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Mack E7 350 coolant temp gauge isn't working

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Ok so I have this 06 Mack 688 and the temp gauge is inop, (pegged out at 260°). I've tested the wires behind the gauge. 


White wire/ I= battery voltage 12v

White wire/ S= signal from sensor

Black wire = Ground

So I disconnected the S wire and tested. Reads a solid ground with the temp sensors unplugged or plugged in, at the water manifold. 

  The gauge it self works, needle goes back to cold when I disconnect the S wire. Then I tap the S wire to the S post on the gauge and it jumps right back to pegged out. 

So I'm thinking the S wire is shorted.

So I go to the sensors and test the bigger one unplugged, One wire is 12v and the other is neither ground nor power but shows voltage on me power prob so I know it's not broken. 

I'm just wondering what sensor is the one that operates the gauge itself. I read somewhere on here the one that pionts to the back of the truck is the ecu gauge and the one that pionts to the driverside is the gauge.

Also would like like some more input on why it might be working.

Thanks in advance!



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