water temperature gauge gone wacky

[93Leg-c]

I need help troubleshooting the water temperature gauge in my '98 Legacy wagon. Here's what's happening:

The needle does a few different things, such as going past "H" and staying there, or going to "H" and bouncing around, bouncing around the 3/4 hot area, or sometimes reading normally. I replaced the coolant temperature sensor (the temp gauge is connected to this sensor) with one that worked from my ss (parts catalog shows they are interchangeable) and there was no change to the needle's readings.

I searched and searched everywhere I could think of and finally found a thread with a post that suggested disconnecting the sensor's wire and if the needle stayed at "C" it would mean one thing and if it went and stayed past "H" it would mean another thing (although I forgot which reading would indicate a faulty sensor and which would indicate wiring troubles; and I'm not yet able to find that thread again). So, I disconnected the wire this morning and drove a few miles. The needle never moved past "C". "Great!" I thought. I went into the store, got back in the car, started it, and the needle immediately swings past "H" and bounces around the "H" and stayed there while I did other errands and on the drive back home (about 10 miles of driving).

If the sensor is disconnected, how is it that the gauge is apparently getting varying voltage to it? Is it faulty wiring or a faulty gauge? Or something else or a combination? How do I troubleshoot these errant needle readings?

Any help is much appreciated! Thank you!

[Legacy777]

The newer cars get their signal for the temp gauge from the ECU. So I would suspect either the output from the ECU is not right, or the control in the gauge cluster is messed up.

I would suggest getting an OBD2 scanner and reading what the ECU thinks the engine temp is. If it is reading properly then you can eliminate the sensor and the wiring to the ECU.

If I had to guess, I'd think the problem is in the gauge cluster.

[93Leg-c]

Josh, I got to thinking about your post above--about the gauge getting its signal from the ecu. I'm a bit puzzled but if that is the case, then I'm wondering why there is a sending unit in the cross over pipe specifically for the temp gauge (and another sending unit next to it that goes to the ecu)? Wouldn't that indicate that the gauge gets its signal from the sending unit?

[Legacy777]

So even on the 98 Legacy there are two senders? If there's two senders, then yes the smaller sender sends its info to the gauge. Maybe it wasn't until 2000 that they did away with the second sender.