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Discussion Starter #1
I am hoping someone here can straighten me out on a few questions I have regarding the LT's cooling fans. I thought I had an issue that the cooling fans were running constantly. That's only part true.
This morning I started the bike and just let it idle. The fans kicked in at the midway point of the temp gauge instead of nearly into the red which is normal. Also they shut off when the gauge dropped about a 16th of an inch below the middle mark. A very small on-off range. This cycle only goes on until the engine gets warmer, then they never shut off because it can not get that cool anymore. So basically when I am riding and the engine is hot they are on constantly. I know we have two sensors, one at the pump and another on the rear engine. Which ones turn the fans on and off? Or does the front one turn them on and the rear off? Could just my front sensor be bad? I hate changing things for the heck of it and hopefully one of you can steer me in the right direction. I would think the relay is fine since they do go on and off.
 

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1999 LTC
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Not being a qualified fan wrench, first suggestion I'll make is to check your coolant level. Second is to clean out the radiator fins as good as possible. And third, think about verifying the thermostats operation - (nope, don't know how, butt it's a thought!!). Good luck on the solution, Wally.
 

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Wrencher Extraordinaire
2005 K1200LT
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The sender at the pump only feeds the gage. The one at the rear of the cylinder head feeds the Motronic brain and tells it the temp of the engine for fuel management and in turn drives the fan relay at the appropriate time.
 

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What about resetting the motronic by disconnecting the battery for a few minutes and reconnecting? Just a WAG for something easy to try...
 

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Discussion Starter #5
jzeiler said:
The sender at the pump only feeds the gage. The one at the rear of the cylinder head feeds the Motronic brain and tells it the temp of the engine for fuel management and in turn drives the fan relay at the appropriate time.
Thanks for the input John. Would you happen to know if the rear sensor in a coolant path? What I am getting at is if I change it will I be losing coolant? If so I'll drain it first.
 

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Wrencher Extraordinaire
2005 K1200LT
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Yes it is but if you are quick you won't loose much. More of a pain to drain unless you are due (every four years).
 

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Discussion Starter #7
Just thought I would post an update in case anyone else has the same issue I am. Further troubleshooting is definately pointing to the rear engine temp sensor. I brought an infra red temp sensor from work and when pointed at the sensor base it shows the fans coming on at around 83 degrees Centigrade. 22C less than it should be when the fans come on and 2C short of the temp the thermostat should be opening which explains why the radiator above the thermostat barely gets warm now. It never really gets a chance to open because the fans kick in so early. I ordered one from the new BMW of Countryside dealership and am hoping to have it Tuesday. After I change it I'll let you know the final results. That should take care of it but I suppose the Motronic could be having an issue if the temp sensor does not.
On another note BMW of Countryside is looking to be a great dealership in the coming weeks. Right now they are still settling in but it's shaping up nicely. June 25th is their demo truck weekend and they will also have a BMW stunt rider there. Their first 1600 is suppossed to show up next week.
 

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Discussion Starter #9
Here are my final results. Installed the new engine temp sensor yesterday. Started it up and when it got to the point that the fans would start, the temp gauge actually dropped a bit as the thermostat opened on the second radiator. Very good sign. Then it continued to rise and the fans came on near the top as they should be. Everything is back to normal.
Some have said the sensor is a glorified thermocouple...it will either work or not. WRONG. Apparently this proves it can somehow lose a set range.
I did discover an easy way to eliminate the relay as the problem. With the engine running disconnect the plug from the temp sensor and your fans will come on. Much easier than stripping the whole bike and changing the sensor needlessly.
Anyway I am back to normal now. Hopefully this info will help others with fan issues.
 

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2005 K1200LT
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Thanks for posting your success Wally. I am sure it will help others. Forgot about the "unplug" the sensor trick. That is a good one to know.
 
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