Weird Temp Gauge issue

[Dazguzzi]

Hi, recently I removed the temperature sender unit/drain plug in the water pump on my 94 K1100RS and went on to change the thermostat, temperature sensor ( in the water housing ) and refilled with correct ratio antifreeze once I had reinstalled the temp sensor/drain plug
I filled the system directly via the top radiator inlet where the housing with the cap goes on and slowly added it until coolant appeared before refitting this housing that incorporates the cap.

I ran the bike up to temperature with the cap off and kep adding coolant until the system seemed fully bled.

I went out on it and did 200 miles and everything was fine.

Today I went out on the bike and when it reached normal operating temperature I noticed that if I accelearted hard in a low gear the temp gauge needle fell way back towards cold and instantly righted itself again when I eased off the throttle. It will also do this whilst stationary if I just rev the bike quite hard.
Other than this issue, the gauge reads correctly and if left ticking over, the fan kicks in at the correct point, cools the bike and switches itself off


Thanking that I might have an air lock I rode the bike home and it did not overheat, smell hot or in any way misbehave, it was absolutely fine apart from this issue with the gauge.

Once home, I eased the tank back and removed the cap when the bike had cooled. The coolant level was perfect  :dunno2:

Any ideas or could the sender unit be playing up.

Thanks Darryl

[rbm]

The sender in the water pump drain hole drives the temp gauge.  It's not connected electrically in any way to the temp sensor in the stand pipe which drives the temp relay controlling the fan and the ECU.

You're insinuating that the gauge temp sensor is reading the current coolant temperature at any time and that the coolant suddenly drops a few tens of degrees and then recovers.  I doubt that the sensor has that instantaneous a response. I believe that thermal mass of the coolant will not allow it to change temperature that quickly. I tend to believe that there is an electrical problem.  The gauge is indicating a measured resistance by deflecting the needle of the gauge a proportional amount.  If the gauge suddenly drops, there is increased resistance somewhere in the circuit (the temperature sensor is Negative Temperature Coefficient (NTC)).  Maybe check the connections for security and cleanliness first.

If you suspect an airlock in the coolant and your electrical checks don't turn up anything, massage the return pipe at the lower right corner of the radiator while the bike is idling to expel trapped air and see if the problem disappears.

[Dazguzzi]

Thanks for that Robert.

I will clean the terminals on the sender in the waterpump as you suggest.

I have had the bike running again and it is definitely not overheating and I am now certain that I do not have an air lock so your theory sounds promising/

Cheers Darryl