Found it!

Have a look here...page 14, post 671. This is the mod as mentioned above.

Michael,  

Looks like you have a thermostat on offer.  I can arrange to collect it from Charlie.  

I can probably give you a hand Thursday or Friday this week.  Just need to know which day, so I can plan stuff for the other.  Weekend is clear so far.  

So, I'll throw in a Temp relay and I have new Temp sensor aluminium crush washers as well.  Anything else you may need?  I don't have a sensor readily available.  

If I remember I can collect a thermometer so that we can check the temperature at which the thermostat opens in a pot of water.  (Charlie it came from that school and no I did not borrow it.  My best mate did  ) 

Cheers, Dave.

Hi Dave, that would be great, very kind of you.  Yes would like to try Charlie's thermostat please.  
Friday or the weekend is okay, I will be home so whatever suits you.
I have a new temp sensor on the way in the mail, hopefully it should arrive sometime this week.  Got it very cheap  from China  
Only other thing I can think of is a gasket for the thermostat - Charlie said he didn't have one.  I have read other forums where nobody can find a suitable replacement gasket for BMW K100 thermostats, seems they never sold them as a separate part, only in conjunction with the thermostat.  Anybody know if the Tridon TTG26 gasket will fit the BMW thermostat?
Plan is to test the thermostat first, go for a bit of a ride together, maybe down to Canungra or further if need be.  We can have lunch there.  If still no good, then go back home and on to testing the relay &/or sensor.  Test one thing at a time.
Just wondering, if it turns out to be the relay, is it for sale or do you want to keep it for a spare yourself?
I might ask you to help me taking the mirrors off as well, so get some practice in.  I have ordered some replacement mirror gaskets as mine are worn out and causing some vibration, and also want to paint them while they are off. Stupid me cracked the RH mirror glass trying to take it off, but I have spent the $100 and have a new one coming in the mail.
Where do you live, is it far to travel?  I will send you my address by PM.

charlie99 wrote:I have a spare thermostat here (original type ) but you will still need a new oring around it

I have seen a few radiators with issues , like silicon sealant remains , all in the input side of the radiator , from past works I guess ...

could this be a contributing factor ?

but you need to get the thermostat working correctly in the first place

I wonder if an oil  pump seal job has been done and the impeller is not fitted correctly ?

or the fluid filler cap seal is incorrectly fitted - missing the second seal (quite common issue resulting in fluid escaping as it heats )

give me a message if you want to borrow - whatever a thermostat

Thanks Charlie, I believe Dave will collect that thermostat from you.  Will the Tridon TTG26 gasket fit your OEM 'stat, or if not, any info on the brand/model of gasket that will work?
Will need to check condition of the impeller, but the radiator itself looks very clean inside and out.
I already purchased and installed a brand new BMW radiator cap a month or so ago, so the seals are all good.

Taken from Charlie's post...

"I could see that the new device threw further into the housing and closing off the bypass quicker (maybe not so good) and that the spring-tensioned bypass restrictor could offer a little too much back pressure before opening fully (as I suspect, the BMW one was failing to do on a regular basis and letting uncooled fluid circulate through the system).

I noticed how much change there was between closed and open distances on both the BMW one and the new device, hmm...flared copper..I wonder...yep cut it off at the flare to allow me to a modify it a little. (A Dremel type tool is what I used: Blakey)

So I cut off about flush with the flare of the rear retainer, I then cut a few slots about 5 mm into the length of the flange. The aim was to peen these over and retain the blocking plate again. Also I cut about 5mm out of the spring to match the relative tensions that I thought would work out right without presenting too much pressure against the opening mechanism.

The rear plate although smaller would still do the job admirably after measuring the bypass port."

yeah the tridon seal will fit the bmw thermostat

okay thanks Charlie.

Just in case anyone is interested in comparing the Tridon prices, they are: 

• TT214-180    $25.19 
  
• TT2014-180  $40.99
  
• TTG26          $13.29 
  

That's from my local SCA.  All in stock except for the gasket.  

The high flow TT2014 is advertised with 30% extra cooling flow.

