Apparently I haven't been paying enough attention to my disco because today my ultra gauge started acting funny. It locks up on a blank screen while the ignition is on and stays powered when the ignition is off.

I haven't checked the ultra gauge with another car yet to make sure it's not at fault but thought it wouldn't hurt to check what the collective wisdom has to say.

 

OBDII diagnostic port has +12 volts on pin 16, to fuse F20 under dash to fuselink 3 under hood to battery. So it would have power even when truck is off. It does not draw enough to kill truck under normal conditions (parked for a week, etc.).

 

Yep, what he said. That port always puts out power. Running, with just the electric on, or even when there's no key in the ignition, always has power.

 

Right as listed above. My ScanGuageII turns off when the ignition turns off. But I also have the FS3 Volo fuel saver chip tapped in from the wires behind the OBDII plug. The chip shows LEDs lite up when the Discovery is turned off. It has never killed my battery for 2 years...as LEDs take little to no voltage.
The ScanGuage must have an auto off feature on the OBDII port.

 

I checked the gauge with another car and it seemed to work fine. When I connected it back up to the rover, I only found 18 gauges instead of the usual 30+. I had it rescan again for gauges and it found the right amount.

Could a loose wire be causing a partial ECU signal or could the ultra gauge still be suspect?

 

Take a glance at this drawing from the RAVE electrical manual. You will notice that every computer on the truck outputs data to that one single wire connection. The Ultra Gauge is only looking for codes from the ECU, and most likely does not understand the dialect of the other modules. While it could be a bad connection, it could just be signals for other modules making interference, or wiring to those modules grounding out, or a bad other module.

Kinda like the old party line phone systems, and the fastest grapevine communication system ever invented, the infamous Tell-a-Woman-with-a-Telephone.