Sorry if this topic has been covered before.
 I have an '85 K100 RT and last summer I took the fairing lowers off to aid leg coooling while riding in France during the summer. I've left them off as I like the look but also to ease cleaning and servicing.
 The bike was left standing for a couple of months over winter and since then the engine's temperature gauge hardly gets above the two blue lines and when I stop at traffic lights or whatever the fan cuts in. The gauge used to sit steadily at near half way and only occaisonally have the fan cut-in in hot weather, whilst riding in central London for instance. The engine shows no sign of overheating or being in distress.
 Has anybody else had this phenomena and is there a cure?

yes indeed ...change the thermostat in the first instance

next is to clean up the air paths through the core of the radiator ...that can accumulate a lot of spray and dirt between the fins of the core of the radiator cooling path ...go steady you don't need to use a high pressure water sprayer,,,just something that degreases and lifts the gunk  out .

hope that helps ..!

Hi there, if the fan is cutting in, under the right conditions, and the needle is still on the cool zone, then I would think the problem is the thermometer. Check the temp gauge system, this is a separate circuit from the temp control and has nothing to do with the engine management.
If the fan is cutting in and out and there are no sign of overheating, then I would think that your cooling system is ok and would not work with it until clearing the temp gauge circuit.
Hope this helps
Best regards
JAP

Thank you both for your replies. I've just ordered a new temp.sensor from Motorworks.
 My thinking is that it's not the thermostat as it's default position is open so that if that was faulty and closed permanently,then the bike would over heat rather than the reverse. Similarly the blocked with dirt radiator idea.
 This leaves the temp. switch relay or the sensor in the water system. As the switch is still working, i.e. making the fan come on, all be it at a lower temp. then I think it must be the sensor, which is most likely to be corroded after 97k miles and probably 30 years of service in hot,cold,hot water conditions. Is it a bi-metallic strip system?
 Anyway a new one is only £35, so I'll let you know how it goes on when I put it on.
Cheers, Chris.

Along with Charlie's thinking, looking at the age and mileage, I would be cleaning the external surface of the radiator and also giving it a good flush out. You would be amazed at the difference a new/clean radiator makes. 

I have not heard much here about sensors failing, so perhaps these are one of those rare failures?

japuentes wrote:Hi there, if the fan is cutting in, under the right conditions, and the needle is still on the cool zone, then I would think the problem is the thermometer. Check the temp gauge system, this is a separate circuit from the temp control and has nothing to do with the engine management.
If the fan is cutting in and out and there are no sign of overheating, then I would think that your cooling system is ok and would not work with it until clearing the temp gauge circuit.
Hope this helps
Best regards
JAP

To second this. I recently cleaned up the ground area under the tank on my bike. Afterwards I noticed that the temp gauge was going higher. Initially, I thought that something else was causing the bike to overheat. Now I realize that the gauge is probably working more in spec. Luckily for the health of the bike, the temp gauge does not control when the fan comes on or off. I would check the gauge.

If you have access to an infrared thermometer check the actual engine temperatures and the coolant temps going into and out of the radiator. Let it stand idling till the fan cuts in and do the checks again.  I think you will find the temp gauge system is at fault.
As for cleaning the radiator, If you have a leaking fork seal the oil will often spray back into the radiator fins and then collect all sorts of road grime and block the air flow.
I spent almost a full day getting rid of oil and dirt and a heap of grass seeds that had collected over the years and it made an amazing difference (it worked like new or maybe even better).

klamityboy wrote:

japuentes wrote:Hi there, if the fan is cutting in, under the right conditions, and the needle is still on the cool zone, then I would think the problem is the thermometer. Check the temp gauge system, this is a separate circuit from the temp control and has nothing to do with the engine management.
If the fan is cutting in and out and there are no sign of overheating, then I would think that your cooling system is ok and would not work with it until clearing the temp gauge circuit.
Hope this helps
Best regards
JAP

To second this. I recently cleaned up the ground area under the tank on my bike. Afterwards I noticed that the temp gauge was going higher. Initially, I thought that something else was causing the bike to overheat. Now I realize that the gauge is probably working more in spec. Luckily for the health of the bike, the temp gauge does not control when the fan comes on or off. I would check the gauge.

to add to this ...it is always a good policy to clean up the main battery earth connection to that gearbox stud .
there are cable upgrades as well for this and the main battery power to the starter relay ...amongst other things
if perhaps you do find corrosion amongst any of the connectors ...might be a good idea to attend to some of the multi pin connections that feed the instruments ...likewise the spade connections that connect to the temperature sensor for the gauge and the oil pressure sensor ...right next door ...on the water-oil pump assembly ..
we have found a product called "deoxit " works very well (lots of posts throughout the forum about this amazing product ....just use the search feature )

good luck !

