Nervous coolant temp needle

[molasses]

The other day I removed some of the wiring that goes to the headlight area (front brake switch etc) because I never use it (track only bike).

After that my coolant temp gauge went jittery. When the bike idles all is fine but when the tach reaches approx. 5K the coolant needle jumps about 10 degrees up and then settles to the right temp after that.

I thought it might be a loose connection to the coolant temp sensor on the engine but it wasn't

The battery is fully charged so it's not for lack of spark juice...

Do any of you know what causes this?

Thanks!

[Fariborz]

Sounds like you have a floating ground connection to the sensor. There are two wires on the sensor. One goes to the gauge and one should be connected to the Battery Ground through the harness. I do not have a 998 wiring diagram but on most bikes the black wire from the sensor is the ground.

You can temporarily T off a new wire from the ground wire and directly connect it to the battery ground. If that solves your problem then you need to trace that wire and see where you have a bad ground connection.

-Fariborz

[molasses]

Fariborz, you continue to amaze me. I am VERY impressed by your comprehensive insight into technical matters related to Ducatis.

I will follow your advice and let you know how it went.

Thank you so much, Mate!

[molasses]

Fariborz,

I've done the following:
1) Took the coolant temp sensor wire off the sensor while the ignition was turned on. The needle dropped to the bottom of the scale.

2) I held the sensor wire against a grounded portion of the engine and the needle instantly went to the top of the scale.

3) I reconnected the sensor wire to the coolant temp sensor and pulled the ground wire from the underside of the coolant gauge. Next, I ran a separate wire from the battery ground to the ground connector on the gauge.

I still get that nervous, jittery behaviour when I rev the engine

Could it be a faulty coolant temp sensor?

Or could it be that I've overtightened the nut on the underside of the gauge? Would that result in such strange behaviour?

Thanks!

[molasses]

I've investigated further and I'm not so sure it's the coolant temp sensor after all.

When I measure the voltage across the sensor as the coolant temp rises it all seems nice and steady. There's no sudden voltage drop across the sensor while the needle flickers.

So I guess it's safe to say it's not a faulty sensor.

Next, I'll measure the positive lead going into the back of the coolant temp gauge against a ground wire going straight to the battery to see if that's the cause of the nervous needle.

Annoying stuff!