Solved MC19 - temp gauge wire shorts

Not knowing which specific gauge you're looking at, I will assume the MC19, there's a coloured MC22 wiring diagram in the resources section.

In that schematic for the temp gauge, there is one actual direct to ground wire (for reasons not entirely clear to me) That's the third wire.

This is a little long as I'm working through making sense of it as I type this, bear with me...

It also shows green/blue going to the thermo unit
It does show green/black as being the ground

Also in that schematic there seems to be a 12 volt source, brown/blue which is shared with three globes in the dash. it originates with a fuse as the power supply (fuse C 10A - labelled as other in the schematic).

The bit with three ground wires reading on your multimeter is tricky, this part may help.

EDITED THIS FOR ERRORS
Those three globes which share the same power wire are:
For each of these globes when power is on, there will be 12 volts on the brown/blue wire

Neutral pilot lamp - when the power is off and the bike is in neutral, that globe provides a pathway to ground via the through the brown/blue power wire and then through the globe filament

Oil pressure lamp - same as above, except that when the engine is running and there is oil pressure, there will not be a pathway to ground through the filament of this globe, plus the brown/blue will have 12 volts.

Side stand pilot lamp, when the side stand is down, the globe filament will provide a pathway to ground through it's filament.

When there's any of those globes with their pathway to ground available and the power isn't switched on, you will get a reading to ground on the brown/blue wire.
I had to think that part through as I typed it.

So it might be worthwhile verifying that this power wire, brown/blue, has 12 volts when the key is switched on, if you can that's your 12 volts.

Then the above will make sense.

I'd say that if you're getting a reading of hot on the gauge with the reversed green/blue (thermo unit) and green/black (ground) wires the way they are now, the 12 volt supply is correct.

Also your fuse hasn't popped.

So with the bike switched off (no neutral globe power) it will show a short to ground through the globes from that shared power line, because when globes aren't illuminated they're zero resistance, plus there's no 12 volts on the brown/blue

Now for the not getting gauge movement currently...
From what I know, an NTC (negative temperature coefficient) temperature sensor, it's resistance lowers as the temperature rises, so it's wire connection isn't an actual signal from the sensor, it's a variable resistance to ground which allows more current to flow, therefore voltage rises, which then enables the gauge needle to move.

Once you figure that difference in direction and what's happening when the gauge moves, it starts to make more sense.
The gauge has 12 volt potential and the thermo unit changing resistance is what allows the gauge needle to move.

So, theory is that you've switched to the wire which goes straight to ground for the one which goes to the actual temperature sensor wire, which has a variable resistance path to ground.
Your gauge goes to a full hot reading.

Other part of the theory is that the problem wasn't with the gauge but the temperature sensor, I actually think you've proven that because you thought both gauges were dud, switching those wires shows the second gauge works, I expect that the original one will also shoot straight to hot if you wire it the way you have the replacement one wired now.

Upshot, I think you only need a new temperature sensor - you could probably test the theory, if you have another temp sensor of any type, wire to it from the gauge using the breen/blue wire, have a wire to ground going from it's 'body', put it into hot water and see if the gauge moves.

hope this helps

 

trace grn/blu wire and make sure it only goes to sensor.

test sensor and verify its resistance varies with temp properly

 

If you removed the connections to those three globes I suspect that you will no longer get pathway to ground via blue/brown, but testing for 12 volts with the key switched on is probably quicker and less fiddly.

It was the wiring schematic which was at first confusing to understand what you were getting readings wise.

I think it's fairly well proven with responses you're getting from the gauges, they both work (move) and @DannoXYZ test for the thermo sensor is an excellent one, quick an easy to do.