Isx 870 engine coolant/oil temp gauge/Jake brake

[Crossfire]

I’m looking for assistance with 2006 KW T800 with an ISX 870 de-mandate from Running Rough with a few different issues that may or may not be related:

The Jake brake will not actuate and the engine coolant temp and oil temp gauges bounce around and are erroneous.

This truck is new to me so I’ve been correcting problems as they have arisen. Before I purchased the truck, the ECM has been replaced about 800hrs ago (unsure how many years ago). The ECM harnesses have also been replaced recently. I’ve cleaned all the grounds and positive connections. New batteries have been installed. I’ve checked the ECM connectors, the cab harness connector, and various switch connections for corrosion and continuity. The coolant temp and oil temp seem to be within range when checked through Insite. The clutch pedal and brake pedal switches also are working when using Insite. The Jake brake selector switch shows 66% on level 3, 66% on level 2, and 33% on level 1. The cruise control does work properly.

How do the temp gauges work? Are they sent a 5v signal from the ecm or ground from the ecm?
What are some of your thoughts on things I may be missing?