and maybe one of the Kweensland K Kluster has one, so we might be able to arrange another workshop day. I'll call around. 
Regards Martin.

Did you buy part number 61 31 1 459 197 or part number 13 62 2 242 184 from Euro Motoelectrics?  Both sensors look the same and mechanically fit the same but electrically they are completely different.  The former is for your bike and the latter is for 16V bikes like K100-4V and K1100.

You said you "measured the correct Ohms".  What exactly did you measure and what was the value(s)?

At room temperature (20°C), you need to measure 2.5Kohhms between any one of the two contacts and the body of the sensor.  And you need to measure 5Kohms between the two contacts.  

If you measure 5Kohms between the contacts and infinite Ohms between any contact and the body, you have the wrong sensor.  This is meant for the 16V bikes.

robmack wrote:Did you buy part number 61 31 1 459 197 or part number 13 62 2 242 184 from Euro Motoelectrics?  Both sensors look the same and mechanically fit the same but electrically they are completely different.  The former is for your bike and the latter is for 16V bikes like K100-4V and K1100.

Turns out I'm an idiot and did indeed order the 13 62 2 242 184 part ... Completely overlooked this and just saw suitable for a k100 so pushed the button thinking it'd be good. 

Righto! I'll replace with the correct part and report back. Thanks for setting me straight!