Overcooling/ defective temp gauge?

[J900090]

Recently my 2004 ninja 250R temp gauge has been showing that it is cold even though I know it is fully warmed up. Usually drops instantly to cold at Higher RPM's (above 3-4k) and when I come to a stop/ idle it will jump back up showing it is at normal temp. The gauge doesn't always show cold at higher RPM's, sometimes it will work normally for a while before dropping back to the position that you would see if the bike was off. I have searched for days on the internet with no answers and was hoping someone might know what is causing this.

[Motofool]

Welcome, Justin !!!

Your problem is that there is some faulty electrical contact between your gauge and the temperature sensor.

[jkv45]

As Motofool noted, most likely there is a connection issue. If that checks out, it's possible the temperature sending unit itself is faulty.

[greg737]

Your EX250's temperature sensor, the little item that is screwed into the thermostat housing, is just a "thermistor" which is an electrical resistor whose resistance is greatly reduced by heating. The hotter the thermistor gets the more amperage it will allow to flow to ground (the EX250 temperature sensor is an "old school" type of sensor. It is a 1-wire sensor which means it grounds directly through the metal body of the thermostat housing into the bike's frame).

Your EX250's temperature gauge is a "Galvanometer" (which is a type of ammeter). Movement of the Temperature gauge needle is caused by amperage flowing through the instrument to the variable ground provided by the Temperature sensor thermistor in the thermostat housing. A factory-calibrated amount of electricity (DC current) flows through a small set of copper windings that when energized act as little electromagnets to exert a pull-force against the return (to zero) spring-loaded Temperature gauge needle.

So you want to make sure of the following:

1. that the Temperature gauge in the bike's instrument binnacle has good/correct voltage input to it (it's 12volts like everything else on the Pre-Gen EX250). You can test the wire with a multimeter.

2. that the Temperature sensor is correctly installed in the thermostat housing and that the threading it is screwed into isn't corroded (which would inhibit proper grounding).

3. that the wire running from the Temperature gauge in the instrument binnacle to the Temperature sensor in the thermostat housing is firmly attached at both ends. Also make sure that the attach-point on the Temperature sensor hasn't been bent so much that the metal is breaking (it's made of soft metal and is the most delicate part in the circuit).

4. that the thermostat housing is firmly bolted to the bike's frame. If it isn't, if the thermostat housing is just hanging loose on its coolant hoses, there won't be a proper pathway for the grounded power from the Temperature sensor.

[Ghostt]

is there a thermostat?

[greg737]

Originally Posted by Ghostt

is there a thermostat?

Can't tell if joking...

if not joking, yes the EX250 has a thermostat. it's located in an aluminum housing (silver in color) that sits dead-center under the fuel tank, bolted to the bike's frame.

Google "EX250 thermostat" for pictures.

the actual "thermostat", the part that uses a reactive wax core to open and close based on coolant temperature, is contained within the thermostat housing.

[Ghostt]

Originally Posted by greg737

Can't tell if joking...

if not joking, yes the EX250 has a thermostat. it's located in an aluminum housing (silver in color) that sits dead-center under the fuel tank, bolted to the bike's frame.

Google "EX250 thermostat" for pictures.

the actual "thermostat", the part that uses a reactive wax core to open and close based on coolant temperature, is contained within the thermostat housing.

Not joking, I should have said is the thermostat installed in the housing? If so test it to make sure it's operating properly.

[greg737]

Originally Posted by Ghostt

Not joking, I should have said is the thermostat installed in the housing? If so test it to make sure it's operating properly.

That thermostat is hell to remove, I'd make an effort to confirm all of the wiring before going that route.

The odd, intermittent nature of the issue, as described by the OP, suggests a wiring/electrical problem.

[Ghostt]

Very true, and agree, if all else fails, then the OP needs to look at the thermostat.

That's assuming that a previous owner hasn't removed the thermostat.

[dcj13]

Or put a piece of black electrical tape over the gauge.

[alex.s]

there must be some kind of intermittent sensor connectivity issue, captain. try to polarize the eps conduit and adjust for the phase divergence. it might be necessary to run a bypass line to get around the eps rupture.