The sensor arrived this morning, so having nothing better to do I decided to buckle down and do the replacement.
 After doing all the removals I came to the coolant drain plug and found that that was a sensor too, and it was corroded; intact, but still needed a good scraping on both the plug and connector.
 Anyway, the old sensor came out easily and the new one went in. All the pipe work and connectors refitted, the worst bit being the rubber bar on top of the radiator. Coolant replaced, tank back on, the engine was started.
 One pipe had a slight leak, luckily it was one on the right hand side so easily accessible and only needed the clip rearranging.
 I left it to tick over and the needle went passed the first white line ( where the problem fan starting was) and it carried on all the way to the third white line and past for a few millimetres and then the fan cut in, took the temp. back down past the white line and then stopped. Hurrah!!
 Right, on with the riding gear and go and have a bacon sarnie at Cranfield airfield cafe. On arrival the gauge was as it should be, just under the second white line and no sign of dripping water.
 Now back home and all seems well....fingers crossed; but the problem could still have been the sensor OR the corroded connector.  
 Either way, I'm happy and many thanks for the input from all of you.

The sensor in the drain plug is the one for the temp gauge.

Ok. 
 Then what does the sensor above the engine sense? Does that one turn the fan on and the one as a drain plug drive the temp.gauge?
 Why does my Haynes manual only show the drain plug sensor as a drain plug undone by an allen key?
 I'm going to disconnect the drain sensor lead and see what happens...  Don't wait up, I might be sometime.

jayseabea wrote:Ok. 
 Then what does the sensor above the engine sense? Does that one turn the fan on and the one as a drain plug drive the temp.gauge?
 Why does my Haynes manual only show the drain plug sensor as a drain plug undone by an allen key?
 I'm going to disconnect the drain sensor lead and see what happens...  Don't wait up, I might be sometime.

The upper sensor tells the computer the temperature of the coolant and turns the fan on and off. I think it also might also have something to do with feeding info to the computer for the fuel injection. 

On k-bikes without temp gauges (which are apparently most of them according to the manual publishers) there is just a drain plug that uses an allen key to remove. Thus since such a small percentage of bikes have the temp sensor at the drain location there is absolutely no reason to mention it at all.  

Yes, I've just come to that conclusion about the different functions. Good insight into the drain plug conundrum though.
 So the conclusion, to my mind, is that the corrosion on the lower sensor hindered the function of the temp gauge and the fan coming on early might/might not have been an upper sensor fault or was only responding to the poor connection on the lower.... 
 Isn't life wonderful. 

The sensor on the water riser behind the radiator has 2 sensors in one housing.
One connects to the ECU which controls mixture and the temp light the other the fan on and off the sensor on the pump only works the temp gauge and has nothing to do with the ECU at all.

Dear All, 

My bike does a strange thing (woman speaking about mechanics, this is embarassing)  However, I decided to be able to fix my own bike with time and I have to start at some point. I have one Kexpert at home, but I would like to have my own opinion on the subject.

When I get it started and go for a ride, after riding for some time on a motorway and going into regular traffic it often goes off. 

I wonder what can be the reason for that. From what I have already read it can have something to do with the thermostat (please pardon my 'ignorant woman talking about serious matters'syntax). I will ride again tomorrow, so if you have any hints to what I should pay special attention - please write.

Hi Magda,

How do you mean goes off? Do you mean bike stops or fan comes on?

Was good to meet up last week too!

hi magda

if the fluid levels were low this could well happen
poor circulation could mean that you get  a overheat situation as the cooling function would seriously be hindered,
once air gets in there the pressure can be exteeme as air expands far quicker than fluid  

but it may be something else ..just offering some where to start looking

by the way having fuel hoses about and inch and a half extra length (main one being the verticle hose to the injector rail ) will allow you to swivel the tank and get to the fluid filler cap without disconnecting the fuel hoses .

just a tip

good luck

Thanks for the hints! 

Olaf, it was good to meet you indeed. By goes off I meant engine stops working'. 

The indicator of the temperature never crosses the half of the range.

Hi all,

Has anyone found a substitute new thermostat or are we locked into the expensive BMW job.?

Cheers Jeremy

I think Charlie have used a thermostat from a Corolla......search the forum.

your right inge ...yet again ...!!


try here for the modified thermostat

  https://www.k100-forum.com/t1833p650-85-rt-resto-my-attempt-hey-im-not-into-making-a-show-bike#88242