Electrical WTF, help please?

[jeepdreamer]

Hi folks,
Stuck scratching my head here. Working on an M1087A1P2 ExpandOvan which is basically brand new at 197mi.
With vehicle running (which it does just fine) I get a Check Engine (CEL) and Parking Brake (normal and ok). **With vehicle off and master power at the battery box off**,the gauges don’t “park”. Instead, they read the previous levels as before. Coolant temp gauge has somehow gone all the way to the right, past max and never moves. But every other gauge reads something. Pump the brakes to delete air and the gauges still read at about 120.
Trying to decide what’s causing this but I’m hitting a wall.
Opened the dash and see this:

Pretty sure the black box to the left is the MMDC but I have no idea what this grey-ish rectangular box is. What I do know is the wires coming from the left side (numbered ?3660 ) go to a connector that continues as J917. None of those #s appear in the schematic. Also, if you zoom in you can see where the wires into the connector look blackish/burned?

Also, there are 3 wires (B1,B2,and B3) coming out of the right side of that box. The connector they go to looks burned as well.
So…
What is that rectangular grey box? And where are those wires going (again, not in my schematics). And, is this a possible cause of the gauge issue?
FWIW, I opened the grey box and there are two small relays inside as well as two mini 30a fuses. Fuses are good. I do see some sort of circuit board behind all the wires.
I’d appreciate any thoughts or ideas… thanks!

 

[GeneralDisorder]

The burnt look is due to the vulcanizing process of the rubber onto the wires - that's typical of that type of connector and doesn't indicate a problem in my experience.

The MMDC calibrates the gauges when powered up. They "park" when powered on, not when powered off. So I don't think that's an issue.

Two thing immediately come to mind - the coolant temp gauge sounds broken. Especially if it doesn't calibrate when the MMDC resets all the gauges when you turn the ignition on.

I've seen a LOT of the MMDC's with bad pressure transducers - it's a weak point of the design especially if the air dryer isn't maintained. I added inline desiccant "air tool" filters to my truck after repairing the MMDC pressure transducers - I bought a couple extra MMDC's from ebay and every single one of them had one bad pressure transducer - one had only 35 miles on it (mileage is stored in the MMDC).

Another thing I've seen is a bad frame to cab ground connection on a brand new A1P2. They didn't properly remove the paint under the cab side of the strap when they installed it at the factory. It's on the passenger side behind the grill. We had a huge voltage drop across the ground when testing from the PDP ground at the grab handle to the alternator ground strap and traced it to the cab ground strap behind the grill.

It sounds to me like you have a bad gauge and bad transducers in the MMDC. Try a gauge and an MMDC from another truck.