Yes, I just bought the TTG26 online from Super Expensive today.  They agreed to price match it to a price of $8.60 I saw online 

So normally the TT2014-180 + TTG26 is $55, versus $94 for an OEM from Munich Motorcycles.  And I'm not sure how well it's going to work without doing Charlie's mod, coz nobody has commented about that yet. Plus I only bought the regular version as I wasn't aware of the hi-flow at the time.

In hindsight I probably would have bought the OEM from the start, saving myself a lot of time researching this online and having piece of mind it's going to work out of the box.

Last night I fitted the new Tridon TTG26 gasket to my TT214-180 'stat.  Took it for a 30 km ride and appeared problem solved - no more overheat light and the fan only came on momentarily a couple of times when idling.  So I am thinking having no gasket caused the overheating by allowing coolant to bypass the radiator...HOWEVER...

...Just took it for another 30 km test ride in the middle of the day. Before leaving I let it warm up but after about 10 minutes the light+fan came on again at exactly the same time...so back to the same old problem.  With light+fan still on I rode down the mountain I live on and cruised about 20 km on flat ground down the bottom - the light+fan stayed on about 95% of the time.

Now get this...as soon I began climbing the mountain on the way back home, light+fan went out.  Even while I sat for 5 minutes at traffic lights half way up the mountain, with hot air around my legs, still no light or fan.  I got to the top of the mountain and decided to ride a bit more to see what it does.  A few minutes after easy riding on flat land the light+fan came on again and stayed on all the way home.  With the bike idling and the light+fan still going, it did not feel hot.  I sprayed the radiator & engine with water, but absolutely no steam.

So in summary, today the overheating light + fan remained on most of the time during easy riding,  but went away during hard work going up a mountain.

...just tested my temp sensor the EFI Plug.  It was 2470 ohms cold, 280 ohms hot.  Exactly as it should be.  Bike took 10 minutes to warm up, before warning light and fan came on at exactly the same time.
...will now test at the relay...

It appears that somewhere along the fan wiring path, gravity or motion might be acting on a defect in the wiring causing the connection to intermittently short to ground. That might be occurring within the temperature sensing relay or elsewhere along the line. You might be able to reproduce the failure by tapping on the relay.

I'm still not convinced that the cooling system has been filled to the correct level and purged adequately but that's my problem to resolve.

I agree laitch

discussed this with tackler this afternoon , with the same thoughts in mind

he will be attending tomorrow some time , with a spare temperature relay unit I believe.

but a few measurements need to happen to,  "prove" the fault ....


got my fingers crossed

good luck guys

...just done all the tests in the "Fan System Diagnostics for Two-Valve K Bikes" document, everything is normal.

Just thought I would update this thread.  Dave came over and let me test two of his spare Temp Relays, but this did not make any difference on the test rides.

I also attempted to test with a new Chinese sourced Temp Sensor, but this seems to be faulty as the bike would not even start with it fitted.  A resistance test between the pin and screw threads showed an open circuit, compared to 2550 ohms in my original Sensor.  Also the test at the control box Temp Relay socket between E & 9 also gave an open circuit, compared to 3800 ohms (cold) with my original sensor.  Lesson learnt not to waste time buying the cheap Chinese version if you need one in the future.

However, a day or so after doing the tests with Dave, with my original Temp Relay & Sensor refitted, I took the bike for a couple of long rides and the bike suddenly began behaving normally.  The overheat light no longer comes on, and the fan only comes on briefly during prolonged idling.  

I have no idea why the problem just dissapeared.

michaelvass wrote: 
I have no idea why the problem just dissapeared.

Where is the level of the coolant reservoir tank's gauge now when the engine is cold and the bike unridden?

G'day Laitch, I have emptied and refilled the coolant about half a dozen times in the past few months of troubleshooting this problem, what with the radiator flushing, thermostat & gasket changes and temp sensor changes.   I have always kept the cold coolant reservoir level about half full after each refill, and it stays at that level.

michaelvass wrote:G'day Laitch, I have emptied and refilled the coolant about half a dozen times in the past few months of troubleshooting this problem, what with the radiator flushing, thermostat & gasket changes and temp sensor changes.

If this celebrated overheating condition has finally been eliminated, my guess is the last of the half-dozen or so flushings and refills finally might have eliminated a troublesome air pocket. That would be nice.

Enjoy it while it lasts anyway.