I bought what I thought were all the bits and bobs needed to fit the temp. gauge found on later models to my 1985 K100RS. The parts fiche does not make clear how one wires the instrument. Was a different wiring loom used on the models that were delivered with the temp. gauges?
I expected to find a connector behind the inner panel to match the wires on the gauge, but on my machine there is nothing there.

Since this model never included a gauge, the wiring for one is not present.
I'm sure others will chime in on this with more accurate info. Possibly someone will direct you to a previously posted topic that describes the procedure exactly.
It's generally a fairly simple circuit..... One side of the meter is probably wired to 12V + through an (Ignition) switched circuit, so it turns on when the engine starts. The opposite side of the meter obtains it's (variable) ground through the temperature sensor which for this application is mounted in the water pump drain hole in place of the Allan head drain plug that is original equipment.
As the coolant temperature rises the variable (Thermistor) in the sensor lessens (or gains??? not sure which way) the resistance in the wiring's path to ground, hence moving the dial on the meter.
I think a high resistance to ground is Hot and a non resistive path to ground is Cold, but it could be the other way around.....lol.
I think the most difficult part might be doing a clean install of the grounding wire, in parallel to the Hall effect sensor wiring, down through the front timing cover area.
IMO it's more trouble than it's worth to install a temp gauge. I think it's likely to set you to worrying about all kinds of minor variations in the temperature which are completely normal, where presently without the instrument, you can continue along in relatively untroubled mental bliss, not having a fictitious issue to fret over.
You will find others here and around..... who jump through all kinds of Hoops trying to install colder thermostats & bypass fan control switches to try to force the engine to run cooler than it's supposed to. That....is a very slippery slope indeed.
Install the gauge if you wish, but don't let it's readouts worry you to death until you have 10's of 1000's of test miles so you establish a feeling for what's normal and what might actually be a defect.
These engines appear to be running very HOT....... when they are, in actuality, running as per design.
Happy Motoring.
ibj......Phoenix (spelled HOT), Arizona

below is list you need all bought from Motorworks
Coolant temperature gauge sensor wiring loom ......connects from gauge to sensor
Coolant temperature gauge sensor ...... mounted to drain plug in water pump
Fuel level and coolant temperature gauge wiring loom ....this connects to loom under fuel tank to gauge

hope this helps cheers Tony

It should be there but it will be under the tank and taped to a part of the loom (probably to the offside of the fame backbone tube).

The wiring diagram for the early models shows the connection for "additional instruments". The wires are Green/Black, Brown & Grey/Green.

Funny mine is a 84 model K100RS and the temp gauge and bits appear OEM.....certainly the temp sensor on the water pump is not a bolted on after thought.....there are times though when I wish the gauge were not there...mid summer in the city for example.

all verry true ibj

put simply the guage needs 4 wires to it

1> b+ from after the ignition switch ...hopefully fused

2>a good earth wire

3>a light illumiation + wire (pick up from the parking light for this one )

4> the sensor feed ..as described above

there are kits for the additional wiring as klompy describes so if you look around you might get lucky and pick up a accesories wiring kit available on ebay ?

i have noticed that the bmw type guage (original ) is an expanded scale type guage with the centre at around 85 -90 deg c ...thats my best guess, so many folks just see the range from about 60-70 degrees c to a little over 100 c for the red indicator region (smart guys these bmw blokes )so probably tops out at about 110 deg c ( the thermostat is rated for opening at 82 c -180 f)

so beware on after market type guages, many are just a more linear type scale that go from about 40 deg c to about 120 c so not a lot of information around the normal operating temperature area of the bikes cooling system

i have chosen to install a bypass switch on the temp relay input .......to excersize the fan

many of our cooler climate members hardly ever see the fan start up untill they get into stopped traffic or ..on occasion a really warm day .......there are a few horror stories of overheat events on here .....many found to be a siezed fan from ...no excersize at all for many years .

be sure you get the right sensor for the water pump region to match the guage ...they are not all the same but usually the sensor for the brand of guage matches

i recon its nice to have some feedback on whats happening down there

hope that helps .

Thank you for all the helpful responses. I've since discovered I'd bought an incorrect sensor unit, which partially explains my confusion. I've decided to put this project on the back burner for now. As was noted, the temp gauge is not an essential instrument, so I've decided to sort out some other issues in the near term.