Coolant temp gauge stuck in red for no apparent reason... - - Excellence in Motion
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post #1 of 6 (permalink) Old Mar 29th, 2008, 3:51 pm Thread Starter
Join Date: Apr 2007
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Exclamation Coolant temp gauge stuck in red for no apparent reason...

Within 2 minutes of leaving my home today (65 degrees) my coolant temperature gauge on my K1200GT jumped to the highest postion in the red and stayed there. Occassionally the needle would rapidly fluxuate up and down over 2-3 seconds but always return to the maximum position.The coolant warning light did not come on and the fans did not come on. The engine ran fine and did not feel excessively hot. Coolant levels were slightly below the low level in the reservoir so I topped it off but it made no difference. My gut feeling is the gauge is wrong but I don't know.

Any thoughts on this?
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post #2 of 6 (permalink) Old Mar 29th, 2008, 4:48 pm
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Not too much.

The sensor for the gauge is independant. There's a separate sensor that lights the lamp on the gauge panel. The Motoronic brain runs the fans, but I'm not sure what it uses as an input. All that to say it might be the sensor as much as it might be the gauge.

Gauges usually have some sort of shunt or protection, such that if the shunt fails, the gauge will generally go full-scale in almost no time. Had a truck where the fuel gauge lost its shunt, and when the needle started coming off of "Full", you had about 1 gallons of gas left. Which in your case would mean that the water isn't cold anymore...

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post #3 of 6 (permalink) Old Mar 29th, 2008, 9:56 pm Thread Starter
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Thanks for the input thus far. An update-- after sitting for several hrs with the engine at room temp, the gauge still shows the needle in the red. So presumably this tells me that either the coolant temperature sensor has failed or there is a short in the wiring somewhere, is my logic correct here? Any suggestions on troubleshooting that?

Assuming then that the error is in the sensor or gauge, I am hoping I can continue to ride until I need new tires in a month or so and rely on the coolant temp warning light to indicate the unlikely event of a second unrelated failure causing actual overheating. This would save me an extra trip to the dealer. Is this safe?
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post #4 of 6 (permalink) Old Mar 30th, 2008, 1:38 pm
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Coolant sensor or sensor wire short most likely, There are two similar sensors around the water pump/oil pump and a single wire from each goes into a junction terminal in the right side electrical connection box. I seem to remember it takes a tortuous route through the front housings, so if there is bad insulation, pull a new wire through using the old one.

The water temp sensor is the one marked with a BLUE sleeve (The other is the oil pressure switch). A small terminal like a spade 'faston' is fitted on the end of the wire and the sensor has a brass 'button' on the end. It's exposed to road crap so could make bad connection. Of it's loose, carefully squeeze both sides of the faston with fine pliers to it pushes back tight.

This experimental result is for my KRS so your sensor may be different. At 20 deg C I get 620 ohm and at 90 deg C I get 50 ohm. Your gage is hard in the red so either the gage is faulty or you have a short on this wire or it dropped off the sensor terminal and is shorting to ground.

The fans and overheat are controlled by a separate second sensor in the rear of the cyl. head which connects to the Motronic. It's also used for engine temperature ECU mixture/cold start as well.

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post #5 of 6 (permalink) Old Mar 30th, 2008, 7:22 pm Thread Starter
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That's exactly what I think it is after some testing-- the water temp gauge has failed as when I removed the wire the needle dropped.
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post #6 of 6 (permalink) Old Apr 6th, 2008, 10:58 am Thread Starter
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To follow up, I replaced the coolant temp sensor, only lost a few ounces of coolant in the process, and the problem is fixed .
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