Monitoring the final drive oil temperature

As all of us BMW K 1200 LT riders know the final drive is something that can create headache. The construction was used already many years before the LT was created and the added weight of this bike added stress to the main bearing of the unit. 

I suppose all of us have heard horror stories of the units failing, typically somewhere on the road and not so often in the garage. 

Even though my bike is of the newer generation when the factory had already paid better attention to the preload setting of the unit during assembly I decided to be safe with my final drive(s). 

At quite early stage, maybe in 2007 or so I was able to purchase two almost new final drives for 400 usd that were taken off from triked LT:s. The other drive went to colleague of mine and I kept the other one (9 miles on it) in my garage as a spare one. 

I took the habit of changing the whole final drive at maximum 50.000 km without any further checking or analysis. I just throw the new one in and change the main bearing to the spare one in the winter. 

In addition to this (no no , I'm not paranoid!) I decided to install an oil temperature gauge as I had some space in  my dash for another gauge.

VDO has a gauge that has a range of 20 - 100 deg C. A gauge with max 80 deg C would be even better since the max normal operating temperature what I have ever experienced seems to be at 65-70  deg C. 

There are two options for installing the sensor. The main thing is to make sure the sensor is not too long so that it would touch anything inside the drive unit.
My first sensor was too long to be installed in the fill plug so I installed it in place of the drain plug. This of course ensures that the sensor is in the oil but my problem was that I never got this sensor to be 100% oil tight. Even if I had a copper washer the sensor / plug was slightly seeping oil which was not very nice.

Later I found a VDO sensor which was short enough to be installed in place of the fill plug. The oil temperature readings are well comparable to the bottom sensor readings so I put the original drain plug in place and changed my fill plug to this.

The sensor needs two wires. One goes to the sensor and one is for grounding. The grounding must make sure it has proper contact with the (in this case) the final drive itself.

At first I used the breathing plug as grounding point but I really did not find a proper way to fix the wire in the plug so I changed the grounding point to the disc brake assembly. At this time unfortunately I don't have a picture of that but if I recall correctly there is the ABS sensor grounding point available and I simply put another ring connector under the same screw.

This final drive oil temperature gauge installation was made in 2008 and so far I have learned that the highest temperatures so far (about 68 deg C) have been when riding on German Autobahns at 130 km/h average speed two up and pulling Uni-Go trailer at + 28 deg C weather. Actually the final drive oil temperature follows very well the engine oil temperatures only at slower pace. This meaning that that engine oil temperature goes up faster than the final drive oil but the in longer ride they both end up to very close readings. The final drive  gauge also shows how efficient cooling water provides. In rainy weather the oil temperature gauge barely moves up regardless of the speed.