Help FZR250 Temp gauge rises when lights are turned on?

He's done that, gauge correctly reads full deflection to hottest temperature

https://www.2fiftycc.com/index.php?...lights-are-turned-on.10840/page-6#post-132953

So there's some other resistance to ground somewhere which is giving higher than actual readings

 

And the question on everybody's lips, we're all quivering with suspense

Does it deflect further up the range when you turn on the lights?

 

Don't apologise, we've eliminated the ground wire being the problem, now the puzzle becomes how are the lights interfering with the gauge current flow increasing when they're switched on and it isn't the ground through the sensor.

So is there voltage at both the brown and green with a red stripe?

 

The confounding part of this is:
V=IR
Assume resistance to be a constant for calculation;
Voltage and current are proportional, so when the lights come on, the voltage drops at the gauge, BUT the needle goes higher - that is counterintuitive

Wondering if that power wire to the gauge is inappropriately going through a headlight circuit
Positive line direct direct from battery positive

I think the green with a red stripe plays a role as it goes to ground with the key off and is connected to the negative on the gauge, what does it go to or through

 

OK, so here's the error, not in the reasoning, but in the assumption that the black is the earth signal wire for the temperature sensor unit - it's NOT, it's the green with a red stripe

Check this schematic, it's for the FZR400, but I think you'll find exactly the same colour assignments on the FZR250 - the legend on the side for wire colours designate that same wire as green with a red stripe - hasn't your system already got a hard wired switch for the cooling fan installed

It's not the temperature sensor, it's wherever that green/red goes that's been tampered with, that then goes on to the temperature sensor

 

Go back one step, put the loom black wire back onto the gauge, then connect the single distinct earth/signal wire between the terminal currently occupied by green/red wire terminal on the gauge directly to the temp sensor

I don't think you will see gauge deflection any more when the light go on

 

Agreed, but for one thing, gauge rises with lowering resistance - that element doesn't make sense

Jut read this thread - the temp sensor if it's got two connections seems to be responsible for the current flow as well as the fan turn on, do they have two, differentiated/labelled connections on top?

 

Clearly some confusion regarding wire colours. I would not trust the FZR400 wiring diagram as it shows a 2 terminal temp sender. I recall Brandon indicated he only has one wire going to his temp sender. Please confirm that you are looking at the correct sender. There are often 2. One is for the gauge and the other is a thermo switch for the radiator fan.

Try disconnecting the temp sender wire. Turn the ignition on then measure the voltages at the 3 terminals on the rear of the gauge. Measure with the negative lead of you meter connected to the negative terminal of you battery.
I expect you will find that 2 terminals appear to have zero volts and one has battery voltage.

Now turn off the ignition. Put your meter on the resistance range (lowest ohm range) and measure between the wire that would normally connect to the sender (it should still be disconnected) and each of the terminals on the rear of the gauge in turn. The connection with the lowest resistance should be the direct connection from sender to gauge.

This should allow you to deduce all connections.
One will connect to battery voltage (since it does not look like there is an sort of separate voltage regulator for the instruments), one will connect to ground and the last one will connect to the temp sender.

The suspicion that everyone seems to share is that voltage at the ground connection to the gauge is rising above zero when the lights are turned one (bad earthing somewhere) or the battery voltage connection is dropping when the lights are on (bad supply connection somewhere). You should be able to check this with your meter.

Report back with all findings and measurements.

Peter.