K1200RS/GT Cooling Fans - BMW Luxury Touring Community
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post #1 of 6 Old Feb 5th, 2019, 10:04 am Thread Starter
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K1200RS/GT Cooling Fans

Is there any way to get the electrical cooling fans to kick in quicker (read lower temperature) on these bikes...Both mine are 2003's The RS's which I have had always creep up temp wise in stop and go traffic but the fans don't kick in until close to the red zone....

Thanks, Joel

Joel Talka
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2007 K1200LT - Blue
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post #2 of 6 Old Feb 5th, 2019, 2:21 pm
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Re: K1200RS/GT Cooling Fans

Quote:
Originally Posted by jtalka View Post
Is there any way to get the electrical cooling fans to kick in quicker (read lower temperature) on these bikes...Both mine are 2003's The RS's which I have had always creep up temp wise in stop and go traffic but the fans don't kick in until close to the red zone....

Thanks, Joel
On all these K1200 having "brick-engine" (K1200RS , K1200GT , K1200LT), the temp gauge on dash is driven by a separate coolant temp sensor (located in front of engine, below water pump).

ON THE OTHER HAND, the Motronic ECU will start both radiator fans based on the data from a 2nd coolant sensor (located in rear most cylinder head (number 4). You cannot change this sensor for a different type (having a different resistance curve) as this sensor is also use to adjust mixture (injectors time) based on pre-programmed data in ECU tables.

Assuming both sensors are correct (resistance curve), when the radiator fans starts, the Needle/Gauge on dash should be pointing about 3/4 of way between the Middle White line and the beginning of the red zone. At that point, the needle should start to creep down slowly toward the middle white line (with BOTH fans running).

It may take several minutes before it goes down (in hot stop-go traffic) AND it should NOT go into the red zone unless something is wrong.
Defective coolant sensors (either of these 2) is not very common and would often show up as other symptoms (fans start immediately after ignition ON ...and/or... difficult cold start...)

For these K1200, often what I see as the most common cause of a weak cooling system:
- the radiator fins have never been cleaned (a tedious job that needs removal to do a good job - soaking and water pressure). About 40% of cases.

- the thermostat is defective (jammed partially closed). About 15% of cases.

- the coolant is old, or not full (need to check expansion-tank AND also under radiator cap when cold). About 25% of cases.

- the radiator cap is defective with age (it will let go pressure too early). About 20% of cases
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K1200RS (2002 IceBlue/Red - 95,000 miles)
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Last edited by sailor; Feb 5th, 2019 at 6:09 pm.
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post #3 of 6 Old Feb 6th, 2019, 7:05 am
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Re: K1200RS/GT Cooling Fans

One more data point for your concern.

The thermostat opens at 85 C / 185 F

The fans kick in at 105 C / 221 F

The red light comes on at 115 C / 239 F

This will help put what you are seeing on the gauge into perspective.

John
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post #4 of 6 Old Feb 6th, 2019, 1:22 pm Thread Starter
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Re: K1200RS/GT Cooling Fans

Thanks guys for the information. I probably need to do some cooling system maintenance given the lack of documented maintenance on this bike. A system flush and new coolant at a minimum.

The temperatures are helpful.. If you look at them however, there is only an 18 degree (F) difference from when the fans come on until technical overheating....

Every K12RS I have ever had has always crept up while sitting in traffic, and it just seem like the point where the fans do come on is significantly higher (36 degrees F) than intended operating temperature...

From what I can tell, the K12GT I just picked up is working in line with these numbers, at least from what you guys have described. My concern is that I am now riding down in South Florida where is gets really warm out there...quickly...

I guess the real question is can I/should I do something with that sensor to get the fans to kick in at a lower temp?

Joel Talka
Glen Gardner, NJ

2007 K1200LT - Blue
1978 Corvette Pace Car Replica - ZZ383/M22
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post #5 of 6 Old Feb 6th, 2019, 1:49 pm
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Re: K1200RS/GT Cooling Fans

Quote:
Originally Posted by jtalka View Post
Thanks guys for the information. I probably need to do some cooling system maintenance given the lack of documented maintenance on this bike. A system flush and new coolant at a minimum.
..
....

I guess the real question is can I/should I do something with that sensor to get the fans to kick in at a lower temp?
NO, YOU CANNOT ;-)

In my previous post, I had "indirectly" answered your question.
From my earlier post above:
"... Motronic ECU will start both radiator fans based on the data from a 2nd coolant sensor (located in rear most cylinder head. You cannot change this sensor for a different type (having a different resistance curve) as this sensor is also use to adjust mixture (injectors time) based on pre-programmed data in ECU tables."

MORE DETAILS to better explain my "NO, YOU CANNOT":
1) the K1200RS/LT coolant sensor (in cylinder head) has an inverse resistance curve: as temp go higher, the sensor has less resistance. This is a common automotive design for such sensor.

2) When the resistance of this sensor reaches 178 Ohms (+/- 5 error), the ECU assumes the engine is at 105 celcius (221 F) and will start both radiator fans. It will wait until the temp has come down to a specific point (about 260 Ohms / 194 F) to shut the fans OFF.

3) The same coolant sensor curve (each resistance matches a specific temp) is also used by the ECU to adjust Fuel injection mixture (injector time).

4) If you try to fool the ECU by adding OR removing resistance in this circuit you might be able to trigger the fans earlier, but you will also completely mess-up the injector time (mixture control).

There is a way out of this, but it requires implementing a separate temp sensor that would trigger the radiator fans RELAY (instead of letting the ECU trigger the same RELAY). This modification is not something that I would recommend unless you are very handy with electronics AND can take the risk to overheat the engine (if your modified system fails).

YOUR BEST BET is always make sure the cooling system is maintained / stays in good shape (rad fins, thermostat, fluid level , Rad-CAP, sensors...). Regulary confirm If BOTH rad fans start and stop at position outline earlier on gauge, check fluid level is stable in expansion tank (when cold).
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K1200RS (2002 IceBlue/Red - 95,000 miles)
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Last edited by sailor; Feb 6th, 2019 at 5:44 pm.
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post #6 of 6 Old Feb 6th, 2019, 4:23 pm Thread Starter
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Re: K1200RS/GT Cooling Fans

Thanks John, Got it... Great explanation BTW, one of the benefits of this and other boards...you run into guys who really know the nitty gritty .... Anyway, It's not worth the risk to mess with it. Looks like I will be doing that cooling system maintenance sooner than later! :-)

Best, Joel

Joel Talka
Glen Gardner, NJ

2007 K1200LT - Blue
1978 Corvette Pace Car Replica - ZZ383/M22
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