I should probably mention the first time I had the message I quickly checked the oil level so I was comfortable knowing I wasn't demolishing the case. I also don't see any odd leaks or drips on the garage floor

 

I think your 1st guess is the right one, one of the connectors is letting water in and causing an open, or a short.

Inspection is the only thing you can do.

 

Thanks ray, good to know I’m not crazy. Your response implies there’s more than one connection on the T-case. Is that correct or just one harness at the case?

 

I have to play dumb, I only remember, one harness with a number of connectors; but there could be 2.

 

Here is some schematics to run off of.

Good close up high quality pictures of the transfer case connector male and female would help also.

 

only when the undercarriage is wet.

Also, just thinking, check any grounds on the frame under the cab and anywhere the harness is down low.

 

Good ideas. Thanks! Those schematics I believe will be extremely helpful. It’s supposed to be warmer this weekend so I’ll look at it probably Sunday. It’s almost due for an oil change too so I’ll just add this to the service of the day.

 

Ok, I’ve dug into this. There appears to be a connection that appears to be a speed sensor. Then there is a second connection that is plugged into the shift motor. Further research shows that some select transfer cases have two speed sensors, for those of you following along at home.

These all looked to be in decent shape, but I cleaned them and put some electrical connector grease at the boot

I did see this connector that was laying on the front axle and was loose, pointing at the ground and gross looking. I cleaned it too but I’m not sure what it serves.

I also cleaned the two grounds that are directly behind the driver front wheel, side by side. Not sure if there’s much else to do until I have a full failure. Any other thoughts?

Before photos:

 

With the snow we had here on Wednesday I was able to take it out in the slush yesterday afternoon. I did still get the "service 4wd" message and it wouldn't shift modes, it wouldn't even try (blinking light). I intended to read the code with the scan meter when I got back from the hardware store but when I shut the truck off and then turned it right back on the parking lot, everything had reset and I could select auto and 4wd again. No error codes when I returned. I guess I'll have to wait until a terminal failure to find the problem, or I completely missed something on Saturday

 

I did manage to get it to hold the code long enough to read it yesterday. C0306 Range actuator control circuit short to ground.

I believe that’s an issue with the transfer case encoder motor. Or there’s a harness/wire issue I couldn’t see in my first inspection

 

Closing this out. I got the wild hair yesterday to start tracing the wires back from the sensors and motor controls. Found where three of the wires in the main harness nearby had rubbed through the plastic wire mold and the insulation jacket. Not sure what caused it, but possibly rubbing from the front driveshaft based on the wear pattern. I repaired the three wires with waterproof seal, then repaired the wire mold. The used tape to seal it up again and found some harness attachment clips to support the wires off the transfer case.

Then I realized I never took a photo for anyone. Sorry all. I tested the system and confirmed it was working, of course it's 75 degrees and bone dry out. The probably is likely fixed however, based on Rick's wire diagram and the colors of the wires that were damaged.