The temperature gauge on my dash has suddenly started staying near to zero even when the engine is warm. It might occasionally creep off the bottom to about 5%. The engine seems to be running no differently, and I plugged in a OBDII tool to check what temperature that reads, and that says around 70-80 degrees after a bit of a drive, so that seems OK. So is the temperature gauge feed off a different sensor from the one the ECU uses? I'm guessing it's on the pipe that runs across the top of the engine, but where in the pipe is it? I want to eliminate whether its the sensor, gauge, or wiring that's at fault.

The car is a 2005 WRX with a EJ205 standard engine.

 

V5 onwards the temp sensor was combined to just the one sensor before that there was two.

 

Item 12 - sits behind the alternator.
Always use the copper washer - Never use ptfe tape on the threads of the sensor as the gauge circuit grounds via the sensor body to earth

 

Thanks for those digrams, very useful.
I just measured the resistance between each of the 3 pins without knowing how they are connected. Didn't think to measure the resistance between pin 3 and the body (ground).
Do you know if the ECU takes the coolant temperature from the same pin 3, or if pins 1 and 2 are used? There's wires going to all 3 wires, so I assume they are all used.

 

ECU uses pins 1(common or shield) pin2 (signal)
Relevant Engine Wiring pages





ECU connections

 

Nice one. I think I've got enough info to get probing away now. Just need a dry day to have a go at it.

 

The temperature suddenly started working again today! I haven't touched a thing, so maybe a loose connection has sorted itself out? Far too wet out to poke around anyway.